Gas phase SN2 nucleophilic displacement reactions at atmospheric pressure by the photodetachment-modulated electron capture detector

Inlernational Journal of MersSpechvmetry and Ion Processes 130 (1994) 227-236 0168-1176/94/$07.00#Q1994 - Elsevier ScienceB.V. All rights reserved

Gas phase SN2 nucleophilic displacement reactions at atmospheric pressure by the photodetachmentmodulated electron capture detector KS. Strode, E.P. Grimsrud” Department of Chemistry, Montana State University,Bozeman, MT 59717, USA

(Received9 July 1993;accepted 8 October 1993) Abstract Rateconstants for the reactions of Cl- ion with 20 different alkyl bromides (RBr) have been measured at 125°Cin an atmospheric pressure buffer gas by the photodetachment-modulated electron capture detector (PDM-ECD). By this method, the relative concentrations of the reactant ion, Cl-, and the product ion, Br-, are spectroscopicallymeasured in a steady-state reaction volume with various concentrations of purified RBr introduced to the ion source by gas chromatography. Most of the reactions reported here have not been previously studied and, therefore, the present data set provides additional insight into substituent effectsin gas phase SN2reactions. Key words:Sx2; Nucleophilic displacement;Photodetachment; Electron capture detector

Introduction Very few measurements of ion molecule (IM) reactions have been reported, to date, using buffer gas pressures exceeding a few Torr, in spite of the fact that interesting effectscan be anticipated with use of higher pressures [l-4]. One of the major reasons for the present deficiency of kinetic methods that operate successfullyat pressure as high as one atmosphere is the fact that high-pressure mass spectrometric methods typically rely on aperture sampling of the ion source and at atmospheric pressure this sampling method is known to cause large and uncontrolled perturbations of the detected ions [.5,6].Another reason why studies of ion chemistry at atmospheric pressure are difficult is related to the increased importance of side reac*Corresponding author. SSDI 0168-1176(93)03918-C

tions involving buffer gas impurities whose absolute concentrations are increased in proportion to the total buffer gas pressure. We have recently set out to develop reliable methods for the measurement of rate and equilibrium constants of various gas phase ionic processes under conditions of unusually high pressure and some of our initial accomplishments in this area through an approach involving ion mobility spectroscopy(IMS) have been previously reported [7-91. In the present paper, we describe another, entirely differentapproach to kinetic measurements in an atmospheric pressure buffer gas that is based on the in situ spectroscopicobservation of negative ions by the photodetachmentmodulated electron capture detector (PDM-ECD) [lO,ll]. This method also includes the introduction of the neutral reactant in very pure form by gas chromatography (GC), so that the detrimental

KS. Strode and E.P. Grimurudlht. J. Mars Specirom. Ion Processes 130 (1994) 227-236

228

\

moncchromotor

Fig. 1. Schematicdiagram of the photodetachment-modulated electron capture detector (PDM-ECD)used here. The normal ECD response and the PD-modulated component of the ECD response are simultaneously monitored at pen I and pen 2, respectively.The monochromator is set to 365nm. The chopper frequencyis 23 Hz.

effects of impurity side reactions are greatly reduced. The IM reactions to be investigatedhere include the reactions of chlorideion with 20 differentalkyl bromides(RBr) at 125”C,as shown in reaction (1): Cl- t RBr + RCI t Br-

(1)

