1544
JACK
G. CALVERT AND WILLIAM C. SLEPPY
should again give more information than the average. The helical content and the orientation of the helices in the molecule should be determinable. Acknowledgments.-We wish to thank Dr. E. R. Blout for the gift of the PBLG which made
[CONTRInUTIONF R O M
this study possible. We also are indebted to Prof. M. Calvin for the use of his laboratory and polarimeter. The help of Prof. C. T. O’Konski and Mr. W. H. Orttung, who made their results available prior to publication, is gratefully acknowledged. BERKELEY 4,
CALIF.
MCPHERSON CHEMICAL LABORATORY, THEO H I O
THE
VOl. 81
STATE
UNIVERSITY, COLUMBUS]
A Kinetic Study of the +Propyl Radical Decomposition Reaction BY JACK G. CALVERT AND WILLIAM C. SLEPPY RECEIVED OCTOBER 8, 1958 Thermally equilibrated n-propyl radicals were generated homogeneously through the selective photolysis of azomethane a t 3660 A. in n-butyraldehyde-azoniethanemixtures. The decomposition reaction, n-CaH7 --t CHI CsH, ( l ) , was followed by measurement of the rate of ethylene formation. The steady-state concentration of n-propyl radicals was gauged indirectly by the determination of the rates of butane formation, CHI n-CsH,+ n-C4Hlo (5), and ethane formation, 2CHa -c CZHI(6). From the temperature dependence of the rate function, .Rc~H,~c,E,’/’/R~-c,H~~ = klk&/*/ks and the assumption k6 = &6 = 2.2 X 10” cc./rnole-sec., it is estimated that kl E 2.85 X 10’6e-84**’zT set.-'. From these data and Brinton’s 15.4 e.u. (near 200’). These and accurate thermal data suggest estimate of k-I, we derive: AH1 Z 26.2 kcal./mole; ASlo D C E ~ ED~-c,H,-HS 6.8 kcal./mole; taking D c a I - ~= 102.0 kcal./mole, D h - c I ~ TS - ~ 95.2 kcal./mole. The agreement is good between the present kinetic estimates of the thermodynamic quantities related to reaction 1 and those estimated from “reasonable” thermal data. It is probable that the previous kinetic estimates of E S 19-21 kcal./mole are seriously iii error.
+
+
-
The great divergence between the published kinetic data and “reasonable” thermal data for reaction 1 n-CaH7 +CHa
+ C2H4
(1)
is alarming. There are three independent determinations from photochemical and thermal kinetic experiments which suggest El 19-21 kcal./ From measurements of the rate of methyl radical addition to ethylene, the reverse of reaction (l), Mandelcorn and Steacie‘ derive G I 7 f 1.5 kcal./mole. Recently a more detailed study of this reaction by Brintonb has confirmed the “high” value for G1; he derives El E 8.7 kcal./mole. Thus current kinetic data suggest AH1 e E1 E-1 10-14 kcal./mole. Contrast this estimate with that based on “reasonable” thermal data, AH1 23-26 kcal./mole.6,’ The recent “high” values for 6 1 appear to be well substantiated, but the accuracy of the kinetic estimates of E1 should be questioned, as Mandelcorn and Steacie suggest.‘ Indeed the estimates of E1 seem to be low in view of the recent findings of TrotmanDickenson and The preliminary results of a study of the full arc, high temperature photolysis of n-butyraldehyde give: E1 25 kcal./ mole. Recently a re-evaluation of the earlier work relative to reaction 1 has been made,9 and several possible sources of error have been noted. (1) R. W. Durham, G. R. Martin and H. C. Sutton. Nolure, 164,
The present study was designed to avoid many of the possible complications often encountered in the photochemical studies of radical decomposition reactions. Thermally equilibrated n-propyl radicals were generated homogeneously; the thermal decomposition of n-butyraldehyde was sensitized by the selective photolysis of azomethane a t 3660 A. in n-butyraldehyde-azomethane mixtures. It will be seen that the data from this system remove the apparent conflict between kinetic and thermal estimates of the thermodynamic quantities related to reaction 1. Experimental
