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Rate constants for the reactions of CCl(X 2II) with a series of silanes
Volume 68, number 1
CHEMICAL
RATE CONSTANTS
FOR THE REACTIONS
F-C. JAMES, H-K-T_ CHOI, 0-P. STRAUSZ of Chedst~. Unir ersity of Albert.
Department
PHYSICS LETTERS
OF CCI@II)
Edmonton.
Alberta_
1 December 1979
WITH A SERIES
OF SILANES
Canada T6C 2G2
and T-N. BELL Department
of Chemtit~.
Simon Frazer ffnir etxty.
Burnaby.
Bnful~ Coiunrbia. Canada V5A IS6
Received 15 August 1979
Rate constants for the gas phase reactions of CC1 generated by the flash photolysis of CHBrzCl with a series of silanes have been obtained by kinetic absorption spectroscopy. In general, the rate constants arc very high, and range fmm (4 8 2 0.5) X 10’ (SIHH~)to (6-4 r 0.34) % IO9 for SitHe. CC1 does not insert into the Si-C or primary C-H bonds of silanes and its nte of reaction with tertiary 5-H bonds is 600 times greater than with tertiary C-H bonds. CC1 reacts slowly with the Si-Si bond. kH/k~ variesfrom 1.9 to 1.0 on gomg fmm primary to tertiary Si-H bonds. The electrophilic character of CC1 is manifested, on a per Si-H bond basis, by ew_xIIent correIations between the rate constants and the hydridic character of the Si-H bond, and betxkeen logk and the ionization
potential.
1. Introduction In recent pubiications from this laboratory we have reported on the gas phase reactions of the halocarbynes CBr and CC1 with alkanes, alkenes and alkynes [l-3] _ It was found that CBr(z211) and CCl(%:aIl) reacted primarily via addition across the double or triple bonds of the unsaturated species, and insertion reactions were found to be of minimal importance except in the case of isu-butane [ 11. The electrophilic nature of these reagents is reflected in the correlation between log k and the ionization potential of the substrates [2,3] _ The selectivity of halocarbynes with respect to saturated and unsaturated carbon bonds contrasts with the behaviour of CH(g 2fl) [4-61 and carboethoxymethyne [7] _CH(% ‘II) generated by the gas phase vacuum UV flash or laser photolysis, or pulse radiolysis, of methane
shows
tion and addition
non-selective reactions
behaviour which
occur
in both at rates
inserclose
to collision frequency [4-6]_ Although kinetic data are not available for the liquid phase reactions of carboethoxymethyne, end product analysis indicates that this species participates in both insertion and concerted addition reactions [7] _
These investigations have now been extended to the reactions of CCi@ *lI) with a series of silanes_ It was anticipated that the weak Si-H bond of the silanes [S] would be more amenable to attack by the halocarbyne than is the C-H bond of alkanes. The species CH,(lA,) and SiH,(lA,) are known to insert into Si-H bonds with very fast rates [9], and silanes also act as efficient hydrogen donors in their reactions with alkyl iadicaIs and H atoms [lo-12]_ As in the previous studies [3], the flash photolysis of dibromochloromethane was used as the source of ccl(z2I-I) [13] CHBr,Cl
+ hv --t CHBrClf
CHBrCI’
-I-M +CHBrCl
+ Br , -I-M ,
Cl-I&Cl? + CCl(%2Il)
+ HBr ,
CHBrClr
+ HCl _
4 CBr(%zII)
The CBr(X-2 II) is formed in quantities insufficienf for the successful parallel measurement of its decay kinetics by kinetic absorption spectroscopy. To minimize deactivation of the excited bromochloromethyl radical, helium was used as diluent gas in all experiments131
Volume 68. number 1
CHEWCAL
PlIYSlC LElTERS
I December 1979