Most of the reactions included in the present study have not been previously studied. Therefore, the present measurements provide additional insight into the role of alkyl substituents in gas-phase SN2nucleophilicreactions. Experimental Various forms of the PDM-ECD have been previously describedin considerabledetail [lo-121. A schematic representation of the apparatus used here is shown in Fig. 1.The electroncapture detector (ECD)contains a 63Ni-on-Ptfoil of 9 mCi activity. This foil formsthe interior walls of a cylindrical ion sourceof 2.0cm length and 1.3cm width. Fused silica windowsform the front and rear walls of the ECD so that light can be made to pass through the active volume of the detector. For the present experimentsthe temperatureof the ECD wasmaintained at 125°C.Its pressurewas maintained at the ambient level, 640 Torr. The ECD anode shownin Fig. 1is a &in pin that

penetrates about $in into the source volume from one side as shown in Fig. 1. Positivepulsesof 1ps duration, SOV amplitude, and 500Hz frequency are applied to the anode and the resulting time-averagedelectron current is monitored by the electrometerand recordedat pen 1 of a strip chart recorder. A mechanically chopped (frequency= 23Hz) beam of light, produced by a 1OOOW HggXearc lamp is passedthrough a grating monochromator prior to the detector. In the present application, the monochromator is set to 365nm where the emission spectrum of the Hg-Xe lamp includes an intense peak. Most significantly,light of 365nm is known to cause photodetachment of Br-, but not Cl- [lo]. The bandwidth of the monochromator is 20nm. The beam widthat the point of entry into the ECD is sufficientlylarge so that all negativeions present within the ECD are irradiated by a relativelyuniform photon flux. The alkyl bromides(RBr) of interest wereintroducedto the ECD by a gas chromatograph(Varian 3700) using either a loft x gin packed column (10% SF-96on ChromosorbW) for the more volatile RBr or a 15m wide-bore capillary column (Altech RSL-200 polydiphenyldimethylsiloxane) for the less volatile RBr. Standardized gaseous mixtures of the volatile RBr (having boiling points less than 80°C) were prepared in glass carboys containing superambient pressures of nitrogen. Aliquots of these were introduced to the GC column by use of a 2mL gas sampling loop (Carle, model 8030). Standardized solutions of the less volatile RBr were prepared by dilution into HPLC grade hexane and were injected onto the GC column with a 1PL syringe. The GC carrier gas was a mixture of 10% methane in argon (Matheson)and was typically set to a flow rate of about 30mL min-‘. The GC retention time of all compounds was determinedby parallel analysis of standards by gas chromatography-flame ionization detector (GC-FID) and GC-MS (mass spectrometric)techniques.For many of the compounds studiedhere, the purificationof the reagent neutral that is provided by the GC was found to be essential to the acquisition of meaningful kinetic data.

229

KS. Strode md E.P. Grimsrud/ht. J. Mm Spectrom. Ian Processes 130 (1994) 227-236

For example, FID chromatograms indicated that our supply of bromomethylcyclopropane (BMCP) contained two impurities of significant relative abundance. Analysis by W-MS revealed that these two impurities were monobrominated butenes. As will be indicated in the present study, these two compounds would be expected to be much more reactive towards Cl- ion than BMCP and would have led to large errors in the determination of k, for BMCP had they not been completely separated from BMCP by the present neutral reactant introduction scheme. As indicated in Fig. 1, a makeup gas (also 10% methane in argon and also set to a flow rate of about 30mL mm-‘) is mixed with the GC eflluent flow just prior to the PDM-ECD. A very small amount of CFzClz is introduced to this makeup gas stream from a lecture bottle (Matheson) by control of a needle valve. The needle valve was set so that CFzClz diminishes the ECD standing current to a half of its undoped value. The resulting PDM-ECD spectrum of the ions thereby produced indicated that Cl- is then produced in the ion source, as expected [13], by the following EC reaction: et CFzClz + CF$l t Cl-

(2)

By this method, a constant population of Cl- ions, thought to be roughly equal to the electron population, is continuously maintained in the ion source prior to introduction of RBr. As illustrated in Fig. 2 for the case of l-bromoethane, when an alkylbromide is then chromatographically introduced to the PDM-ECD, two response functions are simultaneously obtained. The lower chromatogram provides the normal EC response to this sample and indicates that lbromoethane does not undergo electron capture and does not significantly reduce the electron population as it passes through the detector (the known retention time of I-bromoethane is indicated by the arrow). This lack of EC response to this and almost all of the other RBr studied here is expected at the concentrations used here due to their very low rate constants for electron capture

Time

(mid

Fig.2.A typicalPDM-ECDanalysisof a bromoethanesample. An approximately equalpopulationof electronsand Cl- ions are continuouslymaintainedin the ion source priorto the introduction of bromoethane. The arrowsindicatethe known retentiontimeof bromoethane.The lack of au EC response to bromoethane indicates that this compound does not attach thermal electron. A strong PD response to bromoethane is caused by its reaction with Cl-.

