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Kinetics of the hydrogen atom reactions with benzene, cyclohexadiene and cyclohexene: hydrogenation mechanism and ring cleavage
Chemical physics 7 (1975) 229-243 Q North-Holland PubUshing Company
KINETICSOF THE HYDROGENATOMREACTIONSWITHBENZENE,CYCLOHEXADlENE ANDCYCLOHEXENE:HYDROGENATIONMECHANlSMAND RING CLEAVAGEl R. KNUTTI** and R.E. BmLER Laboratory fir Physical Uxmistry,
Swiss Federal Institute of Technology, 8006 Zu&h,
&itzaM
Received 26 September 1974
me hydrogen atom raction with benzene and the subsequent elementary reactions with H-atoms were studied in de tail, using a fast gas flow in a linear reactor at pressures in the mbar region, with a mass spectrometer for the product analysis ‘Therateconstant determinations were based on a kinetic model, which includes the strong catalytic H-atom recombination on the wall, caused by adsorbed reactant molecules and also corrects for !he pressure drop within the reactor.The H-atom concentration was determined by scavenging with NO1. The method was checked by determining the rate constant k(H + trun+butene2) = (4.6 i 1.2) X LOaM-’ s-l, which agrees with the literature value of Daby et al. within experimental error limit+ The rate constants determined are: kf.H +c&) = (18r 0.2) x 10’M” s-1, kCH+C&) = (3.4 f 0.6) X lO’ M-’ s-l , &(H+ c+,HB-1.3) = (1.41* 0.26) X lo9 M-’ s-l ,
k(H + c-&&-1.4) = (1.04 t 0.18) X lo9 hl-’ s-l , k(ki + c-c&i],)
= (4.9 f 0.9) X 108 hi-’ s-t
,
k(H + toluene.) = (5.1 f 1.1) X 107hlw1 5-l .
H + benzene is the rate determining step for benzene hydrogenation From the rate constant for H + C& it is concluded. that benzene is.refonncd from some intermediate reaction products (CbH; and/or C&l:). These back reactions should be suppressed at high pressures, in agreement with results by Sauer and Ward (l-54 bar). The mass spectra show that H + benzene at mbar. pressures predominantly initiates ring cleavage to form methyl ndicals, methane and Cz_hydrocarbons as the main product% However for H + cyclohcxene 85% of the products is cyclohexam The results for H f cyclohexadiene are intermediate to these extremes. It is argued that accumulation of vibrational energy over two consecutive reactions must be responsiile for the ring cleavage, which most likely cccurs from CgHi* and CeH;i.
1. Introduction Reactions of hydrogen atoms with olefms have been studied quite extensively during the past few years [ 11.However much less information is available about reactions of H-atoms with aromatic compounds. A few resultshave been published for the reactions of hydrogen atoms with benzene, which were derived from kinetic spectroscopy, electron spin resonance and product analysisin either aqueous solutions [2] or gaseoussystems with pressures above 0.6 bar [3,4]. IIis resear& has been supported by the “Schweizerischer Nationalfonds zur Fdrdenq derwissctiaftichzn Forschuq”. l * Present address: Department of Chemistry, Cornell UI& vasity, Ithaca, N.Y. 14850. l
The primary adduct radicalhas been identified as the cyclohexadienyl radical [3,5,6]. In this paper we report in detail results from 10;~pressure studies (mbar region) with a fast gas flow system [7]. The various consecutive elementary steps, initiated by H-atom addition to benzene were studied separately to gain detailed information on the rate deterknin step, the reaction mechanismand the ring cleavagereactions occuring at these low pressures.A mass spectrometer was used as a particle detector and the time dependence of the educt, transient and product concentrations were measured. The method allows the determination of second order rate constants from about IO6
to 10toM-ls-l, cover@ thereby the full range oT the known rate constants for H-atom reactions with olcfm [ 1,8,9]. The method is not suitable for the detection of unimolecular reactions, e.g., the decay of
230
R Knutti, R.E. IhlerJki-reactions with benzene, c-C&, wtd c_CJf10
vibrationally
excited intermediates, since the accessible first order rate constants are limited to lessthan about lo3 s-l. Since we publishedcur preliminary note [lo] a similarinvestigationof the H-atom addition to benzene has been publishedby Kim et al. [I 11. Our results will be compared to those of Kim et al. and possibleexplanations for the observed difference in rate constant will be given.
[H] : [benzene] : [He] = 1 : 2 X 10B3: 15 .
2. Experimental 2.1. Gasfiw
age reaction time of 12 ms (with a linear flow rate of typically 20 m s-r). The time resolution imposed by the mixing process at the inner tube jet (5 pin-holes, total area of I mm*), that is the time to reach radial homogeneity of 95% [12], is 1 ms in the present arrangement.The pressureas measureddownstream from the detector leak was in the range of 0.4 to 0.7 mbar. The relative concentrations involvedwere typically
The H2-and He-supplyrate were measured individually by rotameter, regulatedby needle valvesand mixed after pressure reduction. The ground-stateHatoms [ 131were produced in the gas mixture by a microwavedischarge(cavity no. 5 in ref. [ 141)powered from a 2435 MHz-magnetronthrough a unihne and an E/H-tuner for optimum coupling.The organic compound was either injected as a liquid from a motor driven syringe through a septum into the vacuum system with the evaporation at the needle tip assistedby infrared light or introduced as vapor through a needle valvefrom a storagebulb at elevated temperature. In the former case the concentration was calculated from
mctor
Hydrogen atoms produced by a microwavedisdiluted with helium and mixed with an organic reactant in a steady flow system of two coaxial quartz tubes as shown in fig. 1. A futed distance downstream from the mixing point, correspondingto a particular reaction time, the system is sampledby the tapered leak of a mass spectrometer (reversed metal nozzle, 0.3 mm diameter, coated inside and outside by DuPont teflon enamel 852-201). The maximum reactor length of 25 cm corresponds to an avercharge are
organic compound
microwauc dlschsrgc
vacuum syslcm
Fig.1.Principleof the gas flow system. Dimensions are:reactortubeinrrctdiameter19mm,innertube outer diameta inner tube jet 5 pinholeswithtotal sea of 1 mm2, MSleaL dimctcr 0.3 mm.
R Knurti, R.E. &ihler/H-reactions withbenzene, EC&
the syringecalibration, in the latter the corresponding masspeak was calibrated relative to the injection method. At very low benzene concentrations the vapor was diluted with He to assurea minimumvelocity at the gas-mixing jet of about 10 m s-! to avoid back diffusion into the jet. To allow fast on-off-switchingof of the organic component a by-passto the reactor was used in the off position. Thus the source flow was kept constant and the disturbance of the supply system could be minimized. 2.2. H-atom yieId Since all experiments were carried out under conditions of H-atom excess, an optimum H-atom yield was important. Maximumdissociationwas obtained by diluting the hydrogen gas with helium, reducing the wall temperature in the dischargeregion by intense air cooling, and not removing traces of water. Whenthe supply gaseswere aried in a liquid nitrogen trap the kinetic data and the relative product yields did not change,however, the H-atom concentration dropped by a factor of 2 to 3. Hence most experi. ments were run without the liquid nitrogen trap. Hatom lass by 3rd order recombination in the gas phase is negligibleunder the conditions of these experiments (half life estimated at 30 s [IS]). H-atom recombination on the quartz surface was greatly reduced by coating with D&Film SC-87.With this coating the H-atom loss was typically 5-10% over 20 cm of the reactor tubing. Other tested coating like phosphoric acid, Kel-F-90,Teflon (DuPont enamel 852201) or treatment with fluoric acid either &owed less surface protection or more rapidly lost their protecting effect. In all experiments the largestH-atom 10s~ was due to the catalytic effect of the unsaturated hydrocarbon, which is adsorbed on the wall during the experiment. As this effect could not be suppressed,it will be included in the kinetic model. The absolute H-atom concentration at the position of the inner tube jet was determined from the intensity difference at m/e = 2 with H-atom production on and off. In order to avoid errors due to H-atom recombination downstreamfrom the inner tube jet, all H-atoms were scavengedwith the very fast reaction [ 16,171 uH t bN0, + cHzO + do, + eN0. Since the stoichiometry of this complex reaction is
and c-Cdl0
231
still debated [ 181,it was checked by variation of several experimental parameters(total pressure,partial H, pressure,intensity of air cooling in dischargeregion and H20-content). Fig. 2 clearly showsthat d/c = 1.0, and with material balance b/u =d/c + 0.5 = 1.5. in agreementwith expectation [16,19].
2.3. Flow conditiorzin the reactor With a typical mean free path of 4 X lUIL cm(O.5 mbar) the reactor flow is stii dominantly viscous.The radialvelocity distribution is therefore nearly parabolic and the dwelling time of gas molecules between the mixing jet and the detector leak would be dependent on their average radial position, unless radial difksion is fast enough to homogenize the concentrations. The condition which must be met for homogeneous flow, thus allowing the system to be treated as a linear reactor, is [ 171
a = D/kc,-,R2 Z 0.5
(1)
(D = diffusion constant; R = reactor radius;k = rate
Fig. 2 Determination of the H-atomconcentration by the ICaction with NO2:Test of the stoichiometry invohrcdfor tie variouswnditiansof the syrtkn.(Variationof total presswe 0, of partialHrprcssun0, of air cooling iatcruity at dip chargel , of water content in dikhargc A_)For details see text hfas9intatsities are calibratedand given in relativeunits.
232
R KnuttL RE.
BihlerlH-reactions
constantof 2nd order reaction and CO= initial cons centration of the speciesinvolved).In all experiments which will be reported here this condition was satisfied [e.g.,0(HtC6H6)=50;O(H+C6H8)~ 1; a(H +C6Hlo) = 2.51.The total pressure of the systern was measuredby a gas independent, capacitive torr-meter (Atlas MCT)downstream from the detector leak. The pressure drop along the reactor length, determined by measuringthrough the inner quartz tube, was linear within experimental error. It arnountcd to 10%at a total pressure of 0.4 mbar and 4.5% at 0.66 mbar.
with benzene, c-C,&
and c-C6HI~
(99.9952, dewpoint 220 K), benzene(Flukapuriss, 99.93%),benzene+ (Flti puriss, 99.5% deuterium atoms), cyclohexadiene-I,3(Flukapurum 98%), cyclohexadiene-I,4 (Fluka purum, 95% with 5% benzene), hexatriene-1,3,5 (Fluka pract., 5% impurities in mass range m/e = IO to MO),cyclohexene (Fluka puriss,99.95%), cyclohexane (Fluka puriss, 99.9%), pans-butene-2 (FIti puriss, 99.8%), toluene (Siegfried puriss, 99.5%).Nitrogen dioxide NO,&04 (Fluka purum) was first treated with oxygen, then sublimed severaltimes in vacuum until the solid N204 was completely white.
