J. ALISTAIRKERRAND JACKG. CALVEHT
1022
The Formation and Decomposition Reactions of the Acetyl Radical and the Heat of Formation of the Acetyl Radical
by J. Alistair Kerr and Jack G. Calvert Evans Chemical Laboralory, The Ohio State University, Columbus, Ohio
4%lO
(Receized Yoeember 2, 1964)
The 3660-A. photolyses of azomethane in the presence of either carbon nionoxide or acetaldehyde have been investigated, and the forination and decoinposition reactions of the acetyl radival have been shown to be dependent on the over-all pressure in the system. The activation energies obtained for the decoiiipositiori arid formation reactions of the acetyl radival are consistent with recent results of O’Neal and Benson. A combination of the present results and recent independent estimates gives AHro(CH3C0) = - 4 i 2 k(bal./iiiole. Results on the reaction (8) CH3 CH3CH0 CH, CH3C0 have been obtained froiii the azoniethane-acetaldehyde system and together with three other deterininations yield the Arrheriius equation: k8 = 1011.5e-6.8/RT cc./mole-see. The approxi(CH3)JY2--L (CH,)&iYCH3 was derived mate rate constant for the reaction (10) CH3 from the data: klo = 1011.7e-7.1/HT cc./inole-ser.
+
-
+
+
Sluch discussion has taken place concerning the thermodynamic properties of the acetyl radical deternlined froin kinetic studies. Most recently O’Neal and Herisoil’ (0 and B) have published results from the 3130-A. photolysis of acetone in the presence of hydrogen iodide which strongly support the “low” values ( ll-ltj kcal./mole) for the activation energy of the deconipositioii of the acetyl radical, as opposed to the “high” values (17-20 kcal./niole). I n most respects the findings of 0 arid B were in accord with the results of (hlvert and Gruver2 (C arid G) who also directly measured the rate of the radical deconiposition from the azoiiiethaoe-sensitized decomposition of acetaldehyde. One major difference appeared in these studies. 0 aiid B observed a pressure dependence of the ratc ronstant for the acetyl decomposition; C arid G did not. C aiid G’s data, however, provided no extensive test of the pressure effect on the acetyl radical deconiposit 1011. The heat of forniation of the acetyl radical derived by 0 and H (-6.4 l;ral./niole) ultimately depends on the activation energy of the reverse reaction (CH3 CO + CF1,CO) as deterniined by C and G. The rate (*onstant for acctyl foriliation showed no signific.ant variatioti nith a iwofold change i n the azonicthatie prcssurc and a fivefold c.hange in the carbon niorioxide
+
Thc .Joiirna/ 01’ Physical Chemistry
pressure. The liiiiited accuracy of the C and G data, however, leaves uncertain the possible presence of a sniall pressure effect. Of course, if the findings of 0 arid B are correct, then one would expect a detectable pressure effect for both the acetyl formation arid decompositioii reactions in the same pressure arid temperature region sirice the same transitiori state for the two reactions seeins probable. Therefore, before the value of the heat of foriliation of the acetyl radical derived by 0 aiid B from their data for the deconiposition reaction and C and G data for the formation reaction can be accepted, the effect of pressure 011 the acetyl fortnation reaction requires clarification. Also the effect of pressure 011 the rate of the decomposition of the acetyl radical in the systein of C and G requires further investigation. Xew data which bear on both of these points are given here; also the first direct determination of the rate constant for r d i c a l addition to the K=X bond in azomethane has been made.
Experimental Apparatus and Materials. The photolysis system was similar to that used by C arid G which has been (1) H . E. O’Nenl and S.W.Benson. J. Chem Phys., 36, 2196 (1962). (2) J. G . Cnlvert nnd J. T Gruver. .I. Am Chem. S O C ,EO, 1313 (1958)
THEFORMATION ASD DECOMPOSITION REACTIOXS OF
ACETYL RADICAL
THE
Table I : The Effect of Pressure ( M ) on the Acetyl Formation Reaction, CH, Run no.
Temp.,
oc.