[14].The large peak observed at a shorter chromatographic retention time in Fig. 2 is due to small amounts of oxygen introduced with the sample. Oxygen has a small, but positive electron affinity and some electron attachment to O2 therefore occurs [15]during its passage through the detector. In the upper chromatogram of Fig. 2, the PDM response to this sample is shown to include a strong peak during the passageof I-bromoethane through the detector. As shown in Fig. 1, the PD response function is obtained by extracting the 23 Hz photodetachment-modulated component of the ECD’s response by a lock-in amplifier. This response function is sensitive to the presence of RBr in the ion source for the following reason. Light of 365nm does not cause PD of Cl-, but does cause PD of Br- (that is, Br- •t hu -+ Br t e). Therefore, when Cl- is the only ion present in the source, no modulation of the cell current is caused by the pulsed light beam. As RBr passes through the ion source, some Br- is produced by reaction (1) and modulation of the cell current is then caused by the pulsed light beam. It is also noted in the upper chromato-

KS. Strode and E.P. Grimsrud/lnt. J. Mnrs Spectrom. ion Processes 130 (1994) 227-236

230

positive ions, reaction (3): Br- t Pt + neutrals

(3)

by photodetachment,and by ventilation out of the ion source with the carrier gas flow. It has been shown [lo], however,that the rates of Br- loss by photodetachmentand by ventilation are of negligible magnitude relative to that of ion recombination under the conditions of the present experiment.Therefore,it can be assumed that the steady-stateconcentration of Br- within the PDMECD is adequatelydescribedby Eq. (4):

0 CZH,Br Concentration (10” molecules cd’) Fig. 3. A plot of the relative PD responseto bromoethane as a function of the concentration of bromoethanein the detector at the peak maximum.The ratio of 61to (61, - U) is taken as a measure of the concentration ratio of Br- and Cl- ions within the detector at any givenbromoethane concentration.

gram of Fig. 2 that the PD response to oxygen is relatively weak. This is expecteddue to the relatively small PD cross section of 0; at 365nm [16]. In Fig. 3, the magnitude of the PDM responses to a widerange of I-bromoethane concentrationsis shown. It is seen that the PDM responsesincrease strongly in the low RBr concentration range and then approach a constant maximumvalue with use of relatively high RBr concentrations. Saturation of the PDM responseis expectedwith use of high RBr concentrations as reaction (1) causesa nearly complete conversion of Cl- to Br-. The magnitudes of the saturated PDM responsesapproached for all RBr were identical, as expected. Analysisof results A model of the events occurringwithin a slightly simpler version of the PDM-ECD has been previously described [IO]. In order to interpret the present experimentsin which a change in the identity of negativeions is occurringby an IM reaction, it is necessaryonly to add reaction (1) to the previous model of PMD-ECD events. In this model, Br- is assumedto be=producedonly by reaction (1). The losses of Br- will be by recombination with

d[Br-]/df = k,[Cl-][RBr]- k3[P’][Br-]= 0

(4)

which leads to Eq. (5): QI= k, [RBr]/P

(5)