2.4. Massspecnomem’c analysis 3. Kinetic mwiel A Bendix time-of-flightmass spectrometer type 12-101with a W-107 ion source was used. It was equipped with a scope output for fast monitoring of the system and with two analog output channels.The mass spectra could be recorded by a fast W-recorder (SE. 2005,4 channels),a dual channel strip chart recorder and an on-line DPDW computer for data acquisition and spectra analysis.A detailed description of the da& system has been given elsewhere 1201.To avoid saturation of the electron multiplier by the ions of the carrier gas (He) all spectra were recorded for electronenergies(EJ below the ionisation potential of He (E, < 24.6 ev). The loss of intensity due to low ionisation energy was additionally advantageous,because this simplifiedthe analysisof overlappingmass spectra of the various compounds present in the reaction system. To verify the peak assignmentsthe correspondingionisation potentials or appearance potentials were determined by the method of Loss@ et al. [22] and compared with literature values.Vibrationally excited reaction products may yield different ionisation potentials and fragmentation spectra than in their ground state. When the reaction products were sampled at the torr meter (fig. 1) no difference ia the spectrum was detected. Since the additional distance traveled by the gas allows for collisionalde-excitation it is concluded that possible viirationally excited .sp~ its do not affect the mass spectra.
In all experiments with excess H-atom concentration a substantialH-atom loss was detected, which was often more than hundred times larger than the consumption of benzene or any of the other unsaturated hydrocarbons. Cyclohexane did not show such an effect. It is concluded that the H-atom loss is due to the catalytic effect of the unsaturated compound on the rate of H-atom recombination. After the benzene supply to the flow system had been switched on, the H-atom concentration at a futed position downstream in the reactor dropped slowly over several minutes (fig. 3). After switchingoff the benzene supply the H-atom concentration returned to the original value within 10-20 minutes. If traces of water were eliminated from the gasesby a liquid trap before entering the reactor then the rate of recovery to the originalH-atom concentration was stowed.Without H-atomsin the system (discharge switched off) the intensity of m/e = 78 did not show this slow response on switchingthe benzene supply. Hence, the catalyzed H-atom recombination is attributed to a surface reac tion and it increasesas the adsorption of the unsaturated compound progresses.The initial reactions for a system with benzerie are therefore gives by reactions (2) and (3): LB
H + CsHs'CaH, ,
(2)
25. Chemkds
H + wall (with adsorbed bewne)5H2.
(3)
The followinggasesand chemicalswere used as supplied: argon (99998%), He (99396%), hydrogen
Since [H] * [benzene] fat all systems, reaction (3) predominantlydeterminesthe time dependence of
R Knutti, RE.
Wihler/fi-reactions
with benzene, CC&
I
I
B
ql
H-atoms only no rc4cllcn
233
and c-C&lo
H + benzene
mlnular
*
benzene only no reXlIon
Fig. 3. Relaxationprocessdue to the benzene adsomtion on the reactorwall, measuredat reactorIen& I= 20 cm with [He]:[H~,:(Bz]l~= 10:1:105.[lndex 10indicatesthatthe concentrationismered atx=I(mass spectrometer)inconditionwithout reaction] ?%escale for [ HJis 10’ times larger than for [ Bz]. At time zero the benzene supply is switched on to a hitherto benzene free system: simultaneouslya slow drop of [If] due to the catalytic waUeffect and a correspondingreductionof benzene consumption A[&] occur. After 5 minutes the hydrogendischargeis switched off: A[Bzj is reducedto zero immediately.
the H-concentration. Although it is a wall reaction, the fast radialdiffusion allowsit to be treated as a
homogeneousreaction. The following6 kinetic models were tested: (A)
-dPW
=~wlWo[Hl 3
@I
-4w~
= k,lw,a1*
I
(C) -43il/d~=~,,,lW[Hl, 03 -4Wd~=$J’W[HI2s 09 -4I-W =0, at constant pressurep = p. ; and (F)
-4Wdr =~,lIW,W1~
withp = pi + p(I-x).
[x = position within the reactor
mixing point at x = 0), I= total reactor length andirgiexforvaluestakenatx=I,pl=gaspressureatt.he reactor end (x = I), and 0 = pressuregradient.] Model (E) neglectsany H-atom loss, making reaction (2) pseudo first order. Although this is a standard procedure for this type of investigations [ 1 l] it does not fit our data. Model(A).(B) and (F) assumethat the effective concentration of benzene on the wall is uniform throughout the reactor, and proportional to the initial benzene concentration [B$. Model(C) and (D) assumeit to be proportional to the actual gas phase concentration of benzene. The best fit with experimental data was obtained with model (F), which is identical to (A) but includesthe effect of a linear pressuredrop along the reactor. For the stationary linear flow reactor without pressuredrop the reaction time f (from mixingpoint to samplingpoint) corre sponds to the reactor length I= uot (u. = constant linear flow velocity). For model (F) the pressure drop
(with
R Knurri.R.E. fihler/H-reactions
234
with benzene,
along the reactor affects both the flow velocity and the concentrations. In this case the rate equations for reactions (2) and (3) are:
cC6H8 and c=Cdflo
zone. With eq. (7) the rate equation (5) for benzene is integrated to give:
(4) with
(3
p=p,+b(l-x)
Y=
(Index 1 labels values for x = I, index 0 values for x = 0.) In integral form, eq. (4) is (7)
or for the ratio of H-atom concentrations at the sampling point (x = Z)with and without benzene:
(8) (the double index l0 labels the concentration at x = 1 for zero reaction, i.e., for pzJ,-, = 0). A test for eq. (8) is given in fig. 4. The slopes increase with time as is expected due to increasingwall adsorption of ben-
4u[B4oP;
( P,z@o- PI) exp -
Aqq)P 1
xl0pLxP(gp2) dP.
(6)
_
1
(93
This result is equivalent to a pseudo first order reaction in 1,if multiplied by the correction term 7. The latter depends only on experimentally known data and was calculated by expanding the exponential function within the integral in a Taylor series (centered at p = pi). By summingover the first 9 integral terms only, an error of < 0.1% remains.Typical values for 7 are between 0.5 and 1.1.
4. Results
Based on eq. (9) there are 3 methods to determine the rate constant kg: A. Variation of the reactor
0
5
10
13
25m
I
Q. 4. Tut of modd (F) far catelytic H-atomIOU ‘lb time dlI?iQ ahidl benzene vas dlowsG to adsxb on the reacttlr wdl is taken u a parameter.Devbions from linearity only OCCU9t IlUXhUIl IUCiOI ISO&.
length
- This
is the
usual
method in fast flow experiments. The benzene consumptioc is determined as a function of the reactor length by on- and-off switchingof the H-atom supply (microwavedischarge).A plot of ln([Bz]lo/[Bz)) versus(yl) yieldskB from the slope (f@. 5). in these experiments H-atom losses,although corrected for, are kept to a minimum.The H-atom concentration is monitored at m/e = 1. Experimental details and re suits for the H-atom reactionswith benzene, the two isomeric cyclohexadicne. cyclohexene and tmnsbutenc arcgivenin table 1. The latter compound was used for a cross check with litcmtnrc data [9] to test the method. B. Relaxation expeninent with fad renctor length (&J 3) - Since the adsorption of benzene on the wail
requires severalminutes to come to equilibrium, k, change with time (f#. 4) after switchingon’the
will
R Knutti, RE.
BihlerJH-wacrionswith benzene, c-C,&
235
and CC~HIO
0.2 -
0
I 5
I lo
I 15
,
I 20
Y-1
25
cm
Fig. 5. Rate constant determinationby the method of variablereactorlengthf Experiment2 of tabk I.
Table 1 Rate constants for H + M, determinedby the method of variableIeactorlength 1at 303 g Exp. 1 2 3 4
5
M
0/\ -
6
0/\
7
Q
8
.
Q
9 :;
/
Pressure Mm)
uo (m/s)
[HeloX 10’ (M)
[HIoX 10’ (MI
[M]oX 109 (MI
A[HII[Hlo kBX lo-’
n
0.42 0.41 0.41 0.41 0.42
19.0 19.3 18.9 19.6 18.2
1.56 1.50 1.50 1.45 1.55
9.8 9.7 9.8 13.5 10.3
25 2.1 1.3 1.2 2.6
0.34 0.09 0.08 0.09 0.20
0.38
20.1
1.40
9.3
4.8
0.28
128
t24
60
0.38
20.1
1.40
9.3
4.8
0.35
99
t16
15
0.38
20,l
1.40
9.3
7.2
0.29
44.6 * 8.2
16
0.42
18.2
1.55
10.3
5.3
0.65
46.3 5 8.8
17
0.42
la.2 18.8
1.50 1.55
10.3 14.1
5.3 5.1
0.37 0.41
43.4 36.0 fe 8.7 ?.6
30 14
(M-’ s-9
1.43t 0.26 1.63t 0.25 1.73L 0.30 1.66i 0.27 1.4ot 0.24
21 37 57 38 32
n = numberof data points, which determinek~. 6[HIfiH] o = averagefractionalH-atomloss Withinthe reactor,due to the catalytic wall effect Enor limits stated are for a condifcncclhit of 95%.
136
R Knutti, R.E. LGhlerJH-reactions with benzene. c-C,&
aI=
Scm
@I I
(x8-
andc-Cdjl~
=lO
cm
@l=lScm 0.7 -
@ l=20cm @ t’25cm
46-
06 t=15cm @ 1=20cm
0.5 -
p3J
t=2os
0.4 I
/
I
5
I
I
lo
15
,Y*l 20
*
cm
Fig. 6. Rate constant determinationby the relaxationmethod. Experiments III of table 2. Lime 8 correlatesall data points taken 20 seconds after the start of the experiment(time duringwhich benzene was allowed to adsorb on lhe reactor wall).
Table 2 Ratsconstantsfor the H-additionreactionto benzene, as determinedby the relaxation method (at fixed reactor leqtb 303 K)
I
25 25 25
PtS.Ql~ (mbar)
L1
[HeloX 10+s 0
[HIOX lo* Ml
[BzloX lO*O Ml
0.68
Il.3
249
221
21
0.69
11.1
253
2.25
4.3
111
slO;’
n
250 * 0.40 2.11*0.34 255 t 0.47
21 15 11
7.17 i 1.90
40
3.08* 0.65
40
25
3.15
l
0.55
40
5 10 15 20 25 15 20
6.45 f 236t 248 f 23 1 i 233 i 234 f 205 f
1.17 0.45 0.62 0.35 0.36 0.36 0.29
21 26 10 41 41 41 47
10 [1
2-i
17.5
0.69
11.2
253
benzene supply in a previously bcnzefie free system. 7, too, will change during this time. A plot of ~JI(~z~~/ @J,) VCISUS 7 (or versus74 since 1is kept constant)
223
26
I and at
again yields the rate constantkg from the slope. Such plots for severalreactor lengths are shown in fig. 6 and table 2 lists the data and results.