2_ Experimental The conventional flash photolysis system wils the same as that used in earlier investigations [l-3] and the techniques employed have been described in detail previously_ Dibromochioromethane (Eastman) was purified by vacuum distiiiation_ Disiiane. disilane-d6, monomethylsiIane, dimethyIsiIane_ril and trimethyIsiIane_ii (Merck. Sharp and Dotrme of Canada Ltd_)_ s&me (Petrarch Systems Inc.). trimethylsihne (Chemical Procurement Laboratories), nmnon~ethyIsilant&3, dimethylsilane, diethylsihme, triethylsilane, hexamethyidisikme, monofluorotrimethylsilane. difIuorodimethyIsiIane and triffuoromonomethylsikme (Peninsular Chem_ Res. Inc.), tetramethyfsihne (City Chemical Carp_) and 2-methyipropane (Phillips) were of research grade and were used after vacuum distillation and degassing at the appropriate temperatures_ Helium (Linde) was purified over copper at 350°C foIlowed by passage &rough molecular sieve_
3_ Results Kinetic measurements were made using the strong absorption band of CCI(kzfI) at 277-7 nm, which has been assigned to the (Qt , 0,O) band of the 2+,2 + *ffuz transition [14]_ For pIate densitometry, the value y = 05 which had been determined previousiy [3,15] was used In the modified form of the BeerLambert law_ The experiments were carried out at 298 t 5 K using helium (50 Torr) as diluent gas and a constant dibromochloromethane concentration (0.30 Torr)_ The decay kinetics of CC1 were monitored after the termination of the photolysis flash (=30~)_ In the absence of substrate CC1 exhibits simple first order decay kinetics which can be ascribed to bimoIecuIar reaction with the parent haloakane or other photolytic products [3]- No attempt was made at product anaIysis_ Addition of substrate led to an enhancement of the decay which can be described by:
Fig- I_ Pseudo fist order decay coefficients k” for CC1 (% Zn) in the prcwnce of s&me (0). dimethylsilane (a), snd triethylsilane (A)_ Pressures of dibromochloromethane and helium were 0.30 snd 50.00 Torr. respectively_Each point represents the average of serertll measurements. A number of points at Iow pressure habe been omitted for clarity.
Table I Iists the second order rate constants for the silanes studied_
4_ Discussion The most noticeable feature of the data presented in table 1 is that the reaction of CCI(~2fI) with most siknes is extremeIy rapid. and in the case of disihne, is one-fortieth that of the collision frequency * _ The rate constants for reaction of Ccl@ ‘KI) with methylated siIanes are of the same order of magnitude as for the Insertion reactions of the species CH2(tAr) [17,18] and SiH2(lA,) [1920]. However, they are S-10 times Iager than for the abstraction reactions of H or D atoms [ 12,211 and up to six orders of magnitude larger than for the room temperature abstraction reactions of methyl radicals [IO] _ No measurabIe rate of reaction was observed with tetramethylsiiane and with the series F4,SiIMeX(< lo6 M-1 s-1). implying that insertion into Si-C and primary C-H bonds [3] is unimportant, and that abstraction
- d [CCi]/d t = d [CCL] + k, [Ccl] [S] _ The second order rate constant k, for different substrates was determined from the least mean square siopes of pIots of the pseudo first order decay coefficient k”= k* f k, [S] versus [S], and some typical decays are iliustrated in fig_ I_ 132
* Collisionfrequmcies at 25OC calculated as 2.56 x 10” (SizH& 226 x 10” (SM& Z-4 x 10” @leSiH& 259 X IO” (Me2StH2); 2.82 X IO*’ (Me3SM) in M-r s-r units. uCC1 = 4.065A [16], oS12H6 =650A, US& =5 04A. obl&as = = 654A. o&lesS~ = 732A (from ref. 1171 580A. 0~~S~* or interpolated from &ta therein)_