Apparatus.-The photolysis system was similar to that described previously.1° One significant change was made to obtain a higher light intensity and higher radical concentrations; these conditions favor the desired radical association products, ethane and butane, from which a measure of the n-propyl radical concentration may be had. An additional light source (Hanovia Type A, S-500, burner) and 3660 A. filter system was placed on the optical path, outside the air thermostat, a t the rear of the photolysis cell. In the runs at the highest temperatures both the arcs a t the front and the rear of the cell were operated a t maximum intensity. Under these conditions the rates of ethane and butane formation were sufficiently raised to make analysis for these products feasible, even though the products of the chain reactions, CO, CaHs, CHa and C2H4, were still dominant. Materials.-Azomethane was prepared and purified as described by Renaud and Leitch.” n-Butyraldehyde was taken from a volatile fraction of the Eastman White Label product and further purified by bulb-to-bulb distillation a t reduced pressure. Standard reference samples of the hy1062 (1949). drocarbon gases were Phillips research grade. (2) S. Bywater and E. W. R. Steacie, J . Chcm. Phys., 19,319 (1951). Product Analysis.-By combined mas spectrometric (3) C. R. Masson, T a m JOURNAL,74, 4731 (1952). and chromatographic analyses, identification of a number (4) L. Mandelcorn and E. W. R. Steacie, Can. J . Chem., 82, 474 of products was made: CH,, Nz, CO, CzH,, CzHe, .C!Hs, (1964). CsHe, n-CdHlo and n-C&14. The products were divided (5) R. K.Brinton, J . Chem. Phys., in press. into three fractions for convenience in analysis. The first ( 6 ) E. W. R. Steacie, “Atomic and Free Radical Reactions,” Vol. 11, fraction, Nz, CO and CHa, was pumped.off. with a Toepler Reinhold Publ. Corp., 1954,p. 584. pump while the trap was maintained at liquid nitrogen tem(7) W. M. D. Bryant, J . Polymer Scl., 6 , 359 (1951). perature. This fraction was not analyzed in most of the (8) A. F. Trotman-Dickenaon and J. A. Kerr, University of Edinruns, since these products offered no unique data which burgh, prlvate communication. would justify the time expenditure required for the analyses.
(9) J. 0. Calvert, “Symposium on Elementary Processes in Chemical Kinetics,” Dlvisian of Physical Chemistry, 134th National Meeting of American Chemical Society, Chicago, Sept., 1958.
(10) J. G. CalverL and J. T.Gruver, THIS JOURNAL, 80, 1813 (1958). (11) R.Renaud and L. C. Leitch. Can. J . Chcm., 82, 545 (1954).
1545
KINETICSOF R-PKOPYL RADICAL DBCOMPOSITION REACTION
April 5, 1959 DATAFROM
THE
TABLE I PHOTOLYSIS OF AZOMETHANE-BUTYRALDEHYDE MIXTURES AT 3660 A.
Run no.
Temp., OC.
-Pressure, MeNx
1 2 3 4 5 6 7 8 9 10 11
198.0 207.6 217.3 223.8 224.8 237.1 241.3 248.4 262.0 266.4 276.6
53.2 53.7 62.6 68.0 70.4 62.0 66.2 64.4 62.0 78.5 69.3
GHJO
,---Rates, CSH8
31.2 41.1 35.3 33.3 30.4 36.0 29.0 37.7 31.3 37.8 30.3
1.35 4.00 2.86 7.66 6.74 8.44 16.7 16.4 42.6 61.3 82.2
mm.--
moles/cc.-sec. X IO1-
CIH~
u-GHI~
0.819 0.876 1.77 2.23 2.83 7.82 10.5 26.6 52.1 106 367
5.93 7.27 6.42 8.86 9.43 8.77 10.9 24.7 33.8 52.0 140
R&~::",RC~II~ x 10' Rc4Rta
0.508 0.763 1.48 2.20 2.46 8.19 12.4 13.8 31.8 50.5 75.1
The second fraction, primarily CIHEand CtH4, was pumped It is highly probable that reaction 1 is the source from a modified Ward still which was regulated automaticof ethylene in this system. Ethylene is undetectB C~HE ally t o -155 f 2". Very small amounts of C ~ Hand were removed also in this fractionation procedure, and suit- able in the products of low temperature (