C. Determination of relative mte constunts - For stu-
dies of the initial elementary reaction only, relative rate constants can be determined by simultaneously injecting two reactant molecules (hit and M2). All conditions are exactly the same for both H-atom reactions From the relative mass peak intensities with and without H-atoms from the discharge,the rate constant ratio is calculated by eq. (10) as derived from eq. (9):
3 = w?qom1
I)
k2 ~W2lolF121) -
(10)
From 43 individualdeterminations at widely varying conditions (pressure, reactor length, H2concentration) the rate constant ratios for benzene~r$benzenehe was found to be y
+w45)
m +c&j)
= 1.90*0.17.
The averagefrom 29 determinations of the H-reaction with toluene versusbenzene gave k(H + toluene) = 2.81 +,0.34 . k(H + benzene) 5. Discussion 5.1. Kinetim of H + benzene
The rate constant data from method A (variable reactor length, table 1) suggestsome correlation with the averageH-atom loss in the reactor (m]/[H]u). The slope of this correlation indicates that model (F) does not yet fully correct for the catalytic H-atom loss, but extrapolation to zero-loss is possible (see below). Method B (relaxation experiment, table 2) gives rate constants which are about 50%higher through. out, all being close to 2.5 X 10’ M-l s-l, except for the very short reactor lengths(results at 5 cm and at 10 cm). The model applied assumesuniform adsorption of unsaturated hydrocarbon throughout the reactor. At short reactor lengths this is a poor approximation, as the mixing region represents a non-negligible part of the total reactor length. The corresponding results are also characterized by abnormally large error limits. Both methods should give the same rate constants.
isentirely based on the H-atom decay, whereas the variable reactor length method uses 7 [eq. (9)] only for correction. It is sr& gested therefore that the method with variable reactor length is more reliable.This is further supported by a different analysisof the relaxation data: the experiments labelled II and III (table 2) allow to correlate data points for equal degree of adsorption with the reactor lengths. In fg. 6 all data points of experiment III, taken 20 s after the benzene source was switched on, are linearly correlated by line 8 . From its slope the rate constant is k(H f C&&) = 1.5 X lo7 M-t s-t, in rather good agreement with the results in table 1. Such rate constants are plotted againstadsorption time in fig. 7. It is straightforward to extrapolate to zero adsorption time, equivalent to zero catalytic effect. The result is 1.6 X IO7M-l s-t for experiment Ill (fig. 7) and 2.0 X IO7M-t s-l for experiment II. Fig. 8 now correlates all rate constants from method A (variable reactor length) and method B (as derived for zero adsorption) with their corresponding H-atom losses.The linear extrapolation by the least square method then yields the best value for zero Hatom loss: However the relaxation method
k(HtC6H,)=(1.8*0.2)X
107M-‘s-t.
In table 3 our resuh is compared with literature data. Coveringthe pressure range from IO-3 to 54 bar, the valuesare surprisinglyclose, escept for the value by
Fig. 7. Rate constanu derived from rchxation upaimencs as dctemhcd fmmthef&pcdcncc at cqupldegreeaf benzene adsorption(experimentIII, tlblc 2). plottedagainsta&qtion thG TheLinareXtJapohffon yiddr kBfor zeroadlolption
238
R Knutti, RE. Bihler/H-reactiom with benzene, c-C&
k, * 107IA3.0 ”
1.0-’ a[Hl
1 0
ar
a2
0.3
0.4
-IHI,
Fip. 8. Extrapolationof all rate L’ONIM~S to zero H-atomloss. b: Damfrom variablereactorlength (experiments1-S of table 1); l : data from relaketionexperiments(II and 111of table 2) alreadyextrapolatedto zero adsorptiontime (ix., zem H-atom loss).
Girouard [23] which we believe to be erroneous [24].
The very recent result of Kim et al. [ 1I] which is about three times larger than ours, was determined by a similar variable reactor method, but based on a different technique to determine the initial H-atom concentration and does not include a correction for Hatom loss andpressuregradient along their50 cm flow reactor, Allen et al. [25] used rhe MoO2-method to measure H-atom concentrations. This method is known today to be unspecific in some cases. Yang’s result [4] from competition studies in gas phase radio-
and c-&Hlo
lysis is close to our value, but no error limits are given. The high pressuredata givenby Sauer and Ward[3] derived from observingthe transient absorption of the cyclohexadienyl radicalin pulse rarholysis,compared to our data imply a rather low pressureeffect. The rate constant increasesby a factor of 2 only for a pressureincreaseof 1000. The results from relative rate constant determinations (method C) with benzenedb show that there is no primary isotope effect, in agreementwith H-addition to the benzene ring [reaction (I l)]. The rate constant ratio observed is k,/k, = 1.90 f 0.17, which correspondsexactly to the value determined by Kim et al. [1 I]. Eq. (10) showsthat the ratio of the rate constants is neither a function of the initial H-atom concentration, the total pressure, the flow velocity, nor of effects from H-atom loss and pressuregradient within the reactor. Wesuspect therefore one of these factors to be responsiblefor the discrepancyof the absolutevalues.Weexplain the kD/kwratio by isotopic exchange,due to reactions (I le), (12e) or (13e) which reform benzene.
H t C6H6+$H;%,H,
(11)
H t C,H,%,H;=+C,H,
t H,
L 12w C6Hs Table 3 Cone&d rate constants for the H-atomaddition reactions(comparedwith literature data) Reaction
Pressure(mbar)
kg (M-1s-11
Reference
H + benzene&
0.4 - 0.7 0.59 0.3 - 0.96 a.7 660 1 -54bar
(1.8 f 0.2) X 10’ x 109 2.8 (;.; f 1.0) X 10’ X 100 x 10’ (i.7 * 0.6) X 10’
this work Girouardet aL [23] KbnetaL[ll) AllenetaL [25] Yang141 Sauer,Ward[ 31
0.4 - 0.7 0.4 - Q7 11 -54bar
(3.4 t 0.6) X 10’ (5.1 f 1.1) X 10’ (1.0 *0.1) x 100
this work this work Sauer,Ward[3j
H + benzene-de H + toluene H + cyclobexadienrl, 3 H + cyclobexzdicnoI,4 H + cyclohexene
0.4 0.4 0.4
(1.41* 0.26) x 109 (I.04 f 0.18) X lo9 (4.9 f 0.9) x 108
this worka) tis worka) this work a)
H t rmnobutennc-2
0.4 1.3
(4.6 * 1.2) X 10” (5.3 * 0.4) x 10’
this worka) Dabyet al. [91
a) Correctedfor H-atomloss (7+ctext).
02)
l f3f H + C6H,---K,H **%C6H6 + H,
13e’ other dissoc.products 03)
e 13wC6Hs.
For deuterated benzene these back reactions also yield C&IDS,which is easily detected in the product mass spectra (fig. 9b, peak m/e = 83). As the first elementary reaction of H-addition to benzene is rate determining for the hydr~enation of benzene (see table 3), all intermediatesare of very low concentration. The apparent pseudo 1st order rate constant kobs(Ht C&J is therefore easily correlated to kt tt by the steady state assumption:
atom fess (cf., fig.8) wasnot feasible. However a correction of +lt% was estimated from the H-atom iOsS for the rate constants with cyclohexadiene. cyc!ohexene and frus-butene-2 (table 3). The rate constant for H + Izrms-butene-2co~e~onds to the literature value within experimental error. This is support for our kinetic model and for the accuracy of the concentration measurementof the H-atoms.
k llw h2w k obs=kllf kttetk 11w+k13~~k12e+k12w
1
kl3lWl
h3e*+k13w
+klle+kllw+k13fiHIk13e+k13e’+k,3w
I *
WI
At the high pressurelimit we expect kobS= k, Ifsince the rate constants for coilisiond deactivationkr rur klzw and R13w becomeJominant.Comparingour data with those of Sauerand Ward13J which were ob-
tained at high pressures (l-54 bar), we find kobs(Sauerand Ward) =52. k&$his work) This indicates that at least one of the three back reactions(1 le, 12e or 13e) must have a rate constant of the same magnitude as its competing reactions. No preference for reaction (1 le), (12e) or (13e) can be derived.The SSRdetection of H-atom from the reaction E)+ C&e by Rim et al. [ 1I] might be indicative, but their reaction conditions are quite different from our ([C,H,] about 100 times larger, [H] about 100 times smaller).These different conditions strongly a.f feet the collisionaldeactivation and the competition between the reactions (1 le), (1 lplr)and (13f) is expected to be different.
Therate constants for H-addition to nans-butene-2 (table 1) did not show proper correlation to the Hatom loss in the reactor. An extrapolation to zero H-
d
!’ 20
! -
30
’
40
“’
sb
II 04
70
80
-ma
-r_L 20
30
40
50
00
70
ea
-80
-ea
-do
1*25cm
10
20
30
40
a0
60
-20
70
60
Fig,9. Massspectraof the reactionproducts (electronenergy
18 VI from H-additionto benzene&. and berJzened6,as measuredat different reactortengtttsL Ihc total extent of the reactions,given by AIBz]l[Bzfo isO.O7,0.11,0.13,0.t3,
0.19and0.27for tic sppectra fromtop to bottomrcspectie ly. Inatl spectratfrefragmentationspectnun of benzene and the backgtoundspatnun are subttaeted For the masssifec tsaof the individualcompounds at low electron energincsee table4.