Volume
68, number 1
CHEYICAL
PHYSICS
1 December
LETTERS
1979
Table 1 Bimolecular --
rate const.mtsn) Substmte
mz3SiH
%iejSi EtaSiHa Et&II
Pressure mnge of substrate
120
O-0.81
1-4.5X 109 4.5 x109
CTorr)
(G118+0.05)~ (1.7 r0.2) (2.5 +0-l) (2.8 +0_2) no nkvztion (2.9 20.3)
10’ x109 XI09 xlOg x109
264 288 128 128 120
(45
x109
240
O-O.16 o-O-18 O-O.16 O-62 O-O.18 O-O.16
+0-l)
kt per Si-H bond
0.12x 109 0.8% lo9 IA x109 2.8 X109
(0.2.5*0 03)X 109 (0.98=0.04)x 109 (1.8 50.1) x109 (2.8 503) x109
152 296 272 192
O-0.83 O-0.28 O-O.16 O-O.16
0.06x IO9 0-33x 109 o-9 x109 2.8 ~10~
SizH6
(6.4720.34)X IO9 I5.6150.32)~ IO9 (25 t0.3) x10=
120 9s 120
O-0.08 o-0.13
I.1 x109 0.93x 109
no renction
408
O-45
-
(4.52*0
128
O-86.5 ___-
-
hle,SiF4
.---
______
Number of mensuremerits
SiD, YeSiDa >tcgiiDa We3S1D
Sia D.5 %le&ia
-
uith siknes at 298 R
81
SiH4 .?IcSiHa ~k&H~
___
for the reactions of CClf%‘H) __
11e3CH
a) In units of bl-‘s-r
__r
44)X 106 -.--
o-4.4
_
from C-H bonds must be extremely slow, if it occurs at nit. Experiments with fro-butane indicate a me.rsurable but very slow, 4.5 X 106 ?cl-1 s-1, rate of reaction with the tertiary C-H bond. This can be contrasted with the reactions of CH(?2lT) which inserts into primary [4] (and presumably secondary and tertiary) C-H bonds at rates close to the collision frequency. A comparison between the rates of reaction of CC1 with Me3SiH and iso-butane shows that the reactivity of CC1 for tertiary Si-H bonds is 600 times greater than for tertiary C-H bonds. Deuteration of the srlanes features an isotope effect that decreases from I.9 with primary Si-H bonds to 1 .O with tertiary Si-H bonds. The isotope effect exhibited by CH2(rAl) insertion into primary Si-H bonds is lower at 1.15 [Is] while that for abstraction reactions by methyl radicals is higher at ~2-7 [IO] _ The reaction of Ccl@ 211) with hexamethyidisilane must involve reaction with the Si-Si bond. The value of the rate constant, 25 X lo7 M-l s-l, may be low because of steric hindrance caused by the methyl groups_
However, the much larger rate constant for reaction with disdane, 6.47 X IO9 M-l s-l, is not due to enhanced reactivity with the Si-Si bond, smce as shown below. it correlates well with the rates of Si-H insertion of monosilanes. Hence, it may be concluded that reaction with the Si-Si bond of S&H, contributes little to the overali reaction rate. The results in table 1 are more meaningful if compared quantitatively on a per Si-H bond basis, and this is shown in the fiial column of table 1 where the trend of increasing reactivity with increasing alkylation becomes apparent_ The Si-H stretching frequencies for a series of silanes with various degrees of methyl substitution have been discussed by Hollandsworth and Ring [22] who concluded that the hydridic character of the H atom attached to Si increases with increasing methylation. This was confirmed by recent molecular orbital calculations by Perkins et al_ 1231. who obtained values for the negative charge density on the Si-H hydrogen that increase from =-O-OS to =--O-l 1 in going from SiH4 to hle$ZH. I33
Volume 68. number i
CHEMICAL
PHYSICS LETTERS
1 December 1979
the same, it is possible that they differ by 0.1-0.2 eV_ if this is the case, then the plots for methylated silanes in fig. 2 become aImost coincidental_ The transition state model we propose would be favoured by increasing Si6*-H6charge density scparation resuIting in a hydride ion transfer with more or less simultaneous back donation of the lone electron pair of carbon to the silicon.