----IMezNz1
pmoles/oc,------[COI [MerCl
-------Rates RCzHs
M 12 I1 10 36
24 24 23 25
4 4 5 6
1 1 1 1
30 38 27 38
11 37 10 38
24 24 23 27
4 5 3
1 3 1 3
38 01 27 09
10 42 40 41
23 26 23 26
5 9 2 9
1 27 1 44 1 31 1 35
=
4.43 4 88 7 27 7 64
16 26 14 22
7 6 6 6
14 1 10 8 10 0 8 56
M 1 63 3 04
4 50
formation, moles/cc.-sec. X
8 2 1 7 =
=
lOlz-----
7
RI'
RA
0 0 0 0
051 093 146 286
0 0 0 0
009 027 086 234
0 !I02
42 93 36 92
0 0 0 0
093 090 146 228
0 0 0 0
027 031 086 080
36 77 92 21
0 0 0 0
146 156 138 218
0 0 0 0
086 063
1 42 2 36 3 40
2 4 1 9
R1
co 0 834
17 16 14 10
27 42 47 25
+ CO + CHICO
RU~~CO
64 43 27 4
2 4 7 12
M 0
of
1023
Me2N1 1 1 2 2
1 57 2 68 4 15
0.081 0,088 0.098 0.103
1 2 2 3
57 08 68 31
0,088 0.083 0.098 0.091
2 2 2 2
68 05 22 67
0.098 0.097 0.108 0.146
Me& 2 1 1 2
080 123
+
CH3CO CHI --+ CH3COCH3 described previously. 2, Azomethane was prepared (5) and purified as described by Renaud and L e i t ~ h . ~ 2CH3CO + (CH3CO)z (6) Other materials were obtained as beforee2 2CH3 +CzHfi (7) Product Analysis. With the azoniethane-acetaldeThis is essentially the niechanism proposed by C and G, hyde system the products were separated by lowwith the addition of reactions 3 and 4 to allow for prestemperature distillation, in the usual way, into two sure effects on the radical decomposition. Other fractions. The first fraction, noneondensable a t - 150°, possible reactions of the acetyl radicals such as those consisted of CH,, CO, XZ,and CzH6and was analyzed given below were considered by C and G, who conby gas chromatography3 on a 3.3-m. column (0.63 cm. diameter) packed with 30-60 mesh niolecular CH3 CHIC0 --+ CH, CHzCO sieves maintained a t 7.5'. Previously C and G had CHzCO 2CH3CO +CHBCHO partially separated this fraction and analyzed by mass CHIC0 (CH3)ZNz +CHsCHO CHzSzCH3 spectrometry. The technique employed here is certainly more convenient and undoubtedly more accurate. cluded that they were negligible in this system. The second fraction, consisting of (CH3)&0 plus minor Disregarding the pressure effect on the acetyl deproducts and unreacted starting nisterials, was also coniposition reaction for the moment, it can be shown analyzed by gas chromatography on a column similar that kllk7"' = (R(cH~)?co ~ R C H G O ) ~R d l to that described before. The asomethane-carbon = F1, where R, = R c ~ ~ ~ I " [ C=O ]R'/RC~H~"'[CO] monoxide system was analyzed in a similar fashion. rate of formation of X and where Rz = [ R ( C H ~ ) ~ O /
+ +
+
+
+
+
+
Discussion T h e Formation of the Acetyl Radical. When azomethane is photolyzed in a large excess of carbon monoxide, the reactions I and 1-75 are important with regard to the formation of the acetyl radical.
(CH3)zi"Tz CH3
+ hV
42CH3
-k
sz
+ CO +CH,CO*
+ CO CHsCO* + AI CH3CO + AI CHaCO + M +CHsCO* + A [ CHsCO* + CH3 4
(1) (1)
(2)
(3)
(4)
Table I lists the various terms in this expression, calculated as before,2 and the final rate constant ratios of a series of runs at about the sanie temperature but with varying pressures of carbon nionoxide, azomethane, and added neopentane.fi The ( 3 ) J . A. Kerr and J. G. Calvert, .J, A m . Chem. Soc.. 83,3391 (1961). ( 4 ) R. Renaud and L. C. Leitch, Can. J . Chem.. 32, 545 (1954) ( 5 ) M = cnrbon monoxide, azomethnne, or neopentane.