240
R Knutti, R E. Bdhler/H-n?mions with benzene, PC~ Ha and c-Cdl 10
Table 4 bhs spectra of individualproductmolecules for various clect~o~~energies(for comparison with l&r 9 and 11) BtIlZUlC de
Cyclohexadienel,3 3ov
2OV
26 27 36 37 37.5
0.9
38 30.5 39 40 49 50 51 52 53 61 62 63 73 74 75 76 71 78 19 80
1.4
5.9
3.9 0.7 4.3
0.4
120 14.2 0.9
1.8
0.7 0.7 3.9 100 6.6
24 11.9 100 7.0
75V 1.7 1.9 0.4 27 1.0
de
2ov
Cyclohcxadiene-1.4 30 V
75 v
m/e
zov
3OV
75Y
2.5
26
11.1 20 0.6 4.2
27 37 38 39
2.5 125 21 4.5 19.3
1.6 27
0.6 17.2 1.1 3.9 3.6
39.5 40 41 49 50
1.7 4.2 4.6 1.3 122
1.9
6.6 8.9 5.0
1.4 11.0 16.4 11.8 5.7
51 52 53 54 63
0.8
1.9
1.9
4.3
2.5 0.8 20 5.4 0.8
65 67 74 71 78
2.3 0.9 0.6 31.6
79 80 al a2
26 4.7
27 37 37.5 38
3.8 0.s 10.3 a.5 2(1
38.5 39 39.5 40 41
13.5 16.8 15.4 0.8 0.5
49 50 51 52 53
0.6 2.4 1.4 4.1 1.8
54 62 63 65 73
2.8 11.8 100 6.8 0.4
74 75 76 17 78
11.1 3.5
29.8 10.9
79 80 81
73.2 100 6.2
100 86.8 5.0
9.7
The rate constant H f cyclohexene has the same magnitude as the one for H + fiumbutene-2, whereas the constants for H + 9k+CeHg-1,4 and H 4 cycloChHgl,3 are higher by a factor of 2 and 3, respectively. The H-addition to toluene is almost 3 times faster tbarr to benzene. Most likely this is not due to additional H-abstraction from the methyi group, as Hardwick [26a] found an abstractioujaddition rate constarit ratio of C 0.03 in liquid phase (23°C) and Bennett et al. [26b] could not detect benzyl radicals by their ESR cryostat method. An increased reactivity of Qeortho-position towardsH-addition[26b] seemsto be the more likely explanation.The high pres sure valueby Surer and Ward[3] (table 3) is againtwice
1.7 0.9 0.9
7.2 9.4 5.3 3.4
17.9 13.8 6.7 5.8 20
1.8
4.5
12.4 3.2
31.4 13.5
6.3 4.2 28 44.7 14.1
58.8 100 6.1
89.2 100 6.4
100 99.2 6.7 21
9.9 100 76.3 5.0
our value, as was found for benzene. The same argu ments concerning coUisionaldeactivation of the viiratiordly excited intermediates do hold here, as was discussedfor benzene in connection with reactions (lG-(13). 5.3. Me&at&m
ofbenzene
hydrogenation
The rate constant for H-addition to benzene is much smallerthan for the correspondingH-addition to cyclohexadiew (both isomers)and cyclohexene (table 3). The iuitial reaction step (H + C&) is therefore rate determining for the benzene hydrogenation. The product mass spectra for the H-addition to bea zene (18 eV electron energy) at various distances
R Kmtti. RE. BSer/H-reactions with benzene,c-C&
241
and c-C6Hlo
Table 4 (continued) Cyclohexane
Cyclohexene
mle
2ov
3ov
15
75 v
1.1
mle
20 v
30 v
1.0
15
26
21 29 37 38 39 40 41 42 50 51 52 53 54 55 63 65 66 67
4.8 1.7
3.2 13.1
2.1 59.0
68 77 18 79 80
6.4
81 82 83
9.2 69.3 4.6
21 17.5
Its” 25 1.1
;
3.8
29 38
5.4
86 1.6
39 40 41 42 43
5.1
20.7
22
3.0 320 5.5 36.2 28
8.3 73.8
5.2 8.3 4.1 11.6 78.7
50 51 5.? 53 54 55 56
1.1 1.7 100
5.8 1.1 2.8 2.2 100
61 68
6.5 4.1 1.4 6.1 1.1 11.9 43.1 2.8
15.7 2.2 29.7
3.2
100
75 v
6.1 2.2 5.2
10.9 46.7 3.5
downstreamfrom the mixing point are shown in fq. 9a. The strongest peaks are due to methane (m/e= 16, IS), CHrradical (m/e = 15), ethane and ethylene (m/e = 28). The experimentally determined ionization potentials for the masspeakssupport these assignments (fig. 10). Moreovermethyl radicalsare indicated by the changeof the intensity ratio of m/e 15 and 16 with the reaction time (fe. 9a) and by the formation of smallamounts of toluene (m/e =91 and 92, not shown in fig. 9a). The appearancepotential measuredfor m/e = 28 is intermediate to ethane and ethylene, thus probably both compounds are formed. The product distribution (icorrected for the number of carbon atoms withii the product molecule) re-
22
5.0
43.7 27.2 13.2
55.0 28.8 13.6
2.2 6.5
2.1 2.9 1.3 4.5 6.5
15.9 12.1 3.5 3.5
31.3 99.5 4.9 3.5 1.6
33.8 100 5.0 3.1 1.6
69 70 77 79 80
184 I.2 1.5 7.0 12.4
21.3 1.3 1.9 5.1 5.4
20.9 1.3 1.6 26 21
81 82 83 84 85
1.2 21 4.4 100 7.0
5.4 100 7.0
5.5 94.8 6.5
51
11.6 16.1 1.9
3.1
veds a mptisingly IOW yield fm%iOn of cycb~g products (about 30%).The remaining7% of smaher product moleculesmust be due to ring cleavagereao tions. No dimers(Ct2
R Knutti,RE, BihIer/H-reactions wirhbenzene, c-C& and CC&,,,
242
8 I
0
0b
I
H+eyelo-C&V
H+cyclo-C,,H&l
41 c
9 Q
11
13
15
eV
Fig.10. The ionization potential of theproductmasspeaks (IP& determinedby the method of Losshaget al. 1221, comparedwith literaturevalues (molecules given in brackets). The points are markedby their respectivem/e value.
arrangementno H-atom reaction with cyclohexane could be observed. This agreeswith known rate con. stants for hydrogen abstractions from comparable alkanes [ 11. From these r*sultsring cleavageis expected to occur at some stage between benzene and cyclohexene. Estimated excess energiesfor the individualH-add& tion steps are given in table 5. They indicate that a singleexcitation step does not lead to cleavage,since H t C,H,t with 94 kcal/mol givesmainly CeH,,. Rather, excess energiesseem to accumuiate over more than one elementary reaction at 0.4 mbar pressure.
Therateof quenching of theexcessviirationalenergymay be estimated as follows: helium, the main colhsional speciesin the system [27] is known to quench about 0.1 to 0.6 kcal/mol per collision [28]. The Haddition to a double bond (except to benzene) has a rate constant of about 109hi-l s-l and the H-combination with an htennediate radical about lOto M-1 s-1. With[HI= iOy6 M the pseudo 1storder half lives are IO-3 and 10-4 s respectively. During these times I@ and lo2 collisionswith helium are effective, correspondingto an averagequenching of 300 and 30 kcal/mol respectively.This indicates that the Haddition to radicals is fast enough to compete with the colcollisionalquenching of the excited radical, thereby hulas excess energy on the product molecules [C& or C6H;i; see reactlon (13)]. The energetical-
Fig. 11. Massspectraof the reaction products (electron energy 30 V) for the H-additionto cyclohexadiene-I.4 (a), to cyclohexadien~l.3 (b) and to cyclohexene (c) for I = 25 cn~ The total extent of the reactions,given by A[M]dh&, M being the reactantmolecule, is 0.9,0.95 and 0.8 for the spec tra a-c reqxctively. The spectrumof M and the background spectrumare subtracted.For the mass spectraof the individual compounds at low electron energiessee table 4.
Table 5 Estimated
excess energies
on the H-additionproducts
Reaction
Excess energy (ke&moU
H+C&6
28 IO
H+wL1 H+W9
48137 a) 83194a)
H+Wto H+WII
38 94
ff+GHs
a) Fii numberfor cydohexndienel,3, second number for the Wiimcr.
ly most favorable ring openingproduct from CeHi* seemsto be hexatriene. Further reactions with Hatoms can easily explain the breakdown to methyl radicals(comparable to known breakdown processes of .
R. Knurti. R.E. BChler/H-reactions with benzene. c-C&
ad
c-C~H,O
243
[ 101 R. Knutti and R.E. Buhler, Chimh 26 (1972) 624. [ 11) P. Kim, J.H. Lee, R.J. Bonanno and RB. Timmons, I. ulem. Phyr 59 (1973) 4593.
[ 121 The mixing process was studied in scphate experiments 20
30
40
50
60
70
‘O7 -60
I: 20cm
-
40
- 10
IO
20
30
lo
50
60
10
Fig. 12. Mass spectra of the reaction products (electron energy 18 V) for the H-addition lo rmns-butene2 at two reactor lengths The total extent of the reaction, given by A[butene]/ [buten& is 0.75 and 0.9 for the spectra at 5 and 20 cm respecbely. The Irons-butene2 spectrum and background spectxum are subtracted.
other excited radicalst&9,29]). Preliminary expetiments with H + hexatriene and also the product spectra of the reaction H + Runs-butene-2(fii. 12) seem to support this mechanism.
References
[l] W.E. Jones, SD. Macknight and L..Tcng, Chem. Rev. 73 (1973) 407. [2] B.D. Michael and E.J. Hart, J. Phys Chem. 74 (1970) 2878. [3] UC Sauer Jr. and B. Ward, I. Phys. Chem. 71(1967) 3971. [4] K. Yang, J. Am. Chen~ Sot. 84 (1962) 3795. ]5] H. Fischer, J. Chem. Phys 37 (1962) 1094. 161 R.W. Fesscnden and RH. Schuler. J. Cbem. Phys 38 (1963)773. [7j R Knutti, Thesis No. 5031, Swiss Federal Irstitute of Tezhnology, Zurich, Switzerland (March 1973). [S] J.A. Co&r, D.G. Keil, J.V. Michael and C. Yeh, J. Phys. Chem 75 (1971) 1584. 191 E.E. Daby, H. Niki and B. Weinstock, J. Phyr Chcm. 75 (1971) 1601.
by the luminescence from the reaction 0 + NO + NO2 + hv in an equivalent flow system [R Knutti (1966), unpublished]. [ 13) The lifetimes of electronically excited H-atoms are less than about lo6 s [HA. Bethe and EE Salpeter. Quartturn mechanics of one and tw*eJectron atoms (Springer, Berlin, 1957)]. Therefore all H-atoms rt~~ch ground state before tcaction starts [ 14J F.C. Fehsenfeld. K.hI. Evenson and HP. Broidz Rev, Sci Instr. 36 (1965) 294. [ 151 D.O. Ham. D.W.Trainor and F. Kaufman, J. Chem. Phys. 53 (1970) 4395. [ 161 LF. Phillips and H.1 Schiff, J. Chem Phys. 37 (1952) 1233. [ 171 R.V. Poirier and RW. Carr Jr., J. Lltys Chem. 75 (1971) 1593. I181 A Mckenzie. hLF.R Uulcaby and J.R. Steven. J. Chem. Sot. Faraday I 70 (1974) 549.
[ 191 M.F.R. hlulcahy and R.H. Smith, J. Chem. Phyr 54 (1971) 5215. [20] R Knutti and R.E. Buhler. Chimia 24 (1970) 437. 121) A. Savitrky and M.J.E. Golay, Anal. Chem. 36 (1964) !627. (221 F.P. Lossing, A.W. Tickncr and W.k Brycc, J. Chem phyr 10 (1951) 1254. [23] H. Cirouard, FM. Graber and B.F. hleyers, NASA-CR52376. [241 H. Cirouard et aJ. 123) determined the rateconstant by measuring the H-atom consumption They mostlikely neglected the strongcatalytic H-atom Ion 1251 P.EM. AUen, HW. Melvilleand J.C. Robb, Rot Roy. Sot A218 (1953) 311. [26] a. T.J. Hardwick, J. Phyr Chem. 66 (!962) 117; b. J.E. Bennett and 8. Mile, J. Chem. Sot. Faraday 169 (1973) 139g. [ 271 Quenching by H. Ha or benzene is negligfbfe, although partty more efktiw per cotlkian. since their mol frxnctions to helium are only 6 X lO_‘, lo-’ and lo4 rcSpAVely.