wherea = [Br-]/[Cl-] and /I = kl[Pt]. The magnitude of k, can then be evaluated from the present measurementsin the manner describedbelow. It is necessaryto know the concentration of RBr in the PDM-ECD at the time correspondingto the maximum of the chromatographic peak. This determination can be made if the GC peaks are assumed to have gaussian shape (171.For each GC peak, the 4a peak width, W(s), was measured. Since95.5%of RBr is expected[17]to pass through the cell during the time of W, the averageRBr concentration over this time is @ven by [RBr], = 0.955x (molecules injected)/(W x F), where F (cm3s-l) is the total (GC plus make-up gases)detector gas flow rate measured at the exit port of the ECD. For a gaussianpeak, the concentration of RBr at peak maximum will thereforebe given by [RBr]= 1.67x [RBr], [17]. Measurementsof cr as a function of [RBr]were obtained by analysis of the plots of PD responses versus[RBr],suchas the one shownin Fig. 3. It was assumed that at a given [RBr],the measured PD response 61 is proportional to [Br-] and that the difference between the maximum obtainable responseSi,,,,, and 61is proportional to [Cl-]. Therefore, LYwas determined from the relation QI= 6i/(61mar- 61) at each [RBr]. The optimal value of 61,, was selectedso that cywas propor-

231

K.S. Strode and E.P. Grimwud/Inf.J. Mass Specrrom.Ion Processes 130 (1994) 227-236 Discussion

0.d 0

C,H,Br

2

4

Concentration

6

8

(1012 molecules cc-‘)

Fig. 4. A plot of 0 = U/(SIm, - 61) versus [CzHsBr]usingdata in Fig. 3 for which a < 1.2.

tional to [RBr] over a wide range of [RBr], as required by Eq. (5). The values of o thereby determined were then plotted against [RBr], as shown in Fig. 4 for the case of I-bromoethane. In theseplots, only the data points for which o < 1.2 were used, since these measurements of a: are less sensitive to any error that might have occurred in our choice of SI,,,. AS predicted by Eq. (5) and as shown in Fig. 4, o was found to be linearly related to [RBr] for all compounds investigated here. The slopes of the lines formed by these plots are expected to be equal to ki/P (Eq. (5)). Since the magnitude of fl= ks[p’] has been previously shown [lo] to be equal to (100 f 50) s-’ under the present conditions, the magnitudes of ki are thereby independently obtained from the slopes of the o-versus-[RBr] plots. The results obtained in this way for all RBr are listed in Table 1. The greatest uncertainty in these ki determinations is thought to be associated with the uncertainty of /3 indicated above. Therefore, the absolute magnitudes of the rate constants listed in Table 1 are considered to have an uncertainty of about 150%. However, in comparing the rate constants within the present set of measurements, the uncertainty of the relative magnitudes of the rate constants is estimated to be significantly smaller, about f20%.

The absolute magnitudes of the ki values measured here by the PDM-ECD can be compared with those reported recently for the reactions of four alkylbromides (1, 2, 4, and 11 in Table 1) under identical conditions of temperature and pressure by the kinetic ion mobility mass spectrometry (KIMMS) technique [7]. It is seen that these four rate constant determinations by the PDM-ECD are about 20% greater than those determined previously by the KIMMS and, therefore, these two sets of measurements differ by significantly less than the estimated uncertainty (3~50%)of the present measurements. The fact that all the PDMECD measurements are consistently about 20% greater than the corresponding KIMMS measurements possibly indicates that our estimate of a /I value for use in Eq. (5) is about 20% greater than the actual value. In order to compare the effectsof the various R groups used here on the rate constants of reaction (l), it is first desirable to remove from consideration the relatively trivial effect of the R group on the rate of collisionsbetweenCl- and RBr. Reaction (1)is thought to proceedthrough an intermediateion complex,as shown in reaction (6) [18-201: k, Cl- f RBr + Cl-(RBr) kb

+

products

(6)