( 6 ) In calculating the v:ilues of R2, the pressure dependence of kzkr'/?,'
ks was not tnken into account.
To simplify the calculations, the experimentnlly determined value (see later), 2.9 X 103e'3.7RT (mole/ cc.-sec.)'/j, was used; since this is n correction within a correction term, the error introduced is not lnrge a t the lower temperatures. The result of mnking the correction is n sinall increase (a few per cent a t most) in the mngnitude of the observed pressure effect.
Volume 69. .\'umber
3
March 1966
1024
J. ALISTAIRKERRAND JACK G. CALVERT
rate function F1 increases with increasing carbon monoxide or neopentane concentration, but is surprisingly insensitive to changes in azoniethane pressure; compare runs 11 and 37, and 10 and 38 in Table I. This observation is in agreement with that found by C and G: it led to their erroneous conclusion that the acetyl formation reaction was in the second-order region, and hence the acetyl deconiposition reaction was assumed to be in the first-order region. The reason for the effect seen with azoniethane is not clear. It would be extremely surprising if the k3 for M = azomethane is significantly lower than that for 14 = carbon monoxide or neopentane. Possibly the result is related to the addition of acetyl radicals to the azomethane double bond which may begin to contribute measurably a t the higher azomethane pressure^.^ Toby and Weiss8 have shown that a t temperatures above 50' and pressures above 50 mm. the accepted mechanism of the photolysis of azomethane breaks down and requires an additional source of ethane. It is possible that we are observing a similar effect in this case, which might explain the lack of a pressure trend with increasing azoniethane concentration. The other results, however, which were obtained a t constant azomethane pressure and varied carbon monoxide and neopeni ane pressure clearly indicate that the rate constant of the acetyl formation reaction is pressure dependent, as one would expect from 0 and B's results of the reverse decomposition reaction. It was not possible to extend the pressure studies over a range of temperatures owing to the analytical difficulties of measuring the small amounts of products a t the extremes of the temperature range. A more quantitative test of the suggested mechanism for the pressure effect can be made for the acetyl formation reaction using data of Tablet I. A st,eady-statetreatment of the above complete mechanism leads to the following Hinshelwood-Lindernann type equation
et
7
I
I
0.5
IO
1 15 I/[M]
a IO-',
I
I
I
2.0
2.5
3.0
cc/molr
Figure 1. Hinshelwood-Lindemann plot for the formation reaction of the acetyl radical a t about 25'.
monoxide and neopentane pressures. In this treatment we have assumed that all of the gaseous molecules act with equal efficiency as AI. From the intercept and slope of this plot we obtain estimates of k7'"/k1 8.83 and k ~ k 7 ~ / ' / k l k% 3 1.72 X these values lead to k2/k3 &Z2.0 X 10+ m o l e / ~ c . ~We can cross-check this value with a similar estimate derived from the data of the reverse reaction, acetyl decomposition. If one accepts the temperature dependence of k4k2/k3 derived by 0 and R (15 kcal./niole) and E4 = 12 kcal./ mole, then extrapolation of our 65' data (see below) from the reverse reaction to 25' gives k q / k 3g 6 X mole/cc. The agreement is reasonable considering the inaccuracies in the data and the methods of their treatment. It is clear, however, that the data for the acetyl formation reaction are internally consistent and confirm the pressure effects observed for the decomposition reaction by 0 and B.
