1184
J . Phys. Chem. 1986, 90, 1184-1 186
starting compound can occur (assuming a rate constant of ca. lo6 M-' s-I as found for another para-substituted azobenzene3).
Conclusions NA undergoes one-electron reduction by e; and a-hydroxyalkyl radicals with reactivities which are strongly enhanced, with respect to other azobenzene derivatives, by the nitro substitution. In fact the rate constants of the reaction of N A with e;, both in i-PrOH and in MeOH, are higher than those found for A and DA in the same solvent^.^ (CH,),COH gives electron transfer to N A with a rate constant k N lo9 M-' s-I, ?orders of magnitude faster than that found for A;3 furthermore C H 2 0 H reduces N A (k 6X lo7 M-' s-'), whereas it was found not reducing A, in the same condition^.^ The above findings can be ultimately assigned to the increased stability of the anion NA-, due to the resonance structure -NO,-'. To the same effect can also be ascribed the acid-base behavior of this radical. In fact.NA- is found to be a weaker base than the analogous A- and DA-. The site of protonation is, in spite of the presence of the -NO2 reducible group, the central azo bond and. the semireduced radical is therefore identified as the hydrazyl, -N-N(H)-. It undergoes in i-PrOH a pseudo-first-order reaction with the starting com-
pound, leading to a dimer. This reaction seems to find unfavorable conditions in MeOH. Disproportionation of the hydrazyl and/or dimer radicals produces the complete reduction of the azo bond. This reaction is found to be more efficient for N A than for other azobenzene derivatives. This fact must be ascribed to the higher concentration of N A H attained, which favors the second-order dismutation, leading to complete reduction without any loss. It is concluded than the -NO2 group plays the important role of antenna for the one-electron reduction, enhancing the reactivity of the molecule. It is proposed that this effect is at the origin of the anomalous photofading of nitro-substituted phenylazo dyes in alcoholic solvents.2 Acknowledgment. This work was supported by the Progetto Finalizzato del CNR per la Chimica Fine e Secondaria. We thank Dr. S. Emmi for the setup of the computerized data acquisition system, and Dr. A. Martelli and Mr. A. Monti for the maintenance of the LINAC accelerator. The interest of Dr. M. Venturi is also acknowledged. Registry No. NA, 3646-57-9; NAH, 99798-28-4;i-PrOH, 67-63-0: (CH,),COH, 5131-95-3;CH,OH, 2597-43-5: MeOH, 67-56-1.
Kinetics of the Insertion of Singlet CH, into Methylfluorosilanes T. N. Bell,* A. G . Sherwood, and G. Soto-Garrido Department of Chemistry, Simon Fraser University, Burnaby, B.C., Canada V5A IS6 (Received: August 5, 1985; In Final Form: September 26, 1985)
'CH2 produced from the photolysis of ketene inserts into the C-H bond of methylfluorosilanes (Me,SiF,-,, x = 1-4), the relative rates being in the order Me4Si > Me,SiF > Me2SiF2> MeSiF3. In one case, that of Me2SiF2,the observed products are also consistent with insertion into the Si-F bond. ,CH2 undergoes reactions leading to hydrocarbon products and a radical exchange process is proposed in its reaction with the silanes.