[281 S.W. Orchard and B.A. Thrush, Proc Roy. Sot A329 (1972) 233. [29] H.G. Wagner and R ZeBner, Ber. Bumengen phyzik. Chem. 76 (1972) 440.
KINETICSOF THE HYDROGENATOMREACTIONSWITHBENZENE,CYCLOHEXADlENE ANDCYCLOHEXENE:HYDROGENATIONMECHANlSMAND RING CLEAVAGEl R. KNUTTI** and R.E. BmLER Laboratory fir Physical Uxmistry,
Swiss Federal Institute of Technology, 8006 Zu&h,
&itzaM
Received 26 September 1974
me hydrogen atom raction with benzene and the subsequent elementary reactions with H-atoms were studied in de tail, using a fast gas flow in a linear reactor at pressures in the mbar region, with a mass spectrometer for the product analysis ‘Therateconstant determinations were based on a kinetic model, which includes the strong catalytic H-atom recombination on the wall, caused by adsorbed reactant molecules and also corrects for !he pressure drop within the reactor.The H-atom concentration was determined by scavenging with NO1. The method was checked by determining the rate constant k(H + trun+butene2) = (4.6 i 1.2) X LOaM-’ s-l, which agrees with the literature value of Daby et al. within experimental error limit+ The rate constants determined are: kf.H +c&) = (18r 0.2) x 10’M” s-1, kCH+C&) = (3.4 f 0.6) X lO’ M-’ s-l , &(H+ c+,HB-1.3) = (1.41* 0.26) X lo9 M-’ s-l ,
k(H + c-&&-1.4) = (1.04 t 0.18) X lo9 hl-’ s-l , k(ki + c-c&i],)
= (4.9 f 0.9) X 108 hi-’ s-t
,
k(H + toluene.) = (5.1 f 1.1) X 107hlw1 5-l .
H + benzene is the rate determining step for benzene hydrogenation From the rate constant for H + C& it is concluded. that benzene is.refonncd from some intermediate reaction products (CbH; and/or C&l:). These back reactions should be suppressed at high pressures, in agreement with results by Sauer and Ward (l-54 bar). The mass spectra show that H + benzene at mbar. pressures predominantly initiates ring cleavage to form methyl ndicals, methane and Cz_hydrocarbons as the main product% However for H + cyclohcxene 85% of the products is cyclohexam The results for H f cyclohexadiene are intermediate to these extremes. It is argued that accumulation of vibrational energy over two consecutive reactions must be responsiile for the ring cleavage, which most likely cccurs from CgHi* and CeH;i.
1. Introduction Reactions of hydrogen atoms with olefms have been studied quite extensively during the past few years [ 11.However much less information is available about reactions of H-atoms with aromatic compounds. A few resultshave been published for the reactions of hydrogen atoms with benzene, which were derived from kinetic spectroscopy, electron spin resonance and product analysisin either aqueous solutions [2] or gaseoussystems with pressures above 0.6 bar [3,4]. IIis resear& has been supported by the “Schweizerischer Nationalfonds zur Fdrdenq derwissctiaftichzn Forschuq”. l * Present address: Department of Chemistry, Cornell UI& vasity, Ithaca, N.Y. 14850. l
The primary adduct radicalhas been identified as the cyclohexadienyl radical [3,5,6]. In this paper we report in detail results from 10;~pressure studies (mbar region) with a fast gas flow system [7]. The various consecutive elementary steps, initiated by H-atom addition to benzene were studied separately to gain detailed information on the rate deterknin step, the reaction mechanismand the ring cleavagereactions occuring at these low pressures.A mass spectrometer was used as a particle detector and the time dependence of the educt, transient and product concentrations were measured. The method allows the determination of second order rate constants from about IO6
to 10toM-ls-l, cover@ thereby the full range oT the known rate constants for H-atom reactions with olcfm [ 1,8,9]. The method is not suitable for the detection of unimolecular reactions, e.g., the decay of
230
R Knutti, R.E. IhlerJki-reactions with benzene, c-C&, wtd c_CJf10
vibrationally
excited intermediates, since the accessible first order rate constants are limited to lessthan about lo3 s-l. Since we publishedcur preliminary note [lo] a similarinvestigationof the H-atom addition to benzene has been publishedby Kim et al. [I 11. Our results will be compared to those of Kim et al. and possibleexplanations for the observed difference in rate constant will be given.
[H] : [benzene] : [He] = 1 : 2 X 10B3: 15 .
2. Experimental 2.1. Gasfiw
age reaction time of 12 ms (with a linear flow rate of typically 20 m s-r). The time resolution imposed by the mixing process at the inner tube jet (5 pin-holes, total area of I mm*), that is the time to reach radial homogeneity of 95% [12], is 1 ms in the present arrangement.The pressureas measureddownstream from the detector leak was in the range of 0.4 to 0.7 mbar. The relative concentrations involvedwere typically
The H2-and He-supplyrate were measured individually by rotameter, regulatedby needle valvesand mixed after pressure reduction. The ground-stateHatoms [ 131were produced in the gas mixture by a microwavedischarge(cavity no. 5 in ref. [ 141)powered from a 2435 MHz-magnetronthrough a unihne and an E/H-tuner for optimum coupling.The organic compound was either injected as a liquid from a motor driven syringe through a septum into the vacuum system with the evaporation at the needle tip assistedby infrared light or introduced as vapor through a needle valvefrom a storagebulb at elevated temperature. In the former case the concentration was calculated from
mctor
Hydrogen atoms produced by a microwavedisdiluted with helium and mixed with an organic reactant in a steady flow system of two coaxial quartz tubes as shown in fig. 1. A futed distance downstream from the mixing point, correspondingto a particular reaction time, the system is sampledby the tapered leak of a mass spectrometer (reversed metal nozzle, 0.3 mm diameter, coated inside and outside by DuPont teflon enamel 852-201). The maximum reactor length of 25 cm corresponds to an avercharge are
organic compound
microwauc dlschsrgc
vacuum syslcm
Fig.1.Principleof the gas flow system. Dimensions are:reactortubeinrrctdiameter19mm,innertube outer diameta inner tube jet 5 pinholeswithtotal sea of 1 mm2, MSleaL dimctcr 0.3 mm.
R Knurti, R.E. &ihler/H-reactions withbenzene, EC&
the syringecalibration, in the latter the corresponding masspeak was calibrated relative to the injection method. At very low benzene concentrations the vapor was diluted with He to assurea minimumvelocity at the gas-mixing jet of about 10 m s-! to avoid back diffusion into the jet. To allow fast on-off-switchingof of the organic component a by-passto the reactor was used in the off position. Thus the source flow was kept constant and the disturbance of the supply system could be minimized. 2.2. H-atom yieId Since all experiments were carried out under conditions of H-atom excess, an optimum H-atom yield was important. Maximumdissociationwas obtained by diluting the hydrogen gas with helium, reducing the wall temperature in the dischargeregion by intense air cooling, and not removing traces of water. Whenthe supply gaseswere aried in a liquid nitrogen trap the kinetic data and the relative product yields did not change,however, the H-atom concentration dropped by a factor of 2 to 3. Hence most experi. ments were run without the liquid nitrogen trap. Hatom lass by 3rd order recombination in the gas phase is negligibleunder the conditions of these experiments (half life estimated at 30 s [IS]). H-atom recombination on the quartz surface was greatly reduced by coating with D&Film SC-87.With this coating the H-atom loss was typically 5-10% over 20 cm of the reactor tubing. Other tested coating like phosphoric acid, Kel-F-90,Teflon (DuPont enamel 852201) or treatment with fluoric acid either &owed less surface protection or more rapidly lost their protecting effect. In all experiments the largestH-atom 10s~ was due to the catalytic effect of the unsaturated hydrocarbon, which is adsorbed on the wall during the experiment. As this effect could not be suppressed,it will be included in the kinetic model. The absolute H-atom concentration at the position of the inner tube jet was determined from the intensity difference at m/e = 2 with H-atom production on and off. In order to avoid errors due to H-atom recombination downstreamfrom the inner tube jet, all H-atoms were scavengedwith the very fast reaction [ 16,171 uH t bN0, + cHzO + do, + eN0. Since the stoichiometry of this complex reaction is
and c-Cdl0
231
still debated [ 181,it was checked by variation of several experimental parameters(total pressure,partial H, pressure,intensity of air cooling in dischargeregion and H20-content). Fig. 2 clearly showsthat d/c = 1.0, and with material balance b/u =d/c + 0.5 = 1.5. in agreementwith expectation [16,19].
2.3. Flow conditiorzin the reactor With a typical mean free path of 4 X lUIL cm(O.5 mbar) the reactor flow is stii dominantly viscous.The radialvelocity distribution is therefore nearly parabolic and the dwelling time of gas molecules between the mixing jet and the detector leak would be dependent on their average radial position, unless radial difksion is fast enough to homogenize the concentrations. The condition which must be met for homogeneous flow, thus allowing the system to be treated as a linear reactor, is [ 171
a = D/kc,-,R2 Z 0.5
(1)
(D = diffusion constant; R = reactor radius;k = rate
Fig. 2 Determination of the H-atomconcentration by the ICaction with NO2:Test of the stoichiometry invohrcdfor tie variouswnditiansof the syrtkn.(Variationof total presswe 0, of partialHrprcssun0, of air cooling iatcruity at dip chargel , of water content in dikhargc A_)For details see text hfas9intatsities are calibratedand given in relativeunits.
232
R KnuttL RE.
BihlerlH-reactions
constantof 2nd order reaction and CO= initial cons centration of the speciesinvolved).In all experiments which will be reported here this condition was satisfied [e.g.,0(HtC6H6)=50;O(H+C6H8)~ 1; a(H +C6Hlo) = 2.51.The total pressure of the systern was measuredby a gas independent, capacitive torr-meter (Atlas MCT)downstream from the detector leak. The pressure drop along the reactor length, determined by measuringthrough the inner quartz tube, was linear within experimental error. It arnountcd to 10%at a total pressure of 0.4 mbar and 4.5% at 0.66 mbar.
with benzene, c-C,&
and c-C6HI~
(99.9952, dewpoint 220 K), benzene(Flukapuriss, 99.93%),benzene+ (Flti puriss, 99.5% deuterium atoms), cyclohexadiene-I,3(Flukapurum 98%), cyclohexadiene-I,4 (Fluka purum, 95% with 5% benzene), hexatriene-1,3,5 (Fluka pract., 5% impurities in mass range m/e = IO to MO),cyclohexene (Fluka puriss,99.95%), cyclohexane (Fluka puriss, 99.9%), pans-butene-2 (FIti puriss, 99.8%), toluene (Siegfried puriss, 99.5%).Nitrogen dioxide NO,&04 (Fluka purum) was first treated with oxygen, then sublimed severaltimes in vacuum until the solid N204 was completely white.