(7) In view of the rate constant for the addition of methyl radicals to azomethane derived in this work, one would expect the addition of acetyl radicals to azomethane to be relatively unimportant a t h I & l ' l R ~ ~ ~ a ) ~ o [R(CH,XO ~ R ( C H E O )25'~ ]and a t the usual pressures of azomethane used in this study. If one assumes that the rate constant for acetyl radical addition '2 MGRc~H~ k3 [M to azomethane is equal to that found here for methyl addition, the rates of addition for the conditions of runs 1 1 and 37 of Table I (at [ M ~ z N z=] 1.38 and 3.01 mmoles/cc., respectively) are 0.12 X For the data a t 25' it can be shown that the fourth 10-12 and 0.27 X 10-12 mole/cc.-sec., respectively. I t is not likely term of this expression amounts to only a few per that the rates are this large, but it is possible that the observed effect is related to the occurrence of this reaction; that is, the increased cent of the rates of acetone formation for our conditions, rate of the formation reaction with increased [MezNz] might be obso that the following approximate relation can be used scured by the increase in rate of addition of acetyl to the azomethane a t the higher concentration of azomethane k7"' k2k7"' R C ~ I I[CO] ~"~ (8) S. Toby and B. €I. Weiss, J . Phys. Chem., 6 6 , 2681 (1962). E - -+ - = F z (9) The data for the highest pressure of neopentane were omitted in [R(CHdzCO 2R(CHcO)zl - kl klkl[nI] the treatment given here as they were considered less accurate than the other data. If one includes this point, the conclusions are not In Figure 1, function F2 has been plotted us. l/[AI] for altered but the estimate of k 2 / h is raised from 2.0 X 10-6 to 3.3 x the data a t fixed azoniethane pressure and varied carbon 1 0 - 8 mole/cc.
+
+
The Journal of Physical Chemistry
+
I
THEFORMATION AXD DECOMPOSITION REACTIONS OF
THE
ACETYL RADICAL
1025
Table I1 : Temperature Coefficient for the Acetyl Formation Reaction a t Constant Pressure Run
Temp.,
DO.
OC.
53 54 55 10 50 48 46 43 47 49 45 63
-wmolea/cc-. [MezNzl 1.19 1.22 1.13 1.27 1.47 1.39 1 40 1.32 1.35 1.34 1.26
0.0 0.0 0.0 23.5 23.8 33.3 34.1 37.4 48.0 55.1 58.1 62.0
1.30
RC~H,
7.07 6.93 7.34 7.27 6.94 6.92 7.59 5.19 7.45 6.58 6.45 6.41
12.3 10.6 13.4 14.1 12.8 9.93 10.3 10.7 8.31 8.26 8.27 9.34
. RI' X 10%
Rates of formation, moles/oc.-sec. X 1019 RY~,CO Ra RAW
I
IC01
1.45 1.29 1.60 2.36 1.94 1.77 2.17 1.52 1.77 1.60 1.50 1.67
To compare the results with those of C and G, the temperature coefficient of the formation reaction was measured at nearly constant pressure. The results are listed in Table I1 and shown in an Arrhenius plot in Figure 2. The rate constants obtained here at constant pressure are about three times as high as those obtained by C and G over a range of pressures. There is no apparent explanation of this difference, but our results are to be preferred since the analytical techniques were more refined and the data more reproducible. A least-squares calculation of the results in Table I1 yields the equation
0.019 0.019 0,019 0,244 0.211 0.480 0.616 0.535 1.44 2.17 2.45 3.29
~
Ri'
0.037 0.034 0.042 0.086 0.064 0.086 0.099 0.046 0.082 0.067 0.059 0.065
RC,Hs*/'
1.54 1.38 1.70 2.78 2.28 2.39 2.99 2.15 2 37 3.90 4.07 5.07
[col
0.062 0.061 0.063 0.102 0.092 0.110 0.123 0.126 0 157 0.207 0.220 0.259
-0.6-
kl/kll" 2 81.3e-3.9/RT (cc./inole-sec.)"l
The activation energy is in good agreement with that of C and G (3.8 kcal./mole). However, these values of El do not correspond to the limiting pressure value which is required for thermochemical calculations. To make allowance for this, about 1 kcal./mole should be added to these estimates, so that El, E 5 kcal./mole. T h e Decomposition of the Acetyl Radical. The mechanism of the selective photolysis of azomethane in a mixture with acetaldehyde was studied by C and G.2 In our considerations here we will accept their niechanism again with the addition of reactions 3 and 4 to allow for pressure effects on the radical decomposition and formation reactions. If one neglects the effect
+ hv +2CH3 + Nz CHs + CH3CHO + CH, + CH&O CH3CO + AI + CH&O* + 31 (CH3)2N,
CH3CO*
+ 11+CH3CO + A I
CH3CO* +CH3 CH3CO
+ CO
(1) (8)
I
o
t
-1.6
L 1
I
I
I
b
I
I
3.2
3.0
I
3.4.