With the very high buffer gas pressure used in the present study, the intermediates, Cl-(RBr), formed in the reactions of most of the compounds studied here are thought [9,19]to be thermal&d by collisions prior to their continued motion either forward (kp) or backwards (kb) along the reaction coordinate. A possible exception to this is the case of bromomethane for which the intermediate might have exceedinglyshort lifetime against backdissociation [9,19] and, therefore, might not be completely thermalized prior to its motions along the reaction coordinate. Application of the steadystate approximation to reaction (6) leads to the following expression for the overall observed rate

232

KS. Strode and E.P. Grimsrud/Int.J. Mass Spectrom. Ion Processes 130 (1994) 227-236

constant, k1 : k, = kc x (k,/(k, t kJ) = kc x EFF

(7)

The chemicaleffectsof interest in this study will be given by the efficiency(EFF) of the reaction, which is obtained from a measuredvalue of kl divided by a calculated value of kc. The collision rate constants kc can be calculatedby Langevin-ADO theory [21], using either known (171or estimated values for the polarizabilities and dipole moments. The following values for kc were thereby deduced: 1.42x 10-9cm3s-’ for 1; 1.6 x 10-9cm3s-’ for 2; 1.7x 10-9cm3s-’ for 12; 1.8 x 10-9cm3s-l for 3, 8, 11, 14, and 15; 2.0 x 10m9cm3 s-l for 20, and 1.9 x 10e9cm3s-’ for all others. For the case of bromomethane,for example, the value of EFF = k,/k, = 0.029 indicates that 2.9% of the collisions between Cl- and CHsBrlead to product formation. In order to facilitate a comparison of EFF for all compounds in Table 1, the term “rel EFF” has been delined as the ratio of EFF for each compound to that observed for bromomethane. The uncertainty of the rel EFF values is considerably smaller than that of the absolute kl values and is estimated to be less than f20%. In interpreting the “rel EFF” valuesin Table 1,it will generallybe appropriate to envision the effects of the substituents, R’, R” and R”‘,on the classical SN2transition state, shown as structure I, in which bond formation to the Cl atom and bond rupture from the Br atom are simultaneouslyoccurring at the a-carbon:

R” P”’

‘\I

Cl--C--Br I R’ In Table 1 compound 2 indicatesthat rel EFF is decreasedby a factor of 0.31 by the addition of a methyl group to bromomethane. This retardation of the rate indicates the increasedsteric hindrance causedby this change outweighsthe potential electron-donating inductive effectof the methyl group,

since the inductive effectwould tend to increasek, by stabilizingthe electron-deficiento-carbon of the transition state. The addition of a second methyl group to the o-carbon (11) causes even greater steric hindrance so that rel EFF is reduced to 0.019. It is expected [20] that the addition of a third methyl group to the a-carbon (14) would cause an additional decrease in rel EFF. In the present study, only an upper limit for rel EFF (less than 0.04)could be determined for 2-bromo2-methylpropane.This is because this compound has a significantly greater electron capture rate constant than the other RBr studied here and, therefore, the sensitivity of the PDM-ECD method to its slow reaction with Cl- was greatly reduced. The upper limit reported here is thought to be much greater than the actual k, for 14. It is also noted that rel EFF = 0.0073for bromocyclopropane (12)indicateseven greatersterichindrance to backside attack for this compound than was observed for 2-bromopropane. This is expected due to’the additional strain associated with the attainment of a planar configuration of the HC(CHa), moiety in the SN~transition state for bromocylopropane. It is interestingto note that lengtheningthe alkyl group by addition of a methyl group to the P-carbon of bromoethane doubles the efficiencyof reaction (rel EFF= 0.62 for 3). This observation follows the same trend previously observed by DePuy et al. [20] using helium buffer gas at 0.5 Torr pressureand 25°C: they report efficienciesof 0.015,0.0025,and 0.007for compounds 1,2, and 3, respectively.In addition, theoreticalstudiesby Jensen [22]predict that steric effectsin the SN2reactions of n-propyl halideswill be lesshindering than those of the ethyl halides. It is noted in Table 1, that branching of a primary alkylbromide by the addition of a methyl group to the a-carbon of I-bromopropane (forming 13, for which rel EFF = 0.020)decreasesefficiency by a factor of 30. Also, the addition of a methyl group to the P-carbon of 1-bromopropane (forming 7, for which rel EFF = 0.16) decreases efficiencyby a factor of 4. The reactivity of 8 (rel