2.4. Massspecnomem’c analysis 3. Kinetic mwiel A Bendix time-of-flightmass spectrometer type 12-101with a W-107 ion source was used. It was equipped with a scope output for fast monitoring of the system and with two analog output channels.The mass spectra could be recorded by a fast W-recorder (SE. 2005,4 channels),a dual channel strip chart recorder and an on-line DPDW computer for data acquisition and spectra analysis.A detailed description of the da& system has been given elsewhere 1201.To avoid saturation of the electron multiplier by the ions of the carrier gas (He) all spectra were recorded for electronenergies(EJ below the ionisation potential of He (E, < 24.6 ev). The loss of intensity due to low ionisation energy was additionally advantageous,because this simplifiedthe analysisof overlappingmass spectra of the various compounds present in the reaction system. To verify the peak assignmentsthe correspondingionisation potentials or appearance potentials were determined by the method of Loss@ et al. [22] and compared with literature values.Vibrationally excited reaction products may yield different ionisation potentials and fragmentation spectra than in their ground state. When the reaction products were sampled at the torr meter (fig. 1) no difference ia the spectrum was detected. Since the additional distance traveled by the gas allows for collisionalde-excitation it is concluded that possible viirationally excited .sp~ its do not affect the mass spectra.
In all experiments with excess H-atom concentration a substantialH-atom loss was detected, which was often more than hundred times larger than the consumption of benzene or any of the other unsaturated hydrocarbons. Cyclohexane did not show such an effect. It is concluded that the H-atom loss is due to the catalytic effect of the unsaturated compound on the rate of H-atom recombination. After the benzene supply to the flow system had been switched on, the H-atom concentration at a futed position downstream in the reactor dropped slowly over several minutes (fig. 3). After switchingoff the benzene supply the H-atom concentration returned to the original value within 10-20 minutes. If traces of water were eliminated from the gasesby a liquid trap before entering the reactor then the rate of recovery to the originalH-atom concentration was stowed.Without H-atomsin the system (discharge switched off) the intensity of m/e = 78 did not show this slow response on switchingthe benzene supply. Hence, the catalyzed H-atom recombination is attributed to a surface reac tion and it increasesas the adsorption of the unsaturated compound progresses.The initial reactions for a system with benzerie are therefore gives by reactions (2) and (3): LB
H + CsHs'CaH, ,
(2)
25. Chemkds
H + wall (with adsorbed bewne)5H2.
(3)
The followinggasesand chemicalswere used as supplied: argon (99998%), He (99396%), hydrogen
Since [H] * [benzene] fat all systems, reaction (3) predominantlydeterminesthe time dependence of
R Knutti, RE.
Wihler/fi-reactions
with benzene, CC&
I
I
B
ql
H-atoms only no rc4cllcn
233
and c-C&lo
H + benzene
mlnular
*
benzene only no reXlIon
Fig. 3. Relaxationprocessdue to the benzene adsomtion on the reactorwall, measuredat reactorIen& I= 20 cm with [He]:[H~,:(Bz]l~= 10:1:105.[lndex 10indicatesthatthe concentrationismered atx=I(mass spectrometer)inconditionwithout reaction] ?%escale for [ HJis 10’ times larger than for [ Bz]. At time zero the benzene supply is switched on to a hitherto benzene free system: simultaneouslya slow drop of [If] due to the catalytic waUeffect and a correspondingreductionof benzene consumption A[&] occur. After 5 minutes the hydrogendischargeis switched off: A[Bzj is reducedto zero immediately.
the H-concentration. Although it is a wall reaction, the fast radialdiffusion allowsit to be treated as a
homogeneousreaction. The following6 kinetic models were tested: (A)
-dPW
=~wlWo[Hl 3
@I
-4w~
= k,lw,a1*
I
(C) -43il/d~=~,,,lW[Hl, 03 -4Wd~=$J’W[HI2s 09 -4I-W =0, at constant pressurep = p. ; and (F)
-4Wdr =~,lIW,W1~
withp = pi + p(I-x).
[x = position within the reactor
mixing point at x = 0), I= total reactor length andirgiexforvaluestakenatx=I,pl=gaspressureatt.he reactor end (x = I), and 0 = pressuregradient.] Model (E) neglectsany H-atom loss, making reaction (2) pseudo first order. Although this is a standard procedure for this type of investigations [ 1 l] it does not fit our data. Model(A).(B) and (F) assumethat the effective concentration of benzene on the wall is uniform throughout the reactor, and proportional to the initial benzene concentration [B$. Model(C) and (D) assumeit to be proportional to the actual gas phase concentration of benzene. The best fit with experimental data was obtained with model (F), which is identical to (A) but includesthe effect of a linear pressuredrop along the reactor. For the stationary linear flow reactor without pressuredrop the reaction time f (from mixingpoint to samplingpoint) corre sponds to the reactor length I= uot (u. = constant linear flow velocity). For model (F) the pressure drop
(with
R Knurri.R.E. fihler/H-reactions
234
with benzene,
along the reactor affects both the flow velocity and the concentrations. In this case the rate equations for reactions (2) and (3) are:
cC6H8 and c=Cdflo
zone. With eq. (7) the rate equation (5) for benzene is integrated to give:
(4) with
(3
p=p,+b(l-x)
Y=
(Index 1 labels values for x = I, index 0 values for x = 0.) In integral form, eq. (4) is (7)
or for the ratio of H-atom concentrations at the sampling point (x = Z)with and without benzene:
(8) (the double index l0 labels the concentration at x = 1 for zero reaction, i.e., for pzJ,-, = 0). A test for eq. (8) is given in fig. 4. The slopes increase with time as is expected due to increasingwall adsorption of ben-
4u[B4oP;
( P,z@o- PI) exp -
Aqq)P 1
xl0pLxP(gp2) dP.
(6)
_
1
(93
This result is equivalent to a pseudo first order reaction in 1,if multiplied by the correction term 7. The latter depends only on experimentally known data and was calculated by expanding the exponential function within the integral in a Taylor series (centered at p = pi). By summingover the first 9 integral terms only, an error of < 0.1% remains.Typical values for 7 are between 0.5 and 1.1.
4. Results
Based on eq. (9) there are 3 methods to determine the rate constant kg: A. Variation of the reactor
0
5
10
13
25m
I
Q. 4. Tut of modd (F) far catelytic H-atomIOU ‘lb time dlI?iQ ahidl benzene vas dlowsG to adsxb on the reacttlr wdl is taken u a parameter.Devbions from linearity only OCCU9t IlUXhUIl IUCiOI ISO&.
length
- This
is the
usual
method in fast flow experiments. The benzene consumptioc is determined as a function of the reactor length by on- and-off switchingof the H-atom supply (microwavedischarge).A plot of ln([Bz]lo/[Bz)) versus(yl) yieldskB from the slope (f@. 5). in these experiments H-atom losses,although corrected for, are kept to a minimum.The H-atom concentration is monitored at m/e = 1. Experimental details and re suits for the H-atom reactionswith benzene, the two isomeric cyclohexadicne. cyclohexene and tmnsbutenc arcgivenin table 1. The latter compound was used for a cross check with litcmtnrc data [9] to test the method. B. Relaxation expeninent with fad renctor length (&J 3) - Since the adsorption of benzene on the wail
requires severalminutes to come to equilibrium, k, change with time (f#. 4) after switchingon’the
will
R Knutti, RE.
BihlerJH-wacrionswith benzene, c-C,&
235
and CC~HIO
0.2 -
0
I 5
I lo
I 15
,
I 20
Y-1
25
cm
Fig. 5. Rate constant determinationby the method of variablereactorlengthf Experiment2 of tabk I.
Table 1 Rate constants for H + M, determinedby the method of variableIeactorlength 1at 303 g Exp. 1 2 3 4
5
M
0/\ -
6
0/\
7
Q
8
.
Q
9 :;
/
Pressure Mm)
uo (m/s)
[HeloX 10’ (M)
[HIoX 10’ (MI
[M]oX 109 (MI
A[HII[Hlo kBX lo-’
n
0.42 0.41 0.41 0.41 0.42
19.0 19.3 18.9 19.6 18.2
1.56 1.50 1.50 1.45 1.55
9.8 9.7 9.8 13.5 10.3
25 2.1 1.3 1.2 2.6
0.34 0.09 0.08 0.09 0.20
0.38
20.1
1.40
9.3
4.8
0.28
128
t24
60
0.38
20.1
1.40
9.3
4.8
0.35
99
t16
15
0.38
20,l
1.40
9.3
7.2
0.29
44.6 * 8.2
16
0.42
18.2
1.55
10.3
5.3
0.65
46.3 5 8.8
17
0.42
la.2 18.8
1.50 1.55
10.3 14.1
5.3 5.1
0.37 0.41
43.4 36.0 fe 8.7 ?.6
30 14
(M-’ s-9
1.43t 0.26 1.63t 0.25 1.73L 0.30 1.66i 0.27 1.4ot 0.24
21 37 57 38 32
n = numberof data points, which determinek~. 6[HIfiH] o = averagefractionalH-atomloss Withinthe reactor,due to the catalytic wall effect Enor limits stated are for a condifcncclhit of 95%.
136
R Knutti, R.E. LGhlerJH-reactions with benzene. c-C,&
aI=
Scm
@I I
(x8-
andc-Cdjl~
=lO
cm
@l=lScm 0.7 -
@ l=20cm @ t’25cm
46-
06 t=15cm @ 1=20cm
0.5 -
p3J
t=2os
0.4 I
/
I
5
I
I
lo
15
,Y*l 20
*
cm
Fig. 6. Rate constant determinationby the relaxationmethod. Experiments III of table 2. Lime 8 correlatesall data points taken 20 seconds after the start of the experiment(time duringwhich benzene was allowed to adsorb on lhe reactor wall).
Table 2 Ratsconstantsfor the H-additionreactionto benzene, as determinedby the relaxation method (at fixed reactor leqtb 303 K)
I
25 25 25
PtS.Ql~ (mbar)
L1
[HeloX 10+s 0
[HIOX lo* Ml
[BzloX lO*O Ml
0.68
Il.3
249
221
21
0.69
11.1
253
2.25
4.3
111
slO;’
n
250 * 0.40 2.11*0.34 255 t 0.47
21 15 11
7.17 i 1.90
40
3.08* 0.65
40
25
3.15
l
0.55
40
5 10 15 20 25 15 20
6.45 f 236t 248 f 23 1 i 233 i 234 f 205 f
1.17 0.45 0.62 0.35 0.36 0.36 0.29
21 26 10 41 41 41 47
10 [1
2-i
17.5
0.69
11.2
253
benzene supply in a previously bcnzefie free system. 7, too, will change during this time. A plot of ~JI(~z~~/ @J,) VCISUS 7 (or versus74 since 1is kept constant)
223
26
I and at
again yields the rate constantkg from the slope. Such plots for severalreactor lengths are shown in fig. 6 and table 2 lists the data and results.
C. Determination of relative mte constunts - For stu-
dies of the initial elementary reaction only, relative rate constants can be determined by simultaneously injecting two reactant molecules (hit and M2). All conditions are exactly the same for both H-atom reactions From the relative mass peak intensities with and without H-atoms from the discharge,the rate constant ratio is calculated by eq. (10) as derived from eq. (9):
3 = w?qom1
I)
k2 ~W2lolF121) -
(10)
From 43 individualdeterminations at widely varying conditions (pressure, reactor length, H2concentration) the rate constant ratios for benzene~r$benzenehe was found to be y
+w45)
m +c&j)
= 1.90*0.17.