I
\ I
3.6
I / T x 105
Figure 2. Arrhenius plot of the rate function F1 = l ~ l / k 7 ' / ~ ; data were obtained from the photolysis of axomethane in the presence of carbon monoxide; open circles, this work, [MIE 8.2 ,umoles/cc.; darkened circles, data of C and G,2 [MI 2.6-16.8 umoles/cc.
(4) (3)
(2)
+ CH3 +CHsCOCH3
(5)
2CH3 +CzHe
(7).
of pressure for the moment, a rate expression ( F 3 ) involving the rate constant for the acetyl radical decomposition is readily obtained. To study the effect of /1 kzki'/l ~- - RcoRczno 1
ks
R(CHWO
= F3
Volume 69, Number 3
March 1966
1026
J. ALISTAIRKERRAND JACK G. CALVERT
Table 111: The Effect of Pressure ( M ) on the Decomposition of the Acetyl Radical Run
Temp.,
no.
"C.
[MezN21
[.4cHI
23 22 24
63.9 63.2 66.6
1.09 1.97 2.93
1.05 1.47 1.82
9.16 15.8 22.2
23 25 27
63 9 67 0
1 09 1 03 1 05
1 05 1 03 1 11
9 16 9 57 9 00
pmoles/oo.
I
1
[MerCl
Rates of formation, moles/cc.-sec. X 1012---
RCORC~H,'/?
I--
RN,
RCH~
RCO
RC~H,
RM,CO
RM,,N~HM~
1.52 2 29 3.09
4.52 7.31 8.53
0.910 1.18 1.29
4.63 6.49
hIe4C 1 52 6 79 1 35 6 01 1 43
4 52 5 20 5 23
0 910 0 598 0 483
x
106
RM~~CO
M = Me2Nzand AcH 7.08 12.6 18.9
J1
63 4
1 44 3 17
0.355 0.525
0 . TOO
=
7 08
0.355 0,513 0.677
pressure on this function a series of runs was carried out a t approximately G.5' with varying reactant pressures and added neopentane. The results are shown in Table 111. It is obvious that F3 increases with pressure, and it is likely that this reflects the effect of 11 on reactions 3 and 4. Froni a steady-state treatment of the above total reaction scheme the following relation is found where [RI] = total concentration in the
system. Thus when R ( C H ~ ) ~ C O / R C O R C ~ His~ " plotted ~ against 1/[11], as in Figure 3, there is a linear relationship with the intercept equal to k3k5/k4kzk7'" and slope equal to k6,lk4k7'". The high-pressure limiting rate constant, k4k2/k3 = k, can be obtained from the intercept and the low-pressure limiting rate constant, k4 = ka, from the slope assuming that k, = k7 = 2.2 X 1013cc./niole-sec.10 The values calculated from Figure 94 set.-' and ko 3 for data a t about 65' are k, 9.4 X lo6 cc /mole-sec. These estimates may be conipared with data of the previous workers by extrapolation of the 0 and B datal obtained a t higher temperatures and that calculated by 0 and B froin the 2.5' acetone photolysis data of Herr and Noyes,ll and Howland and Koyes,12 extrapolated to 65' using the temperature coefficients of 0 and B. The values derived from the respective previous studies are: k m = 4, 50, and 260 set.-'; k~ = 4.5 X lo6, 8.5 X lo6, and 6.2 X lo6 cc./niole-sec. The agreement of our estiniates with ihose derived froin other systems is satisfactory, considering the approximate nature of the data and the theory and the very limited range of prcssure available it1 our experinients. Certainly the pressure effect on the acetyl radical decomposition reaction as observed by 0 and B is confirmed by these results. The sensitivity of the rate constant function F a to pressure change is much greater in these data for 65' experiments than that found for the acetyl formaThe Journal of Physical Chemistry
Figure 3. Hinshelwood-Lindemann plot for the decomposition of t h e acetyl radical at about 65'.