K.S. Strode and E.P. Grim.wd/Int.J. Mass Spectrom.Ion Processes 130 (1994) 227-236

233

Table 1 Rate constants and reaction efficienciesof gas phase reactions, Cl- t RBr -+ RCl t Br-, in nitrogen buffer gas at 640 Torr and 125°C Alkylbromide

k, by PDM-ECDa (EFF)b

k, by KIMMS’

1

Bromomethane

4.1(*2.1) x lo-” (2.9 x 10-I)

3.4 x lo-”

1.00

2

Bromoethane

1.4(+0.7) x lo-” (8.7 x lo-‘)

1.2x lo-”

0.31

3

1-Bromopropane

3.2(*1.6) x lo-” (1.8 x lo-‘)

4

1-Bromobutane

2.8(&1.4)x 10-i’ (1.4 x 10-q

5

l-Bromopentane

1.9(11.0) x lo-” (9.9 x 10-s)

0.34

6

1-Bromodecane

2.6(&1.3)x IO-” (1.4 x 10-q

0.47

1-Bromo-2-methylpropane

8.5(*4.3) x lo-‘* (4.5 x lo-‘)

0.16

Bromometbyl cyclopropane

6.6(*3.3) x lo-l2 (3.7 x 10-q

0.13

1-Bromo-3-methylbutane

1.7(f0.8) x lo-” (9.0 x 10-q

0.31

1-Bromo-2,2dimethylpropane

< 1.0 x 10-13 (< 5 x 10-s)

2-Bromopropane

9.7(15.0) x lo-‘” (5.4 x 10-4)

Bromocyclopropane

X6(11.8) x lo-l3 (2.1 x lo-“)

0.0073

2-Bromobutane

1.1(*0.6) x lo-‘* (5.8 x IO-‘)

0.020

2-Bromo-2-methylpropane

< 2.2 x 10-12 (< 1.2 x 10-s)

3-Bromopropene

8.4(&4.2)x IO-” (4.7 x lo-*)

1.63

16

3.Bromo-2-methylpropene

3.0(+1.5) x lo-” (1.6 x IO-*)

0.55

17

4-Bromo-1-butene

3.7(11.9) x lo-” (2.0 x IO-*)

0.67

I8

5-Bromo-l-pentene

5.1(&2.6)x lo-” (2.7 x 10-2)

0.93

19

8-Bromo-2,6dimethyl-Zoctene

2.7(*1.4) x IO-” (1.4 x IO-*)

0.49

20

Ethyl 3-bromo-propanoate

4.6(f2.5) x lo-” (2.3 x IO-*)

0.80

No.

Rel EFFd

0.62 2.2 x lo-”

0.49

< 0.002 7.8 x lo-l3

See Fig. 5 for the alkylbromide structures. aPresent results; units are cm3 molecules’ s-’ bEFF = k,/k, where k, is the calculated ADO collision rate constant. ‘Previously obtained by the KIM mass spectrometer under identical physical conditions [7). d Efficiencyof reaction relative to that of bromoethane. Estimated uncertainty of Rel EFF values is *20%

0.019

< 0.04

234

KS. Strode and E.P. Grhwud/lnt. J. Mass Spectrom. Ion Processes 130 (1994) 227-236

7

6

5

a

12

Br

L4

16

ia

Fig. 5. Structuresof akylbromides listedin Table 1.

