RATEPROCESSES IN THE ACETONE-SILANE PHOTOCHEMICAL SYSTEM
3945
Primary and Secondary Rate Processes in the Acetone-Silane Photochemical System
by H. Edward O'Neal,* Spyros Pavlou, Tom Lubin, Morey A. Ring, and L. Batt Department of Chemistry, S u n Diego State College, S a n Diego, California 98116
(Received December 17, 1970)
Publication costs borne completely bg The Journal of Physical Chemistry
The photolysis (3130 1)of acetone in the presence of silane has been investigated at 25, 65, and 100'. Rate constant parameters for the H-abstraction reactions from silane by the methyl and the 2-hydroxy-2-propyl radical were obtained: log lel (l./mol-sec) = 8.24 - (6.17 =k 0.37)/8 and log lee (l./mol-sec) = 8.75 - (9.0 1)/0, respectively. Spontaneous decomposition quantum yields were also measured and found to be consistent with the primary process mechanism proposed earlier. Mechanisms for the formations of isopropyl alcohol and tert-butyl alcohol, two unusual reaction products, are proposed, and some values for the various reaction processes involved are deduced.
Introduction Primary processes in the acetone photochemical system have been fairly well elucidated.'J One of the most interesting conclusions of the prior studies was that the acetone triplet could behave much like a free radical. Thus the acetone triplet was observed to undergo H-abstraction reactions from hydrogen bromide with a rate comparable to that of the corresponding methyl radical reaction. The purpose of the present study mas to investigate the radical trapping abilities of silane toward the acetone triplet and also to obtain the H-abstraction Arrhenius parameters for the reaction of methyl radicals with silane.
Experimental Section Silane was prepared by the reduction of tetrachlorosilane with LiAlH4.3 Separation was achieved from the reaction mixture by passing the products through a - 160' trap (which retained unreacted tetrachlorosilane) and into a silica gel trap cooled to - 195". Of the two principal contaminants, HC1 was irreversibly absorbed on the silica gel and hydrogen was removed by pumping at liquid Nz temperatures. The silane recovered on warming was found to be chromatographically pure to within 0.05 mol yo. Experimental procedures employed in the photolysis were, with few exceptions, the same as those described e l s e ~ h e r e . ~The light source was an Osram 200-W super high-pressure Hg lamp. Radiation, passed through a nickel-cobalt sulfate and a potassiu? biphthalate filter solution, was centered in the 3130-A region. Transmitted light intensities were monitored by an RCA-931 phototube whose output was suitably recorded. Analyses were made with a Perkin-Elmer Model 154D vapor phase chromatograph with both thermal and flame ionization detectors. Methane and ethane were analyzed on a PE-J (silica gel) column, and all other products were
analyzed on both a PE-R(Ucon oil LB-550-X) column and a PE-t (P,P-oxydipropionitrile) column. Analytical accuracies were estimated between 5 and 10%.
Results Major products of the photolysis were methane, ethane, carbon monoxide, isopropyl alcohol, and tertbutyl alcohol. Minor products mere hydrogen, disilane, methyl ethyl ketone, biacetyl, acetaldehyde, and ketene. Data are shown in Table I. Reactions 1-12, listed in Table 11, seem to account for the most important features of the photolysis system. Methyl H Abstraction from Silane. Silane is a relatively effective trap for methyl radicals, as evidenced by the marked decrease in ethane yields with increasing silane concentration. If one assumes that methyl radical concentrations are sufficiently lorn to minimize methane yields from radical-radical disproportionation reactions and that methane was produced only via methyl H abstraction from silane and acetone, one obtains from the relative rates of methane and ethane formation (eq 1, 2, 3) the usual expression
where Y(X) is the yield of X in moles per liter. Plots of the data according to eq I were made and the rate constants for H abstraction of methyl from silane were calculated. The data give ki(25') = 3.76
* 0.21 X
10'
and (1) C. W. Larson and H. E. O'Neal, J . Phys. Chem., 70,2475 (1966). (2) H. E. O'Neal and C. W. Larson, ibid., 73, 1011 (1969). (3) A. C. Finholt, A. C. Bond, Jr., K. E. Welyback, and H. I. Schlesinger, J.Amer. Chem. Soc., 69,2692 (1947).