The reactivity of CCl(%zH) with silanes suggests that interaction with the Si-Si bond is not important in influencing the observed reactivity sequence_
AcRnowIedgement
10
II
12
lonrzation Potentrat(ev)
F%_ 2_ Pbt of log (kt per Si-II bond) versus ionization potentiat for CC1 (% *II) reactions with sibmcs
Given our previous observations concerning the eiectrophihc nature of CC1 13). a correlation would be expected between the rate of insertion and the hydridic character of the Si-H bond- Our experimental measurements, tabfe I, do indeed lead to such a concIusion, with the reactivity of CCI foIIowing the order Me,SiH > rMetSiHZ > Si,H, > MeSiH, > SiH, [24] _ Further evidence of the electrophilic character of CQ is seen from the correlation between the ionization potential [2SJ* and logk for the reactions of CC1 with the silanes. When compared on a per Si-H bond basis (fll- 2). a division into an homoIogous series sequence Zsapparent perhaps reffectingspecitic Si-H bond strength changes for the different homologous series of silane~_ AMtough we have assumed that the ionization potentials for deuterated and hydrogenated silanes are * Vahres for ionization potentiahrof the deuterated sihuteswere assumed to be the same as those for the non-deuteratcd sitanes.
134
The authors are gratefir to the National Research Council of Canada for continuing financial support_
References [I]
RS- h1cDanie.LR- Dickson. F-C. James, OP. Strausz and T-N. Be& Chem- Phys. Letters 43 (1976) 130. 121 F-C. James,B. Ruzsicska,R-S. McDaniel. R. Dickson, O-P- Strauss and T-N- Bell. Chem. Phys. Letters 45 (1977) 449. 131 FC- James, J_ Choi. O-P_ Strauss and T-N_ Bell. Chem. Phys- Letters 53 (1978) 206. [4] J-E- Butler, L-P. Goss, b1.C. Liu aud J-W_ Hudgeus.Chem. Phys. Letters 63 (1979) 104. 151 M-W. Bosttali and D. Pemer, 2. Narurforsch. 26a (1971) X768161W- Braun. R-H- Wedgeand J-R. McNesby. J- Cbem- Phys_ 45 (1966) 2650; W- Braun, J.R. McNesby and A.bf. Bass, J. Cbem. Phys. 46 (1967) 2071. [71 O.P_ Strausz. G-LA. Kermepohl, FX. Gartreau.T. DoMirth, B-Kim, S. Vaknty and P.S. Skell, J. Am. Chem. Sot. 96 (1974) 5723. 181 P- Potzinger. A- Ritter and J- Krause, Z. Naturforsch. 30a (1975) 347. 191 I.Bf.T_ Davidson. in: Reaction kinetics, Vol. 1, Specialist Periodical Reports (Chem_ Sot-, London. 197.5) p_ 212.
Volume [IO]
68. number 1
R-E. Berkeky.
CHEMICAL
PHYSICS
I. Safarik, 0 P. Strauss and H.E. Gunning,
J. Phys. Chem. 77 (1973)
1741;
R.E. Berkeley, I. Safarik, H.E. Gunning and OP.
Strausz.
J. Phys. Chem- 77 (1973) 1734; O-P- Strauss. E. Jakubowski. HS. Sandhu and HE Gunning, J_ Chem- Phys- 51 (1969) 552_ [ 111 D_ Mhelcic. V. Schubert. R-N_ Schindler and P_ Potainger, J_ Phyr Chem_ 81 (1977) 1543. and references therein [12] E-R_ Austin and F-W_ Lampe. J_ Phys. Chem. 81 (1977) 1134. [ 131 J-P_ Simons and AJ. Yarwqod, Trans. Faraday Sot- 57 (1961) 2167_ [14] R-D- Verma and R.S. hlulhken. J. Mol. Spectry- 6 (1961) 419_ [IS] W.J.R. Tyerman, Trans. Faraday Sot. 65 (1969) 2948. [ 16 ] R_A- S’vehia. NASA Technical Report R132 (1962).