The averagefrom 29 determinations of the H-reaction with toluene versusbenzene gave k(H + toluene) = 2.81 +,0.34 . k(H + benzene) 5. Discussion 5.1. Kinetim of H + benzene
The rate constant data from method A (variable reactor length, table 1) suggestsome correlation with the averageH-atom loss in the reactor (m]/[H]u). The slope of this correlation indicates that model (F) does not yet fully correct for the catalytic H-atom loss, but extrapolation to zero-loss is possible (see below). Method B (relaxation experiment, table 2) gives rate constants which are about 50%higher through. out, all being close to 2.5 X 10’ M-l s-l, except for the very short reactor lengths(results at 5 cm and at 10 cm). The model applied assumesuniform adsorption of unsaturated hydrocarbon throughout the reactor. At short reactor lengths this is a poor approximation, as the mixing region represents a non-negligible part of the total reactor length. The corresponding results are also characterized by abnormally large error limits. Both methods should give the same rate constants.
isentirely based on the H-atom decay, whereas the variable reactor length method uses 7 [eq. (9)] only for correction. It is sr& gested therefore that the method with variable reactor length is more reliable.This is further supported by a different analysisof the relaxation data: the experiments labelled II and III (table 2) allow to correlate data points for equal degree of adsorption with the reactor lengths. In fg. 6 all data points of experiment III, taken 20 s after the benzene source was switched on, are linearly correlated by line 8 . From its slope the rate constant is k(H f C&&) = 1.5 X lo7 M-t s-t, in rather good agreement with the results in table 1. Such rate constants are plotted againstadsorption time in fig. 7. It is straightforward to extrapolate to zero adsorption time, equivalent to zero catalytic effect. The result is 1.6 X IO7M-l s-t for experiment Ill (fig. 7) and 2.0 X IO7M-t s-l for experiment II. Fig. 8 now correlates all rate constants from method A (variable reactor length) and method B (as derived for zero adsorption) with their corresponding H-atom losses.The linear extrapolation by the least square method then yields the best value for zero Hatom loss: However the relaxation method
k(HtC6H,)=(1.8*0.2)X
107M-‘s-t.
In table 3 our resuh is compared with literature data. Coveringthe pressure range from IO-3 to 54 bar, the valuesare surprisinglyclose, escept for the value by
Fig. 7. Rate constanu derived from rchxation upaimencs as dctemhcd fmmthef&pcdcncc at cqupldegreeaf benzene adsorption(experimentIII, tlblc 2). plottedagainsta&qtion thG TheLinareXtJapohffon yiddr kBfor zeroadlolption
238
R Knutti, RE. Bihler/H-reactiom with benzene, c-C&
k, * 107IA3.0 ”
1.0-’ a[Hl
1 0
ar
a2
0.3
0.4
-IHI,
Fip. 8. Extrapolationof all rate L’ONIM~S to zero H-atomloss. b: Damfrom variablereactorlength (experiments1-S of table 1); l : data from relaketionexperiments(II and 111of table 2) alreadyextrapolatedto zero adsorptiontime (ix., zem H-atom loss).
Girouard [23] which we believe to be erroneous [24].
The very recent result of Kim et al. [ 1I] which is about three times larger than ours, was determined by a similar variable reactor method, but based on a different technique to determine the initial H-atom concentration and does not include a correction for Hatom loss andpressuregradient along their50 cm flow reactor, Allen et al. [25] used rhe MoO2-method to measure H-atom concentrations. This method is known today to be unspecific in some cases. Yang’s result [4] from competition studies in gas phase radio-
and c-&Hlo
lysis is close to our value, but no error limits are given. The high pressuredata givenby Sauer and Ward[3] derived from observingthe transient absorption of the cyclohexadienyl radicalin pulse rarholysis,compared to our data imply a rather low pressureeffect. The rate constant increasesby a factor of 2 only for a pressureincreaseof 1000. The results from relative rate constant determinations (method C) with benzenedb show that there is no primary isotope effect, in agreementwith H-addition to the benzene ring [reaction (I l)]. The rate constant ratio observed is k,/k, = 1.90 f 0.17, which correspondsexactly to the value determined by Kim et al. [1 I]. Eq. (10) showsthat the ratio of the rate constants is neither a function of the initial H-atom concentration, the total pressure, the flow velocity, nor of effects from H-atom loss and pressuregradient within the reactor. Wesuspect therefore one of these factors to be responsiblefor the discrepancyof the absolutevalues.Weexplain the kD/kwratio by isotopic exchange,due to reactions (I le), (12e) or (13e) which reform benzene.
H t C6H6+$H;%,H,
(11)
H t C,H,%,H;=+C,H,
t H,
L 12w C6Hs Table 3 Cone&d rate constants for the H-atomaddition reactions(comparedwith literature data) Reaction
Pressure(mbar)
kg (M-1s-11
Reference
H + benzene&
0.4 - 0.7 0.59 0.3 - 0.96 a.7 660 1 -54bar
(1.8 f 0.2) X 10’ x 109 2.8 (;.; f 1.0) X 10’ X 100 x 10’ (i.7 * 0.6) X 10’
this work Girouardet aL [23] KbnetaL[ll) AllenetaL [25] Yang141 Sauer,Ward[ 31
0.4 - 0.7 0.4 - Q7 11 -54bar
(3.4 t 0.6) X 10’ (5.1 f 1.1) X 10’ (1.0 *0.1) x 100
this work this work Sauer,Ward[3j
H + benzene-de H + toluene H + cyclobexadienrl, 3 H + cyclobexzdicnoI,4 H + cyclohexene
0.4 0.4 0.4
(1.41* 0.26) x 109 (I.04 f 0.18) X lo9 (4.9 f 0.9) x 108
this worka) tis worka) this work a)
H t rmnobutennc-2
0.4 1.3
(4.6 * 1.2) X 10” (5.3 * 0.4) x 10’
this worka) Dabyet al. [91
a) Correctedfor H-atomloss (7+ctext).
02)
l f3f H + C6H,---K,H **%C6H6 + H,
13e’ other dissoc.products 03)
e 13wC6Hs.
For deuterated benzene these back reactions also yield C&IDS,which is easily detected in the product mass spectra (fig. 9b, peak m/e = 83). As the first elementary reaction of H-addition to benzene is rate determining for the hydr~enation of benzene (see table 3), all intermediatesare of very low concentration. The apparent pseudo 1st order rate constant kobs(Ht C&J is therefore easily correlated to kt tt by the steady state assumption:
atom fess (cf., fig.8) wasnot feasible. However a correction of +lt% was estimated from the H-atom iOsS for the rate constants with cyclohexadiene. cyc!ohexene and frus-butene-2 (table 3). The rate constant for H + Izrms-butene-2co~e~onds to the literature value within experimental error. This is support for our kinetic model and for the accuracy of the concentration measurementof the H-atoms.
k llw h2w k obs=kllf kttetk 11w+k13~~k12e+k12w
1
kl3lWl
h3e*+k13w
+klle+kllw+k13fiHIk13e+k13e’+k,3w
I *
WI
At the high pressurelimit we expect kobS= k, Ifsince the rate constants for coilisiond deactivationkr rur klzw and R13w becomeJominant.Comparingour data with those of Sauerand Ward13J which were ob-
tained at high pressures (l-54 bar), we find kobs(Sauerand Ward) =52. k&$his work) This indicates that at least one of the three back reactions(1 le, 12e or 13e) must have a rate constant of the same magnitude as its competing reactions. No preference for reaction (1 le), (12e) or (13e) can be derived.The SSRdetection of H-atom from the reaction E)+ C&e by Rim et al. [ 1I] might be indicative, but their reaction conditions are quite different from our ([C,H,] about 100 times larger, [H] about 100 times smaller).These different conditions strongly a.f feet the collisionaldeactivation and the competition between the reactions (1 le), (1 lplr)and (13f) is expected to be different.
Therate constants for H-addition to nans-butene-2 (table 1) did not show proper correlation to the Hatom loss in the reactor. An extrapolation to zero H-
d
!’ 20
! -
30
’
40
“’
sb
II 04
70
80
-ma
-r_L 20
30
40
50
00
70
ea
-80
-ea
-do
1*25cm
10
20
30
40
a0
60
-20
70
60
Fig,9. Massspectraof the reactionproducts (electronenergy
18 VI from H-additionto benzene&. and berJzened6,as measuredat different reactortengtttsL Ihc total extent of the reactions,given by AIBz]l[Bzfo isO.O7,0.11,0.13,0.t3,
0.19and0.27for tic sppectra fromtop to bottomrcspectie ly. Inatl spectratfrefragmentationspectnun of benzene and the backgtoundspatnun are subttaeted For the masssifec tsaof the individualcompounds at low electron energincsee table4.