tion function F , a t the lower temperatures. This is expected in theory since the middle of the pressure falloff region for the acetyl decomposition and formation reactions is expected to be only -7 nini. a t 2.5'. A series of runs, suniniarized in Table IV, was made a t approximately constant pressure over a range of teniperatures to check the activation energy obtained by C and G. An Arrhenius plot is shown in Figure 4, where our results are compared with those of C and G and with the results froin the 3130-,&. photolysis of acetone derived by Heicklenl3and Ausloos and Borkowski.14 Unfortunately, the results of 0 and B cannot be compared in this way. The rate constants from the different system are in general agreement but the (10) A. Shepp, J . Chem. Phys., 24, 939 (1956). (11) D. S. Herr and W. A. Noyes. Jr., J . Am. Chem. Soc., 62, 2052 (1940). (12) J. J. Howland and W. A. Noyes, Jr., ibid., 66, 974 (1944); 63, 3404 (1941). (13) J. Heicklen, I'h.D. Thesis, University of Rochester, Rochester, N. Y . , 1959. (14) Private communication from P. Ausloos, National Bureau of Standards; the dotted line represents the least-square line of many runs utilizing both CHaCOCHa and CDaCOCDa photolyses; these results are only preliminary ones which were given t o the authors before completion of the detailed study which is continuing.
THEFORMATION AND DECOMPOSITION REACTIONS OF THE ACETYL RADICAL
1027
Table IV : Temperature Coefficient for the Acetyl Decomposition Reaction a t Constant Pressure Run no.
Temp,
31 32 22 21 29 28 33
42.0 56.9 63.3 81 f i 100 8 116 7 121 4
-7.0
"C.
I 2.6
-- --RN,
_-_
___
pmoles/cc [JlezNzj [AcHI 2 1 1 2 1 1
1 1 1 1 1 1 1
05 90 97 01 96
98 2 07
30 30 47 30 40 36 42
15 15 15 16 17 17 17
I 3.0
I
2.8
Rcnr
0
7 9 12 16 20 26 26
4
8 6 1 4 6
I
3.2
Rates of formation, moles/cc -sec X 1012-RCO Rc,n6 Rxelco
0 433
38 21 6 4 7 1
1 2 5 7 11 15
1
I
3.4
I I T x IO'
Figure 4. Arrhenius plot of the rate function Fa = k2k7'"/k, obtained from the photolysis of azomethane in the presence of acetaldehyde, and compared with other results: open circles, our d a t a ; darkened circles, d a t a of C and G*; halfdarkened circles, data of HeicklenI3; these three studies were carried o u t with [MI % 3.4 pmoles/cc.; the dotted line represents an average of d a t a obtaining from a continuing s t u d y by Ausloos and Borkowski14; [MI 1.4 pmoles/cc.
07 29
00 92 6 6
10 9 8 97 7 31 5 52 4 03 3 25 3 28
-_---
R C - ~ R ~ , ~x , 105 '/~
RY~~N,HM~
RM~,CO
1 12
0 161 0 334 0 525 1 08 3 08
0 959 1 18 1 09 0 516 0 492 0 457
7 80 11 4 13 3
4 29 6 18
Thus, allowing for difference in standard states, DCHI-cO E 10.6 kcal./mole, and taking AHf"(C0) = 26.4 and AHfo(CH3) = 33.9 k~al./niole,'~we deduce AHf' (CHaCO) = -3.1 kcal./mole. 0 and B favored a lower value of AHfo(CH3)of 32.0 kcal./niole, which in conjunction with the previous value of E , derived by C and G led to the result AHfo(CH3CO) = -6.3 kcal./mole. Our value AHf"(CH&O) = -3.1 kcal./ mole is in satisfactory agreement with two other recent determinations. O'Neal and BensonI6 have also determined AHfo(C€13CO) = -6.4 kcal./mole from a kinetic study of the reactions of acetyl iodide with hydrogen iodide. Murad and Inghrani17 have found AHf"(CH3CO) = -6.5 kcal./mole from photoionization mass spectrometric studies of acetyl compounds. In view of the evidence at hand, we favor a value of AHfo(CH3CO) = -4 f 2 kcal./mole. Using this estimate, DCHCO-CH~ 79 kcal./niole. Pyrolyses of acetone by the toluene-carrier techniquela* have yielded much lower values (about 72 kcal./mole). It should be noted, however, that many of the early results by the technique have recently been shown to be inaccurate when repeated by an aniline-carrier technique. 2o Furthermore, pyrolyses of diethyl, di-npropyl, and diisopropyl ketones by the toluene-carrier technique have been found to be extremely complex processes from which DC-CO values could not be obtained. 21 A redetermination of the decomposition of acetone by the aniline-carrier technique might help to resolve this difficulty.