LETTERS
1 December
1979
1171 W.L. Hase and J-W_ Simons, J. Chem. Phys. 54 (1971) 1277. [18] [19]
CJ. hlazac and J-W. Sin-tons, J_ Am_ Chem. Sot 90 (1968) 2484. P. John and J-H_ Purnell, J. Chem. Sot Faraday I 69
(1973) 1455B. Co1 and J-H. Purrtell. J_ Chem. Sot. Faraday I 71 (1975) 859_ [21] I(. Obi, H.S. Sandhu. H.E. Gunning and 0-P. Strrus+. J. Phys. Chem. 76 (1972) 3911. [22] RP. Hollandsworth and M-A_ Ring, Inorg- Chem. 7 (1968) 1635 1231 z(_A_ Perkins, P-G_ Perkins and T-N. Bell, unpublished [20]
[24] [25]
data-
hi-D. Sefuk and h1.A. Ring, J. Am. Chem. Sot 95 (1973) 5168. R_ Roberge, C_ Scndorfy, J-I_ hiatthewo and 0 P_ Strausz, J- Chem. Phys. 69 (1978) 5105.
135
CHEMICAL
RATE CONSTANTS
FOR THE REACTIONS
F-C. JAMES, H-K-T_ CHOI, 0-P. STRAUSZ of Chedst~. Unir ersity of Albert.
Department
PHYSICS LETTERS
OF CCI@II)
Edmonton.
Alberta_
1 December 1979
WITH A SERIES
OF SILANES
Canada T6C 2G2
and T-N. BELL Department
of Chemtit~.
Simon Frazer ffnir etxty.
Burnaby.
Bnful~ Coiunrbia. Canada V5A IS6
Received 15 August 1979
Rate constants for the gas phase reactions of CC1 generated by the flash photolysis of CHBrzCl with a series of silanes have been obtained by kinetic absorption spectroscopy. In general, the rate constants arc very high, and range fmm (4 8 2 0.5) X 10’ (SIHH~)to (6-4 r 0.34) % IO9 for SitHe. CC1 does not insert into the Si-C or primary C-H bonds of silanes and its nte of reaction with tertiary 5-H bonds is 600 times greater than with tertiary C-H bonds. CC1 reacts slowly with the Si-Si bond. kH/k~ variesfrom 1.9 to 1.0 on gomg fmm primary to tertiary Si-H bonds. The electrophilic character of CC1 is manifested, on a per Si-H bond basis, by ew_xIIent correIations between the rate constants and the hydridic character of the Si-H bond, and betxkeen logk and the ionization
potential.
1. Introduction In recent pubiications from this laboratory we have reported on the gas phase reactions of the halocarbynes CBr and CC1 with alkanes, alkenes and alkynes [l-3] _ It was found that CBr(z211) and CCl(%:aIl) reacted primarily via addition across the double or triple bonds of the unsaturated species, and insertion reactions were found to be of minimal importance except in the case of isu-butane [ 11. The electrophilic nature of these reagents is reflected in the correlation between log k and the ionization potential of the substrates [2,3] _ The selectivity of halocarbynes with respect to saturated and unsaturated carbon bonds contrasts with the behaviour of CH(g 2fl) [4-61 and carboethoxymethyne [7] _CH(% ‘II) generated by the gas phase vacuum UV flash or laser photolysis, or pulse radiolysis, of methane
shows
tion and addition
non-selective reactions
behaviour which
occur
in both at rates
inserclose
to collision frequency [4-6]_ Although kinetic data are not available for the liquid phase reactions of carboethoxymethyne, end product analysis indicates that this species participates in both insertion and concerted addition reactions [7] _