EFF = 0.13)indicates that the addition of a cyclopropyl group to bromomethane causes approximately the same effect as observed for the structurally-similar 7. Compound 10 (rel EFF< 0.002) indicates that the addition of two methyl groups to the P-carbon of I-bromopropane decreasesefficiencyby more than a factor of 300. It is noted that rel EFF (0.49)for 1-bromobutane (4) is also greater than rel EFF for bromoethane

(0.31).Compound 9 (rel EFF = 0.31)indicatesthat the addition of a methylgroup to the y-carbon of Ibromobutane causes detectable additional steric hindrance. It is interesting to note that 5 and 6 indicate that reaction efficienciesdo not systematically decreaseas the lengthof an n-alkyl substituent is made very large (up to a HI-carbonchain). Compound 15 (rel EFF = 1.6)indicates that the addition of a double bond between the /? and y

KS. Strode and E.P. Grimsrud/lnt..I. Mass Spectrom.Ion Processes 130 (1994) 227-236

235

carbons of I-bromopropane almost triples the reaction efficiency so that rel EFF then exceeds that of I-bromomethane. Two potential explanations for this effect have been considered. One is that the increased electron-donating inductive effect of the a-electrons helps to stabilize the ocarbon in transition-state I and thereby lowers the energy of the transition state. The other potential explanation is that an alternate mechanism, called SN2’[23],becomes operative for an ally1bromide by which attack by the Cl- ion occurs at the 7carbon, as shown by reaction (8):

also appear to be more consistent with the SN2 mechanism in which an inductive effect of the remote Ir-electrons tends to stabilize the electron deficient a-carbon in transition state 1. There would be no corresponding basis for explaining a rate enhancement by the double bonds in 17and 18 in terms of the SN2’mechanism, which is expected to be operative only for an ally1bromide. Compound 19 (rel EFF= 0.49) is a relatively complex primary alkyl bromide including a remote double bond and a methyl substituent at the @arbon. For this molecule, it appears that

By the SN~’mechanism, the Br- ion is eliminated, the double bond is shifted to the adjacent bond and the same neutral product as expectedfor the SN2 mechanism is formed. The rate of reaction for 3bromopropene might conceivably be enhanced by the SN2’mechanism because its transition state involves less steric hindrance to the approaching nucleophile than does the transition state of the normal $2 mechanism. Compound 16 (rel EFF= 0.55) indicates that the addition of a methyl group to the P-carbon of 3-bromopropene (15)reduces the reaction efficiencyby a factor of 3. It will be recalled that the corresponding change to I-bromopropane (compare 3 and 7) caused a similar reduction of reaction efficiency.This observation provides support for the relativeimportance of the SN2mechanism over the SN2’mechanismfor 3bromopropene, since relatively little steric hindrance would have been expectedto be caused by the addition of a methyl group at the @carbon in the transition state of the SN2’mechanism.Also, 17 and 18 indicate that the addition of a double bond at more remote positions along primary bromo-nalkanes also causes small but detectable enhancements of reaction efficiencies.These observations

the rate-enhancing effect of the double bond and the rate-retarding effect of &substitution roughly cancel each other. Compound 20 (rel EFF = 0.80) illustrates the effectof insertion of a carboxyl group at the y-carbon of a straight-chain alkyl bromide. The reaction efficiencyis doubled by this change. Conclusion

A novel method of determining rate constants for ion/molecule reactions in an atmospheric pressure buffer gas has been described. By the PDMECD method, the relative concentrations of the reactant and product ions are spectroscopically monitored without causing any significant perturbations of the reaction mixture while the reactant neutral compound is introduced to the reaction volume in exceedinglypure form by gas chromatography. The rate constants for many of the slow gas phase &2 reactions reported here would be very difficult to determine by most existing methods for gas phase ion chemistry because of the frequent co-presence of structurally similar, more reactive impurities in common sources of organic compounds.

236

KS. Strodeand E.P. GrinwudjInr.J. MassSpectrom.Ion Processes130 (1994)227-236

Acknowledgment This work was supported by a grant from the ChemistryDivision of the National ScienceFoundation (Grant #CHE-9211615).

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