The Journal of Physical Chemistry, Vol. 76, No. $6,1971
H. O'NEAL,S.PAVLOU, T. LUBIN,PI,RING,AND L. BATT
3946
Table I : Data on Photolynin of Acetone in the Presence of Silane Reactants (mol/l.) X 104 CHsCOCHa
r---------
(Siiia/.Ada
SiH4
C Ha
Yields (mol/l,) - - - - -X6 U l CzHs
7
co
teTt-BuOH
i-PrOH
12
1.91 1.47 4.40 3.10
0.37 0.37 0.91 0.44 0.36 1.18 0.27 0.20
0.35 0.84 2.02 0.59 2.34 2.06 8.05 9-05
42 35.2 79 27.6 22.2 93 24.6 27.2
t
x
10-4c
T = 25" 6.80 5.30 9.20 8.87 6.50 33.7 13.8 18.2
4.4 10.2 10.2 10.2 20.3 20.0 43.4 76.5
0.65 1.92 1.11 1.15 3.12 0.59 2.88 4.20
a7.6 22.3 32.3
12.8 18.4 11.6
0.46 0.82 0.36
3.45 8.42 11.9 4.63 8.00 5.14 11.o 6.05
2.13 5.50 9.27 1.15 1.36 0.95 0.48 0.059
T 101 88.4 90.5
a
2.79 7.40 28.2 7.18 10.4 8.50 0.30
0.066 0.204 3.58 0.69 0.54 0.34 0.005
lcl(iooo) = 34.3 for kl in units of l./mol-sec. give
f
0.32
x
=
45.7 40.3 0.603 56.2 9.15 38.6 39.8
103
In Arrhenius form, thcse
(log kl (l./mol-sec)
=
8.8 - (6.99
* 0.56)/0
8.82 - 6.89 rf 0.16)/8
(SJSG) (MT)
In the latter studies methyl radicals were generated using the photolysis of azomethane. The systems mere apparently cleaner and better behaved, and in view of the good agreement obtained, their results should be considered the more reliable. One of the inconsistencies in our data, which may have had some effect on our rate constant determinations of kl, is that appreciably more than stoichiometric amounts of silane were reacted. The measured amount of silane in the system after reaction was always less than the amount expected on the basis of initial concentrations and the The Journal of Physical Chemistry, Vol. 76, N o . 36, 1971
35.8 29.2 28.6
269 297 308
1.77 1.20 1.29
68.8 128 30.6 194 45.1 82.5 48.2
32.5 47.3 1.04 27.1 7.00 23.0 3.24
22.4 38.6 28.0 44.5 18.6 29.6 2.7
352 278 59 238 144 248 186
1.11 1.54 1.79 1-85 0.73 1.08 1.04
100'
Units are (einsteins/l.-see)
The rate constant errors, which represent 95% confidence limits (i.e., & 2u) , are rather large. Good mass balances were not obtained in this photolysis system, and these errors undoubtedly reflect the appreciable experimental scatter in the data. Nevertheless, the Arrhenius parameters for the H abstraction by methyl from silane are in reasonable agreement (roughly within the limits of error) with those obtained in prior studies4t5 =
13.7 10.0 20.2
21.5
log lcl(l./mol-sec) = 8.24 - (6.17 h 0.37)/8
log k~ (]./mol-sec)
69.6 53.4 77.6
7.82
59.5 150 64.0 226 63.4 131 23.2
Average silane to acetone ratios in a run (see text).