These investigations have now been extended to the reactions of CCi@ *lI) with a series of silanes_ It was anticipated that the weak Si-H bond of the silanes [S] would be more amenable to attack by the halocarbyne than is the C-H bond of alkanes. The species CH,(lA,) and SiH,(lA,) are known to insert into Si-H bonds with very fast rates [9], and silanes also act as efficient hydrogen donors in their reactions with alkyl iadicaIs and H atoms [lo-12]_ As in the previous studies [3], the flash photolysis of dibromochloromethane was used as the source of ccl(z2I-I) [13] CHBr,Cl
+ hv --t CHBrClf
CHBrCI’
-I-M +CHBrCl
+ Br , -I-M ,
Cl-I&Cl? + CCl(%2Il)
+ HBr ,
CHBrClr
+ HCl _
4 CBr(%zII)
The CBr(X-2 II) is formed in quantities insufficienf for the successful parallel measurement of its decay kinetics by kinetic absorption spectroscopy. To minimize deactivation of the excited bromochloromethyl radical, helium was used as diluent gas in all experiments131
Volume 68. number 1
CHEWCAL
PlIYSlC LElTERS
I December 1979
2_ Experimental The conventional flash photolysis system wils the same as that used in earlier investigations [l-3] and the techniques employed have been described in detail previously_ Dibromochioromethane (Eastman) was purified by vacuum distiiiation_ Disiiane. disilane-d6, monomethylsiIane, dimethyIsiIane_ril and trimethyIsiIane_ii (Merck. Sharp and Dotrme of Canada Ltd_)_ s&me (Petrarch Systems Inc.). trimethylsihne (Chemical Procurement Laboratories), nmnon~ethyIsilant&3, dimethylsilane, diethylsihme, triethylsilane, hexamethyidisikme, monofluorotrimethylsilane. difIuorodimethyIsiIane and triffuoromonomethylsikme (Peninsular Chem_ Res. Inc.), tetramethyfsihne (City Chemical Carp_) and 2-methyipropane (Phillips) were of research grade and were used after vacuum distillation and degassing at the appropriate temperatures_ Helium (Linde) was purified over copper at 350°C foIlowed by passage &rough molecular sieve_
3_ Results Kinetic measurements were made using the strong absorption band of CCI(kzfI) at 277-7 nm, which has been assigned to the (Qt , 0,O) band of the 2+,2 + *ffuz transition [14]_ For pIate densitometry, the value y = 05 which had been determined previousiy [3,15] was used In the modified form of the BeerLambert law_ The experiments were carried out at 298 t 5 K using helium (50 Torr) as diluent gas and a constant dibromochloromethane concentration (0.30 Torr)_ The decay kinetics of CC1 were monitored after the termination of the photolysis flash (=30~)_ In the absence of substrate CC1 exhibits simple first order decay kinetics which can be ascribed to bimoIecuIar reaction with the parent haloakane or other photolytic products [3]- No attempt was made at product anaIysis_ Addition of substrate led to an enhancement of the decay which can be described by:
Fig- I_ Pseudo fist order decay coefficients k” for CC1 (% Zn) in the prcwnce of s&me (0). dimethylsilane (a), snd triethylsilane (A)_ Pressures of dibromochloromethane and helium were 0.30 snd 50.00 Torr. respectively_Each point represents the average of serertll measurements. A number of points at Iow pressure habe been omitted for clarity.