individual activation energies are not. The best line through all the points yields the Arrhenius equation kzk7'"/ks = 2.9 X 103e-13.5/RT(nioles/cc.-sec.)'". This is also compatible with the results of 0 and B. Under the circunistarices the Arrhenius parameters of 0 and B are to be preferred since they have made the only coniprehensive study of the pressure effect on the rate constant for the acetyl decomposition. Hence the (15) G. C. Fettis and A . F. Trotman-Dickenson, J . Chem. Soc., 3037 (1961). high-pressure limiting activation energy of the de(16) H. E. O'Neal and S. W. Benson, J . Chem. P h ~ s .37,540 . (1962). composit>ionis taken as 15 kcal./mole in subsequent (17) E. Murad and M. G. Inghram, ibid., 41,404 (1964); the authors therniochemical calculations in this paper. are grateful to Dr. Murad for a copy of the mimuscript prior to pubThe Heat of Formation of the Acetyl Radical. The lication. heat of dissociation of the acetyl radical, DCH~XO, (18) D. Clark and H. 0. I'ritchard, J . Chem. Soe., 2136 (1956). (19) M. Szwarc and J. W. Taylor, J . Chem. Phys., 2 3 , 2310 (1955). is readily calculated from the high pressure activation (20) G. Esteban, J. A. Kerr. and A. F. Trotman-Dickenson, J . Chem. energies of the acetyl decomposition reaction (15 Soc., 3873 (1963). kcal./niole, 0 and B) and the acetyl formation reaction (21) G. Esteban, J. A. Kerr, and A. F. Trotman-Dickenson, un(revised value 5 kcal./niole, estimated in this work). published results. Volume 69. ,\'umber
3
March 1966
J. ALISTAIRKERRAND JACK G. CALVERT
1028
-
1.8
1.6 -
1.4
-
1.2
-
2 .s
2.0 I/T I
Figure 6. Arrhenius plot of the rate function klo/k7'/'; data obtained from the photolysis of azomethane in the presence of acetaldehyde.
sN* 1.01
\.
- \
.n
-a
31
IO'
-
0.8
0
0.6
-
0.4
-
0.2
-
0.0-0.2
-
I
i 2.0
1 2.2
1 I 2.4 2.6 I I T x 108
I
2.8
I 3.O
I
3.2
Figure 5. Arrhenius plot of the rate function k s / k r ' / 2 ; d a t a from the photolysis of axomethane in t h e presence of acetaldehyde in this work, open circles; Ausloos and Steacie,za darkened circles; Dodd,Z4 circles with plus sign; Birrell and Trotman-Dickenson,2P half-darkened circles.