65"
15.3
T 31.2 34.6 7.38 13.9 17.2 25.4 37.4
=
a , ,
None 7.15 3.16
2.50 1.so 1.80 1.59 1.42 1.08 2.08 1.60
x
1010. ' Units are sec.
extent of photolysis. Since an appreciable amount of hydrogen and disilane were also produced in some runs, it is possible that some silyl radical induced decomposition of silane was occurring (eq 11). This process has been proposed by Purnell and Walsh6and also by Ring, Puentes, and O'Nea17 to explain the thermal decomposition kinetics of silane. SiHa f SiH4 --t H f Si& H
+ SiH4
-3 SiHa
+ HS
fII1 \
I
P r i m a r y Decomposition Q u a n t u m Yields. Decomposition quantum yields, calculated from the equation =
Y [CH4f 2CzHe
+ tert-BuOH + R'LEK-CO]/I,.t
were found to be much lower than those of acetone wit)hout inhibitors. They were independent of the silane concentration and strongly temperature dependent. Average values at the three temperatures were pod N 0.10 (25'), p d _N 0.22 (65"), and 'pd N 0.37 (100') (see Table 111). These observations are consistent with equations d*, S, and T where spontaneous decomposi(4) 0. P. Strausz, E. Jakubowski, H. S. Sandhu, and H. E. Gunning, J . Chem. Phys., 51,552 (1969). (6) E. R. Morris and J. C. J. Thynne, J . Phys. Chem., 73, 3394 (1969).
(6) J. H. Purnell and R. Walsh, Proc. Roy. Soc., Ser. A , 293, 643 (1966).
(7) M. A . Ring, M. J. Puentes, and H. E. O'Neal, J . Amer. Chem. SOC., 92,4845 (1970).
3947
RATEPROCESSES IN THE ACETONE-SILANE PHOTOCHEMICAL SYSTEM
Table I1 Rate constantsa
Primary processes CHaCOCHsJA) hv (3130 A) +'A*
+
to be quantitatively consistent with the variations of t8ripletstate phosphorescence caused by total pressure, temperature, and wavelength changes.2 ~~
Iab
~~~
Table I11 : Spontaneous Decomposition Quantum Yields
Decompositions 3A*
d",CHa. + CHaCO
Stabilization
3A*
+ M 8 _ aA + iLI
Secondary processes CH3. SiH4 CH4
+
+
2.M
+ SiHa.
ki
'v
1011.M
- 6,33'0c
=
2
CH3. A+ CH4 .CHzCOCHa 3 CH3. CH3. +CzHs CH3CO R . CHzCO CH&O f (31)
+ + +
k 2 -- 108.6 - 8.7/8 d k3 = 1 0 1 0 . 3 4 e
+ RH
+ co + nt
k4
=
T
+
+
3kT
> - 108 - w
kg = 1 0 8 . 7 7
-
eo
8.0/0 c
tert-Butyl alcohol formation 7
- + *A e '(terl-BuO. *)
CH3
-7
+
e
+ + +
+
+
IC1
2 10'2.2C
8
k-, 2 1 0 8 . 7 ~ ks (sec-1) CT 2 - M
+
les 2
'(tert-BuO * j M ---f '(Lerl-BuO.) M 9 '(tert-BuO.) ----f A* CHa. 10 SiH4 l(tert-Bu0.) + tert-BuOH R.AOH A RH 12 Re -I%+ products
+
tpd*
Z*M
Mb
Z
fd
A + SiHa
0.10 0.22 0.37 0.08 0.18 0.26
0.11 0.28 0.58 0.087 0.22 0.33
5.5 7.4 6.7 6.6 6.8 7.5
0.61 2.07 3.88 0.57 1.50 2.48
4.23 6.36 6.35 1.97
A A
+ SiHc + SiH4
A + HBra A + HBr A + HBr
... 2.35
'
M is the average total pressure for all runs at a See ref 1. each of the temperatures indicated. Units are cm. The silane collision efficiency relative to acetone was assumed to be 0.33. "Units are cm. Units of the extinction coefficient ( e ) are 1./mol-cm.
=lollf
Isopropyl alcohol formation SiHa 3A ---f CH,C(OH)CH,( sAOH) SiH4 .AOH CH3CHOHCH3 SiH3.
25 65 100 44 96 126
~
System
C
ks8
CH~.
+. +
k(