Table I Iists the second order rate constants for the silanes studied_
4_ Discussion The most noticeable feature of the data presented in table 1 is that the reaction of CCI(~2fI) with most siknes is extremeIy rapid. and in the case of disihne, is one-fortieth that of the collision frequency * _ The rate constants for reaction of Ccl@ ‘KI) with methylated siIanes are of the same order of magnitude as for the Insertion reactions of the species CH2(tAr) [17,18] and SiH2(lA,) [1920]. However, they are S-10 times Iager than for the abstraction reactions of H or D atoms [ 12,211 and up to six orders of magnitude larger than for the room temperature abstraction reactions of methyl radicals [IO] _ No measurabIe rate of reaction was observed with tetramethylsiiane and with the series F4,SiIMeX(< lo6 M-1 s-1). implying that insertion into Si-C and primary C-H bonds [3] is unimportant, and that abstraction
- d [CCi]/d t = d [CCL] + k, [Ccl] [S] _ The second order rate constant k, for different substrates was determined from the least mean square siopes of pIots of the pseudo first order decay coefficient k”= k* f k, [S] versus [S], and some typical decays are iliustrated in fig_ I_ 132
* Collisionfrequmcies at 25OC calculated as 2.56 x 10” (SizH& 226 x 10” (SM& Z-4 x 10” @leSiH& 259 X IO” (Me2StH2); 2.82 X IO*’ (Me3SM) in M-r s-r units. uCC1 = 4.065A [16], oS12H6 =650A, US& =5 04A. obl&as = = 654A. o&lesS~ = 732A (from ref. 1171 580A. 0~~S~* or interpolated from &ta therein)_
Volume
68, number 1
CHEYICAL
PHYSICS
1 December
LETTERS
1979
Table 1 Bimolecular --
rate const.mtsn) Substmte
mz3SiH
%iejSi EtaSiHa Et&II
Pressure mnge of substrate
120
O-0.81
1-4.5X 109 4.5 x109
CTorr)
(G118+0.05)~ (1.7 r0.2) (2.5 +0-l) (2.8 +0_2) no nkvztion (2.9 20.3)
10’ x109 XI09 xlOg x109
264 288 128 128 120
(45
x109
240
O-O.16 o-O-18 O-O.16 O-62 O-O.18 O-O.16
+0-l)
kt per Si-H bond
0.12x 109 0.8% lo9 IA x109 2.8 X109
(0.2.5*0 03)X 109 (0.98=0.04)x 109 (1.8 50.1) x109 (2.8 503) x109
152 296 272 192
O-0.83 O-0.28 O-O.16 O-O.16
0.06x IO9 0-33x 109 o-9 x109 2.8 ~10~
SizH6
(6.4720.34)X IO9 I5.6150.32)~ IO9 (25 t0.3) x10=
120 9s 120
O-0.08 o-0.13
I.1 x109 0.93x 109
no renction
408
O-45
-
(4.52*0
128
O-86.5 ___-
-
hle,SiF4
.---
______
Number of mensuremerits
SiD, YeSiDa >tcgiiDa We3S1D
Sia D.5 %le&ia
-
uith siknes at 298 R
81
SiH4 .?IcSiHa ~k&H~
___
for the reactions of CClf%‘H) __
11e3CH
a) In units of bl-‘s-r
__r
44)X 106 -.--
o-4.4
_
from C-H bonds must be extremely slow, if it occurs at nit. Experiments with fro-butane indicate a me.rsurable but very slow, 4.5 X 106 ?cl-1 s-1, rate of reaction with the tertiary C-H bond. This can be contrasted with the reactions of CH(?2lT) which inserts into primary [4] (and presumably secondary and tertiary) C-H bonds at rates close to the collision frequency. A comparison between the rates of reaction of CC1 with Me3SiH and iso-butane shows that the reactivity of CC1 for tertiary Si-H bonds is 600 times greater than for tertiary C-H bonds. Deuteration of the srlanes features an isotope effect that decreases from I.9 with primary Si-H bonds to 1 .O with tertiary Si-H bonds. The isotope effect exhibited by CH2(rAl) insertion into primary Si-H bonds is lower at 1.15 [Is] while that for abstraction reactions by methyl radicals is higher at ~2-7 [IO] _ The reaction of Ccl@ 211) with hexamethyidisilane must involve reaction with the Si-Si bond. The value of the rate constant, 25 X lo7 M-l s-l, may be low because of steric hindrance caused by the methyl groups_
However, the much larger rate constant for reaction with disdane, 6.47 X IO9 M-l s-l, is not due to enhanced reactivity with the Si-Si bond, smce as shown below. it correlates well with the rates of Si-H insertion of monosilanes. Hence, it may be concluded that reaction with the Si-Si bond of S&H, contributes little to the overali reaction rate. The results in table 1 are more meaningful if compared quantitatively on a per Si-H bond basis, and this is shown in the fiial column of table 1 where the trend of increasing reactivity with increasing alkylation becomes apparent_ The Si-H stretching frequencies for a series of silanes with various degrees of methyl substitution have been discussed by Hollandsworth and Ring [22] who concluded that the hydridic character of the H atom attached to Si increases with increasing methylation. This was confirmed by recent molecular orbital calculations by Perkins et al_ 1231. who obtained values for the negative charge density on the Si-H hydrogen that increase from =-O-OS to =--O-l 1 in going from SiH4 to hle$ZH. I33
Volume 68. number i
CHEMICAL
PHYSICS LETTERS
1 December 1979
the same, it is possible that they differ by 0.1-0.2 eV_ if this is the case, then the plots for methylated silanes in fig. 2 become aImost coincidental_ The transition state model we propose would be favoured by increasing Si6*-H6charge density scparation resuIting in a hydride ion transfer with more or less simultaneous back donation of the lone electron pair of carbon to the silicon.