The consistency of the ratio of the A factors for the acetyl deconiposition and formation reactions with that expected theoretically from the calculated entropy change for the reaction, and the low absolute values of these A factors have been adequately discussed by previous workers.'P2 The results presented here do not alter what has already been said in this regard. T h e Reactions oj Methyl Radicals with Acetaldehyde. From the results in Tables I11 and IV, rate constants can be derived for reaction 8. Allowance must be CHs
+ CH3CHO --+ CH, + CHaCO
(8)
made for the attack of methyl radicals on azoniethane (reaction 9). From the photolysis of azoniethane CH3
+ CHsK\'=I\'CH3
---j
CH,
+ CH2N=NCH3
(9)
a1one,Z2k9/k.7'/' g 6.6 X 103e-7.3/RT (cc./m~le-sec.)~/', and hence values of ks/k7'l' were calculated from RcH,~~,/Rc,H,,'"[CH~CHO 1 where RCHd.3) equals T h e .Joozirnal of Physical Chemistry
the rate of formation of methane from reaction 8. Previous r e s ~ l t s ~and ~ -our ~ ~ values for this rate ratio are plotted in Figure 5 . The general agreement with our results lends support to the validity of the other results presented here. The best line through all the points in Figure 5 corresponds to the equation ks/k7'/a S 6.31 X 104e-6.8/RT (cc./niole-sec.)'/' and, assuming k7 = 2.2 X l O l 3 cc./niole-sec., this gives ks 3.16 X 1011e - W R T cc./mole-sec. T h e Addition of Methyl Radicals to Azomethane. In a few of the runs with the acetaldehyde-azoniethane system it was possible to determine the trimethylhydrazine product, presumably fornied in reactions 10 and 11. Triniethylhydrazine appeared as a very
+ CHJS=NCH3 + (CH3)2K--SCH3 (CH3)2N=XCH3 + CHlCHO + (CH3)J%"KHCH3 + CH&O CH3
(10) (11)
broad peak after the elution of the acetone in the analysis of the second fraction and therefore could not be measured with great precision. There were also traces of tetraniethylhydrazine which could not be measured at all.26 The rates of formation of trimethylhydrazine (see Tables 111 and IV) can be used to obtain approximate rate constants for the addition of methyl radicals to azomethane (reaction 10) on the assumption that most of the (CH3)&2 radicals react to forni triniethylhydrazine by reaction 11. The small amounts of tetramethylhydrazine produced (22) P. Ausloos and E. W. R. Steacie, Can. J . Chem., 32, 593 (1954), (23) P. Ausloos and E. W. R. Steacie, ibid.,33, 31 (1955).
(24) R. E. Dodd, ibid., 33, 699 (1955). (25) R. N. Birrell and A. F. Trotmnn-Dickenson, J . Chem. Soc., 2059 (1960).
(26) The authors are indebted to Dr. Trudy Enser Smith (Connecticut College, New London, Conn.), who supplied samples of trimethyland tetramethylhydrasine which were used as standards in this work.
THEFORMATION A N D DECOMPOSITION REACTIONS OF
THE
support this assumption; it is also consistent with the finding of Kerr and Trotinan-Dickenson,n who have shown that alkyl radicals produced by the photolysis of ail aldehyde in the presence of olefins yield predoniiriaritly alkanes from the addition of the initial radical to the olefin followed by abstraction of a hydrogen atom from the aldehyde by the ensuing radical. Values of klO/kl1'*were calculated from the expression, kl&/k7"* E R a f e 2 K z ~ ~ e / R[(CH&r\rZ], ~ z ~ ~ G I ' z and are shown in the ilrrheriius plol in Figure 6; these data give k10/k7'" E 3.6 X 104e-7.11RT(cc./iiiole-sec.)'/'. Taking k, = 2 . 2 x 1013cc./i1101e-sec.,this yields kl0 1.8 X 10" e--,. l / R T cc./inole-see. The activation energy here deterniined directly for reaction 10, Elo % 7.1 kcal./ niole, checks reasonably well with that estimated by
ACETYLRADICAL
1029
Jones and Steacie2s using a mass balance technique without direct analysis; they estimate E,, S 6.4 kcal./moJe. Apparently methyl radicals add to the N=N bond in azoniethane at, about the same rate as they add to a C=C bond in olefins.29 Acknowledgment. The authors are grateful for the support given to this work by a grant from the Division of Air Pollution, Bureau of State Services, U. S. Public Health Service. (27) J. A. Kerr and A. F. Trotman-Dickenson, Trans. Faraday SOC., 55, 572 (1960). (28) M. H. Jones and E. W. R. Steacie, J . Chem. Phys., 21, 1018 (1953). (29) J. A. ,Kerr and A. F. Trotmsn-Dickenson, Progr. Ebeactkm Kinetics, 119 (1961).
Volume 63,Sumber S Narch 1365