Introduction The reactions of methylfluorosilanes with the CF, or C H 3 radical result in H abstraction, and such experiments have indicated' a significant polar character of the C-H bond, which varies with the degree of fluorine substitution. In studies of CC1 with the Si-H bonds of silanes where insertion is presumed to occur,2 the rates correlate well with the polarity of the Si-H bond and the electrophilic nature of CCI. In the case, however, of Me,SiCI and Me,SiF, the rates are too small to measure., It was of interest, therefore, to extend the studies of methylfluorosilanes to their reactions with 'CH2 which is electrophilic in character, and to examine the relative rates for insertion as a function of fluorine substitution. During the course of this work, it was also opportune to study the reactions of 3CH2,and these latter proved to be of considerable interest. Experimental Section The photolysis source was a P.E.K. 200-W high-pressure mercury arc, associated with a filter system to isolate 3 13-nm radiation. The reaction vessel was made of quartz, volume 21 5 mL, fitted with planar windows and maintained in a furnace assembly at 25 "C. Light transmitted through the cell was
monitored with a scanning monochromator/photomultiplier system. Materials. Ketene was prepared by decomposing acetone vapor and purified by trap-to-trap distillation. It was stored in the dark at -196 "C. Ketene-d2 was prepared4s5from acetic-d, anhydride, purified,6 and stored at -196 "C. The methylfluorosilanes were obtained from PCR Chemicals, Inc., and contained small quantities of C 0 2 and Me2SiF2 as impurities. A combination of trap-to-trap distillation and preparative gas chromatography sufficed to produce these materials free from contamination. Tetramethylsilane (Alfa Inorganics) was sufficently pure that degassing at -196 OC was the only procedure necessary. Nitric oxide (Matheson Co.) was purified by trap-to-trap distillation and trapped at -196 "C, while the system was continuously pumped. Analytical Methods. Purity checks and product analysis was by use of gas chromatography, mass spectrometry, infrared, and 400-MHz NMR spectroscopy. Having ascertained that the main silicon-containingproducts of the reaction of ketene with the silanes were the ethyl derivatives resulting from the insertion of 'CH2 into the C-H bond, and in one case, (Me2SiF2),into the Si-F bond, these products were prepared in bulk, isolated by gas chromatography, and used for GC calibration purposes. This enabled
(1) Bell, T. N.; Platt, A. E. J . Phys. Chem. 1971, 75, 603. ( 2 ) James, F . C . ;Choi, K. J.: Strausz, 0.P.; Bell, T. N. Chem. Phys. Lett. 1979, 68, 13 1 .
(3) James, F. C.; Choi, K . J.; Strausz, 0. P.; Bell, T.N. Chem. Phys. Lett. 1980, 73. 522.
0022-3654/86/2090-1184$01.50/0
(4) Jenkins, A. D. J . Chem. SOC.1982, 2563. ( 5 ) Hasek, R. H.; Gott, P. G.; Martin, J. C . J. Org. Chem. 1964, 29, 2510. ( 6 ) Laufer, A. H . J . Chromatogr. Sci. 1970, 8, 677.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 1185
Singlet C H 2 Insertion into Methylfluorosilane quantitative analyses to be made on the kinetic experiments.
1.8
Results
F3Si Me
Experiments were carried out to determine the products of the photochemical reaction of ketene with each silane. In all cases, the ethyl derivative was obtained, Le., C2H5SiMeyF3, together with ethylene, ethane, and acetylene. In the case of (CH3)2SiF2, a small amount of dimethyl(fluoromethyl)fluorosilane ((CH3)2SiF(CH2F)) was formed. In the presence of nitric oxide, ethane and acetylene were eliminated and the silicon derivative and ethylene diminished. Experiments carried out with C D 2 C 0 and (CH,),SiF, yielded unambiguous evidence from N M R spectroscopy that the ethyl derivative was structurally CHD2CH2Si(CH3)F2, indicating clearly that insertion of singlet methylene into the C-H bond occurred. The detailed photochemistry of ketene has been well studied and both 'CH2 and 3CH2are produced, there being a wavelength dependence of the relative yields. Interpretations of the mechanism of the primary photochemical process and the manner in which both 'CH2 and ,CH2 are produced vary, as do the methods used for these studies. These range from the classical' to the more recent molecular beam laser excitation methods.8 For the present purpose, a simplified scheme will suffice where K* represents an excited state capable of decomposing to form 'CHI and K**, that state yielding ,CHI, and taking into account only the resulting reactions of 'CH, we have
1.6
-2
1.4
v)
1.2
U
2
a 8
1.0
\
I" 0"
OB 0.6
0.4
0.2
~
hu
K-K*
- + - + + - + + + - + + + + + hu
K
01 0 .1
ICH2
CO
K**
,CH2
CO
K** or K*
M
K
M
12.0
K
ICH2 + Me,SiF4_,
3CH2
Me,SiF4-,
K
lCH2 + Me,SiF4_, ICH2
-+
C2H4
CO
EtMe,-,SiF4-,
+ Me,SiF4-,
06 0.6
1
3CH2
'CH2
05 0.5
04 0.4
Figure 1. Variation of C2H4/Si(p) vs. ketene/silane; a test of eq 13.