The reactivity of CCl(%zH) with silanes suggests that interaction with the Si-Si bond is not important in influencing the observed reactivity sequence_
AcRnowIedgement
10
II
12
lonrzation Potentrat(ev)
F%_ 2_ Pbt of log (kt per Si-II bond) versus ionization potentiat for CC1 (% *II) reactions with sibmcs
Given our previous observations concerning the eiectrophihc nature of CC1 13). a correlation would be expected between the rate of insertion and the hydridic character of the Si-H bond- Our experimental measurements, tabfe I, do indeed lead to such a concIusion, with the reactivity of CCI foIIowing the order Me,SiH > rMetSiHZ > Si,H, > MeSiH, > SiH, [24] _ Further evidence of the electrophilic character of CQ is seen from the correlation between the ionization potential [2SJ* and logk for the reactions of CC1 with the silanes. When compared on a per Si-H bond basis (fll- 2). a division into an homoIogous series sequence Zsapparent perhaps reffectingspecitic Si-H bond strength changes for the different homologous series of silane~_ AMtough we have assumed that the ionization potentials for deuterated and hydrogenated silanes are * Vahres for ionization potentiahrof the deuterated sihuteswere assumed to be the same as those for the non-deuteratcd sitanes.
134
The authors are gratefir to the National Research Council of Canada for continuing financial support_
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Volume [IO]
68. number 1
R-E. Berkeky.
CHEMICAL
PHYSICS
I. Safarik, 0 P. Strauss and H.E. Gunning,
J. Phys. Chem. 77 (1973)
1741;
R.E. Berkeley, I. Safarik, H.E. Gunning and OP.
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LETTERS
1 December
1979
1171 W.L. Hase and J-W_ Simons, J. Chem. Phys. 54 (1971) 1277. [18] [19]
CJ. hlazac and J-W. Sin-tons, J_ Am_ Chem. Sot 90 (1968) 2484. P. John and J-H_ Purnell, J. Chem. Sot Faraday I 69
(1973) 1455B. Co1 and J-H. Purrtell. J_ Chem. Sot. Faraday I 71 (1975) 859_ [21] I(. Obi, H.S. Sandhu. H.E. Gunning and 0-P. Strrus+. J. Phys. Chem. 76 (1972) 3911. [22] RP. Hollandsworth and M-A_ Ring, Inorg- Chem. 7 (1968) 1635 1231 z(_A_ Perkins, P-G_ Perkins and T-N. Bell, unpublished [20]
[24] [25]
data-
hi-D. Sefuk and h1.A. Ring, J. Am. Chem. Sot 95 (1973) 5168. R_ Roberge, C_ Scndorfy, J-I_ hiatthewo and 0 P_ Strausz, J- Chem. Phys. 69 (1978) 5105.
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