240
R Knutti, R E. Bdhler/H-n?mions with benzene, PC~ Ha and c-Cdl 10
Table 4 bhs spectra of individualproductmolecules for various clect~o~~energies(for comparison with l&r 9 and 11) BtIlZUlC de
Cyclohexadienel,3 3ov
2OV
26 27 36 37 37.5
0.9
38 30.5 39 40 49 50 51 52 53 61 62 63 73 74 75 76 71 78 19 80
1.4
5.9
3.9 0.7 4.3
0.4
120 14.2 0.9
1.8
0.7 0.7 3.9 100 6.6
24 11.9 100 7.0
75V 1.7 1.9 0.4 27 1.0
de
2ov
Cyclohcxadiene-1.4 30 V
75 v
m/e
zov
3OV
75Y
2.5
26
11.1 20 0.6 4.2
27 37 38 39
2.5 125 21 4.5 19.3
1.6 27
0.6 17.2 1.1 3.9 3.6
39.5 40 41 49 50
1.7 4.2 4.6 1.3 122
1.9
6.6 8.9 5.0
1.4 11.0 16.4 11.8 5.7
51 52 53 54 63
0.8
1.9
1.9
4.3
2.5 0.8 20 5.4 0.8
65 67 74 71 78
2.3 0.9 0.6 31.6
79 80 al a2
26 4.7
27 37 37.5 38
3.8 0.s 10.3 a.5 2(1
38.5 39 39.5 40 41
13.5 16.8 15.4 0.8 0.5
49 50 51 52 53
0.6 2.4 1.4 4.1 1.8
54 62 63 65 73
2.8 11.8 100 6.8 0.4
74 75 76 17 78
11.1 3.5
29.8 10.9
79 80 81
73.2 100 6.2
100 86.8 5.0
9.7
The rate constant H f cyclohexene has the same magnitude as the one for H + fiumbutene-2, whereas the constants for H + 9k+CeHg-1,4 and H 4 cycloChHgl,3 are higher by a factor of 2 and 3, respectively. The H-addition to toluene is almost 3 times faster tbarr to benzene. Most likely this is not due to additional H-abstraction from the methyi group, as Hardwick [26a] found an abstractioujaddition rate constarit ratio of C 0.03 in liquid phase (23°C) and Bennett et al. [26b] could not detect benzyl radicals by their ESR cryostat method. An increased reactivity of Qeortho-position towardsH-addition[26b] seemsto be the more likely explanation.The high pres sure valueby Surer and Ward[3] (table 3) is againtwice
1.7 0.9 0.9
7.2 9.4 5.3 3.4
17.9 13.8 6.7 5.8 20
1.8
4.5
12.4 3.2
31.4 13.5
6.3 4.2 28 44.7 14.1
58.8 100 6.1
89.2 100 6.4
100 99.2 6.7 21
9.9 100 76.3 5.0
our value, as was found for benzene. The same argu ments concerning coUisionaldeactivation of the viiratiordly excited intermediates do hold here, as was discussedfor benzene in connection with reactions (lG-(13). 5.3. Me&at&m
ofbenzene
hydrogenation
The rate constant for H-addition to benzene is much smallerthan for the correspondingH-addition to cyclohexadiew (both isomers)and cyclohexene (table 3). The iuitial reaction step (H + C&) is therefore rate determining for the benzene hydrogenation. The product mass spectra for the H-addition to bea zene (18 eV electron energy) at various distances
R Kmtti. RE. BSer/H-reactions with benzene,c-C&
241
and c-C6Hlo
Table 4 (continued) Cyclohexane
Cyclohexene
mle
2ov
3ov
15
75 v
1.1
mle
20 v
30 v
1.0
15
26
21 29 37 38 39 40 41 42 50 51 52 53 54 55 63 65 66 67
4.8 1.7
3.2 13.1
2.1 59.0
68 77 18 79 80
6.4
81 82 83
9.2 69.3 4.6
21 17.5
Its” 25 1.1
;
3.8
29 38
5.4
86 1.6
39 40 41 42 43
5.1
20.7
22
3.0 320 5.5 36.2 28
8.3 73.8
5.2 8.3 4.1 11.6 78.7
50 51 5.? 53 54 55 56
1.1 1.7 100
5.8 1.1 2.8 2.2 100
61 68
6.5 4.1 1.4 6.1 1.1 11.9 43.1 2.8
15.7 2.2 29.7
3.2
100
75 v
6.1 2.2 5.2
10.9 46.7 3.5
downstreamfrom the mixing point are shown in fq. 9a. The strongest peaks are due to methane (m/e= 16, IS), CHrradical (m/e = 15), ethane and ethylene (m/e = 28). The experimentally determined ionization potentials for the masspeakssupport these assignments (fig. 10). Moreovermethyl radicalsare indicated by the changeof the intensity ratio of m/e 15 and 16 with the reaction time (fe. 9a) and by the formation of smallamounts of toluene (m/e =91 and 92, not shown in fig. 9a). The appearancepotential measuredfor m/e = 28 is intermediate to ethane and ethylene, thus probably both compounds are formed. The product distribution (icorrected for the number of carbon atoms withii the product molecule) re-
22
5.0
43.7 27.2 13.2
55.0 28.8 13.6
2.2 6.5
2.1 2.9 1.3 4.5 6.5
15.9 12.1 3.5 3.5
31.3 99.5 4.9 3.5 1.6
33.8 100 5.0 3.1 1.6
69 70 77 79 80
184 I.2 1.5 7.0 12.4
21.3 1.3 1.9 5.1 5.4
20.9 1.3 1.6 26 21
81 82 83 84 85
1.2 21 4.4 100 7.0
5.4 100 7.0
5.5 94.8 6.5
51
11.6 16.1 1.9
3.1
veds a mptisingly IOW yield fm%iOn of cycb~g products (about 30%).The remaining7% of smaher product moleculesmust be due to ring cleavagereao tions. No dimers(Ct2
R Knutti,RE, BihIer/H-reactions wirhbenzene, c-C& and CC&,,,
242
8 I
0
0b
I
H+eyelo-C&V
H+cyclo-C,,H&l
41 c
9 Q
11
13
15
eV
Fig.10. The ionization potential of theproductmasspeaks (IP& determinedby the method of Losshaget al. 1221, comparedwith literaturevalues (molecules given in brackets). The points are markedby their respectivem/e value.
arrangementno H-atom reaction with cyclohexane could be observed. This agreeswith known rate con. stants for hydrogen abstractions from comparable alkanes [ 11. From these r*sultsring cleavageis expected to occur at some stage between benzene and cyclohexene. Estimated excess energiesfor the individualH-add& tion steps are given in table 5. They indicate that a singleexcitation step does not lead to cleavage,since H t C,H,t with 94 kcal/mol givesmainly CeH,,. Rather, excess energiesseem to accumuiate over more than one elementary reaction at 0.4 mbar pressure.
Therateof quenching of theexcessviirationalenergymay be estimated as follows: helium, the main colhsional speciesin the system [27] is known to quench about 0.1 to 0.6 kcal/mol per collision [28]. The Haddition to a double bond (except to benzene) has a rate constant of about 109hi-l s-l and the H-combination with an htennediate radical about lOto M-1 s-1. With[HI= iOy6 M the pseudo 1storder half lives are IO-3 and 10-4 s respectively. During these times I@ and lo2 collisionswith helium are effective, correspondingto an averagequenching of 300 and 30 kcal/mol respectively.This indicates that the Haddition to radicals is fast enough to compete with the colcollisionalquenching of the excited radical, thereby hulas excess energy on the product molecules [C& or C6H;i; see reactlon (13)]. The energetical-
Fig. 11. Massspectraof the reaction products (electron energy 30 V) for the H-additionto cyclohexadiene-I.4 (a), to cyclohexadien~l.3 (b) and to cyclohexene (c) for I = 25 cn~ The total extent of the reactions,given by A[M]dh&, M being the reactantmolecule, is 0.9,0.95 and 0.8 for the spec tra a-c reqxctively. The spectrumof M and the background spectrumare subtracted.For the mass spectraof the individual compounds at low electron energiessee table 4.
Table 5 Estimated
excess energies
on the H-additionproducts
Reaction
Excess energy (ke&moU
H+C&6
28 IO
H+wL1 H+W9
48137 a) 83194a)
H+Wto H+WII
38 94
ff+GHs
a) Fii numberfor cydohexndienel,3, second number for the Wiimcr.
ly most favorable ring openingproduct from CeHi* seemsto be hexatriene. Further reactions with Hatoms can easily explain the breakdown to methyl radicals(comparable to known breakdown processes of .
R. Knurti. R.E. BChler/H-reactions with benzene. c-C&
ad
c-C~H,O
243
[ 101 R. Knutti and R.E. Buhler, Chimh 26 (1972) 624. [ 11) P. Kim, J.H. Lee, R.J. Bonanno and RB. Timmons, I. ulem. Phyr 59 (1973) 4593.
[ 121 The mixing process was studied in scphate experiments 20
30
40
50
60
70
‘O7 -60
I: 20cm
-
40
- 10
IO
20
30
lo
50
60
10
Fig. 12. Mass spectra of the reaction products (electron energy 18 V) for the H-addition lo rmns-butene2 at two reactor lengths The total extent of the reaction, given by A[butene]/ [buten& is 0.75 and 0.9 for the spectra at 5 and 20 cm respecbely. The Irons-butene2 spectrum and background spectxum are subtracted.
other excited radicalst&9,29]). Preliminary expetiments with H + hexatriene and also the product spectra of the reaction H + Runs-butene-2(fii. 12) seem to support this mechanism.
References
[l] W.E. Jones, SD. Macknight and L..Tcng, Chem. Rev. 73 (1973) 407. [2] B.D. Michael and E.J. Hart, J. Phys Chem. 74 (1970) 2878. [3] UC Sauer Jr. and B. Ward, I. Phys. Chem. 71(1967) 3971. [4] K. Yang, J. Am. Chen~ Sot. 84 (1962) 3795. ]5] H. Fischer, J. Chem. Phys 37 (1962) 1094. 161 R.W. Fesscnden and RH. Schuler. J. Cbem. Phys 38 (1963)773. [7j R Knutti, Thesis No. 5031, Swiss Federal Irstitute of Tezhnology, Zurich, Switzerland (March 1973). [S] J.A. Co&r, D.G. Keil, J.V. Michael and C. Yeh, J. Phys. Chem 75 (1971) 1584. 191 E.E. Daby, H. Niki and B. Weinstock, J. Phyr Chcm. 75 (1971) 1601.
by the luminescence from the reaction 0 + NO + NO2 + hv in an equivalent flow system [R Knutti (1966), unpublished]. [ 13) The lifetimes of electronically excited H-atoms are less than about lo6 s [HA. Bethe and EE Salpeter. Quartturn mechanics of one and tw*eJectron atoms (Springer, Berlin, 1957)]. Therefore all H-atoms rt~~ch ground state before tcaction starts [ 14J F.C. Fehsenfeld. K.hI. Evenson and HP. Broidz Rev, Sci Instr. 36 (1965) 294. [ 151 D.O. Ham. D.W.Trainor and F. Kaufman, J. Chem. Phys. 53 (1970) 4395. [ 161 LF. Phillips and H.1 Schiff, J. Chem Phys. 37 (1952) 1233. [ 171 R.V. Poirier and RW. Carr Jr., J. Lltys Chem. 75 (1971) 1593. I181 A Mckenzie. hLF.R Uulcaby and J.R. Steven. J. Chem. Sot. Faraday I 70 (1974) 549.
[ 191 M.F.R. hlulcahy and R.H. Smith, J. Chem. Phyr 54 (1971) 5215. [20] R Knutti and R.E. Buhler. Chimia 24 (1970) 437. 121) A. Savitrky and M.J.E. Golay, Anal. Chem. 36 (1964) !627. (221 F.P. Lossing, A.W. Tickncr and W.k Brycc, J. Chem phyr 10 (1951) 1254. [23] H. Cirouard, FM. Graber and B.F. hleyers, NASA-CR52376. [241 H. Cirouard et aJ. 123) determined the rateconstant by measuring the H-atom consumption They mostlikely neglected the strongcatalytic H-atom Ion 1251 P.EM. AUen, HW. Melvilleand J.C. Robb, Rot Roy. Sot A218 (1953) 311. [26] a. T.J. Hardwick, J. Phyr Chem. 66 (!962) 117; b. J.E. Bennett and 8. Mile, J. Chem. Sot. Faraday 169 (1973) 139g. [ 271 Quenching by H. Ha or benzene is negligfbfe, although partty more efktiw per cotlkian. since their mol frxnctions to helium are only 6 X lO_‘, lo-’ and lo4 rcSpAVely.
[281 S.W. Orchard and B.A. Thrush, Proc Roy. Sot A329 (1972) 233. [29] H.G. Wagner and R ZeBner, Ber. Bumengen phyzik. Chem. 76 (1972) 440.