K
ICH,
03 0.3
KETENE/SILAN E
K**
K*
02 0.2
(x # 0)
Me,Si(CH2F)F3-,
3CH2
NO
products
ICH2
NO
products
t
F,Si Me,
t
2.0v
't
8
6.0
I" 0"
4.0
I
,
I
0.4
0.5
0.6
In order to establish conditions whereby the kinetics of 'CH2 reactions could be unambiguously followed, 1.3 torr of ketene was photolyzed in the presence of various amounts of NO (between 0.03 and 0.74 torr). Whereas in the absence of NO, equal amounts of C2H2and C2H6were observed, these products were completely eliminated in the presence of even the lowest pressure of NO used. This is taken as evidence that ,CH, is completely removed from the system. The ethylene formed decreases with increasing NO pressure, an indication that 'CH, is also being removed from the system by NO. This is expected given the magnitude of the rate constants for reactions of both 'CH2 and ,CH2 with NO. This work is well reviewed by L a ~ f e r . ~ Kinetic experiments were carried out in which 1.3 torr of ketene plus 0.15-0.2 torr of NO were photolyzed in the presence of varying amounts of each silane and the amounts of ethylene and silicon containing products measured. The relevant reactions
corresponding to these experiments are reactions 8, 9, and 10. Designating Si1 as the methylfluorosilane reactant and Si(p) as the product of reaction
(7) Zabransky, V.; Carr, R. W. J . Phys. Chem. 1975, 79, 1618. Kelley, P. M.; Hase, W. L. Chem. Phys. Lett. 1975, 35, 57. (8) Hayden, C. C.; Neumark, D. M.; Shobatake, K.; Sparks, R.; Lee, Y . T. J . Chem. Phys. 1982, 76, 3607. Nesbitt, D. J.; Petek, H.; Foltz, M . F.; Filseth, V.;Bamford, D. J.; Bradley Moore, C . J. Chem. Phys. 1985, 83, 223. (9) Laufer, A. H. Reu. Chem. Zntermed. 1981, 4 , 225.
The experimental data are shown in Figures 1 and 2 and verify the expectation of eq 13 and 14, Le., a straight line passing through the origin. Values of the relative rate constants k 9 J k sand k I o J k s were obtained from the slopes and are given in Table I on a per molecule, per bond basis and individual values of k9 and k l obased"
0.1
0.2
0.3
KETENE/SILANE Figure 2. Same as Figure 1 but a test of eq 14.
