Chemically activated acetyl radicals, with an excitation energy of 78 kJ/mole, were formed by the addition of methyl radicals to carbon monoxide. At 273°K the pressure required to stabilize one half of the excited radicals was 500 torr. From measurements of the acetyl radical yield at pressures in the range of 700-2100 torr, and at temperatures in the range of 26&413 K, extrapolations to infinite pressure yielded kinetic parameters for the addition of methyl radicals to carbon monoxide, and for the thermal decomposition of acetyl radicals. The rate constants were found to be log k[cm3/(mole .s)] = 11.2-25(kJ/mole/2.3 R T , and log k(s-') = 13.5-72 (kJ/mole)/2.3 R T , respectively. Estimated thermochemical properties of the acetyl radical are A H~O = -17 kJ/mole and So = 262 J/K .mole.

A new interpretation of the mechanism used by Endrenyi and LeRoy for the calculation of the rate constant for 11-pcntyl isomeriza~ion is proposed. The addition of onc step to thc scheme makes possible a recalculation of the Arrhenius parameters. Significantly different values are found which eliminate the previously reported low A-factor. Canod~an Joulxal of Chcrnis[l.), SO, 3738 (1972) A discrepancy has long been recognized (1) pose into ethylene and ethyl radicals at about between the r e~o r t e d A-factors and those esti-157". mated from reasonable entropies of activation for the thermal isomerization of/?-pentyl(2) and 17-hexyl (3) radicals.The reported A-factors were about 10' s-', whereas the theoretical A-factors for 17-pentyl and 17-hexyl isomerization were estimated to be 1010.5-11 o s -l (lb, lc) and 101Os-l (4) respectively. The purpose of this communication is to suggest a resolution of this dilemma for 17-pentyl radical isomerization.In the case of isomerization of 17-pentyl radicals by 1,4-hydrogen atom migration. a reinterpretation of the original data of Endrznyi and LeRoy (2) suggests a suitable solution. These workers carried out their kinetic runs at temperatures between 165 and 230'. They assumed that the only reactions of .set-pentyl were isomerization back to n-pentyl, and combination with methyl. In other reports of 17-pentyl radical isomerization, both Gordon and McNesby (5) and Wijnen (6) observed that ,rec-pentyl radicals formed from 17-pentyl isomerization at 300' decomposed. Also, Morganroth and Calvert (7) in a study of the photolysis of 17-azobutane observed that il-butyl radicals began to decoinThis work proposes that in the calculations of Endrenyi and LeRoy a significant fraction of .set-pentyl radicals was not accounted for because they decomposed by reaction 3. A propene balance on their data (8) shows this suggestion to be a reasonable one. The three most important propene forming reactions (excluding 3) are Other reactions yielding C,H, can be imagined but their contribution is relatively unimportant. The rate of formation of propene fi-om these reactions can be estimated by the following expression where known o r estimated disproportionation to combination ratios can be used.R,(C,Hl,) is the rate of formation of 17-C,H14 from propyl radical combination. It is estimated from expression (iv) (8), kdj = A(n-Pr, n-Pr) = 0.15 (9); ki was estimated to be 0.04, from Can. J. Chem. Downloaded from www.nrcresearchpress.com by 54.245.13.81 on 05/11/18For personal use only.

Azoethane was irradiated in the presence of carbon monoxide in the temperature range of 238 to 378 K. Kinetic parameters for the addition of ethyl radicals to carbon monoxide and for the decomposition of propionyl radicals were determined. The rate constants were found to be log k(cm3 mol-' sec-l) = 11.19 -4.8/8 and log k(sec-') = 12.77 -I4.4/8, respectively. Estimated thermochemical properties of the propionyl radical are AHf" = -10.6 f 1 .O kcal mol-', So = 77.3 f 1 .O cal K-' mol-I, and D(C,H,CO-H) = 87.4 kcal mol-I.

No abstract

occur t o the 3(~,7r*) state with resulting cis-trans isomerization. It is therefore not surprising that the intersystem crossing rate constant for AIP is greater than the corresponding constants for DBO and DBH, being in excess of 1O' O sec-I.' For bicyclic azo compounds, the lower intersystem rate constants may be understood in terms of a crossing to a (n,p*) rather than a (T,T*) manifold3* and less marked differences between the equilibrium geometries of the states, resulting in less favorable Franck-Condon factors. 35 In this previous work we have used sensitization extensively as a source of triplet molecules. lr7 In Table I the interaction rate constante (kir) of a variety of substrates, including azo compounds, with several triplet (38) M. A. El-Sayed, Accounrs Chem. Res., 1, 8 (1968).Abstract: The vapor-phase photolysis of n-pentylazomethane was studied in the temperature range 24-162 ' . The product rate data fit well the proposed reaction scheme involving methyl and n-pentyl radicals. Evidence for the generation of photochemically activated n-pentyl radicals was found. The implications of this evidence to previous work on the photolysis of azoalkanes are discussed. When the concentration of photochemically activated radicals was minimized, the thermal rate constants for the isomerization andH-atom abstraction reactionsof the n-pentyl radical were determined: n-C5H11 + sec-C5H11 (17), k17 = 3.3 X lo8 exp(-l5.1 X 103/RT) sec-'; n-C5H11 + n- (20), kzo = 4.2 X 10" exp(-7.8 X lO3/RT) mol-' cm3 sec-I. The disproportionation to combination ratio at 25 O for n-pentyl radicals was estimated to be 0.14, and A(Me, n-Pe) = 0.033. C~H I~N~C H~here is almost no quantitative information in the W (10) (a) R.