R c ~ H ~kg [CHICO]
-=-
RSi(p)
k9
[Sill
(13)
and in the one case of fluorine insertion i n t o MezSiFz
-R=c -~ H ~k8 [ C H D I RSi(p) ~ I O [Sill
(14)
1186 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 TABLE I: Relative Rate Constants of Singlet Methylene Insertion at 22 5 1 OC compd
total
klnSlk8a per bond
k,,,,* cm3 molecule
MeSiF, Me2SiFz Me3SiF Me@
Insertion into the C-H Bond 0.14 0.047 0.35 0.058 0.97 0.1 1 1.30 0.11
4.6 x 10-12 1 . 1 x lo-" 3.1 X IO-" 4.2 X IO-"
Me2SiF2
Insertion into the Si-F Bond 0.055 0.028
1.8 X
s-l
Relative rates. *Based on k8 = 3.2 X IO-" cm3 molecule s-'
on k8 3.2 X lo-" cm3 molecule s-I are also given. The results given in Table I show small decreases in silane reactivity with increasing fluorine substitution on the silicon atom on a per molecule basis. These trends are less significant on a per C-H bond basis. Factors which may influence reactivity trends are the C-H bond strength and protonic nature of the H atom, both of which have been estimated' to increase with increasing fluorine substitution on the Si atom. The magnitude of the observed differences for CHI insertion into the C-H bonds would, however, be expected to be small as a result of an anticipated near zero energy of activation for this reaction. We note here that experiments on 'CH2 insertion into hydrocarbons yield rate constants (e5X 10-l2)similar to those obtained in the present work, with small increases as the C-H bond strength decreases. These results are summarized in ref 9. The observation of a product corresponding to insertion into Si-F, only in the case of Me2SiF2,is hard to explain. Presumably combination of all parameters affecting the rate of insertion into C-F or C-H allows both insertions into Me2SiF, to be observed. We do not believe that the lack of observation of insertion into Si-F in the other cases is due to experimental problems in analysis. Experiments carried out in the absence of NO results in the additional reactions of 3CH2and of particular interest are those reactions involving the formation of ethane and acetylene. The steady-state photolysis of ketene has been found to yield C2H2and C2H6in essentially equal amounts, and the same observations were made in this work. A report to the contraryi2based on laser flash experiments is not in conflict and simply reflects (10) Laufer, A. H.; Bass, A. M. J. Phys. Chem. 1974, 78, 1344. (11) Russell, R. L.; Rowland, F. S. J. Phys. Chem. 1979, 83, 2073. (12) Canosamas, C. E.; Frey, H. M.; Walsh, R. J. Chem. Soc., Faradaj Trans. 2 1984, 80. 561.
Bell et al. an alternative pathway13 for the reaction of 3CH2in the laser experiments. The steady-state photolysis mechanism for acetylene and ethane production is generally agreed to follow from the bimolecular reactions of two 3CH2radicals. In previous work, we had noticed that (CH3)2SiF2had a noticeable diminishing effect on the production of acetylene, leaving the ethane essentially unchanged. Through the use of isotopic ketene (CD2CO), the isotopic components were identified, and as a result, a methylene-methyl exchange reaction involving a five-coordinate intermediate was proposed.I4 3CH2
+
/ \
CH Si-
-
c\Hz / $ \,I,
-
/
CH SI2 \
+
CH3 (15)
CH3
In the present work, the same observation was made, namely that C2H2decreased and C2H6was sustained with all the silanes. Quantitatively each silane behaved differently as follows. The C2H2was completely suppressed above 10 torr of SiMe, or 30 torr of SiMe3F. It was considerably diminished but observable at 50 torr of SiMe2F2,while SiMeF3 diminished the C2H2but had the least inhibiting effect. In these experiments ketene was kept constant at 1.3 torr as the pressure of silane was varied. The suppression of the C2H2 can be accounted for by the competition of reaction 15 with the bimolecular reaction, which produces acetylene
-
+
3CHz + 3CH2 C2H2 (2H or H,) (16) The results with the various silanes suggest that the least competition of reaction 15 with reaction 16 occurs with the most fluorinated silane, and thus suggest that the least coordinating ability of the silicon atom occurs with the greatest degree of fluorine substitution. We speculate here that the electron-withdrawing properties of F, fluorine might be compensated for by d r - p r bonding Si which would possibly lead to a decreased availability of the Si d orbitals for forming the five-coordinate exchange intermediates. In contradiction to such a speculation, however, it must be noted that SiF, is a stronger Lewis acid than SiMe,. Steric factors may also play a role.
-
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council for financial support. Registry No. Me2SiF2,353-66-2; Me&, 75-76-3; Me3SiF, 420-56-4; MeSiF,, 373-74-0; methylene, 2465-56-7.
(13) Frey, H. M.; Walsh, R. J. Phys. Chem. 1985, 89, 2445. (14) Bell, T. N.; Soto-Garrido, G.; Sherwood, A. G. J. Phys. Chem. 1985, 89, 1155.
