The relative reactivities in the gas-phase thermolysis of eight allyl alkyl ethers possessing CY hydrogen on the alkyl moiety are separated at most by a rate factor of 4 and are very similar in all other respects, falling largely within a narrow activation energy range of less than 1 kcal/mol. Such insensitivity of rate to extensive variation in substitution can be construed as indicative of a cyclic, concerted transition state. The small effects of substitution on the activation parameters of reaction can be readily explained in terms of a pericyclic, retro-ene structure of the transition state in the general reaction. Thermolysis of the analogous benzyl propargyl ether shows a very similar reaction pattern. However, thermolysis of tert-butyl allyl ether, where an CY hydrogen on the alkyl moiety is not present, results in a reaction with distinctively different activation parameters. The fragmentation reaction products here, which were simple and unique with the other alkyl allyl ethers, are more numerous and complex, the predominant component representing methyl migration in a six-centered transition state. The reaction of the substrates in the liquid phase is little different from the gas phase; the small decreases in activation entropy are not unexpected for a concerted process. The striking increase in the activation entropy of the liquid-phase decomposition of tert-butyl allyl ether, on the contrary, is interpreted as a manifestation of the free-radical character of this reaction. The total absence of a substituent effect on the rates of thermolysis in a series of para (polar) substituted benzyl allyl ethers, in contradiction to an earlier report by Cookson and Wallis, is found to be consonant with the proposed structure of the concerted transition state.he uncatalyzed thermal decomposition of ethers has T not received much attention in the past. Reviews of thermolytic mechanisms by Steacie and by DePuy and King2 reflect the general scarcity of information on this general reaction up to 1960. The succeeding decade has witnessed a greatly accelerated pace of work in this area as indicated by the large amount of data cited in both published3r4 and unpublished5 reviews.Numerous contributions by Stavely and Hinshelwood,6-8 Magram and T a y l~r ,~ Lossing and Ingold, l o,l l Elkobaisi and Hickenbottom, 12, l 3 and Freidlin, Balandin, and Nazarova14 have established the freeradical character of the thermal cleavages taking place in dialkyl, diaryl, and alkyl aryl ethers. Even cyclic
Gas-Phase Thermolysis Kinetics 2817 1.6 X 103 sec-1 calculated from the apparent rate constant for the overall decomposition of methane reported herein.The relationships between the two polyatomic-diatomic exchange systems, CH4 + D2 and CD4 + HC1, and the polyatomic-polyatomic exchange system CD4 + CH4 are worth noting. The reported activation energies for exchange covering the 1600-1750°K range are 52 (CH* + D2), 51 (CD* + HC1), and 70-(CD4 + CH4) kcal mol-1. Although the mixed methane exchange has a somewhat higher activation energy, all temperature coefficients are much lower than the almost equal bond dissociation energies of reactants; namely, 104(CH3-H), 102(H-C1), and 104-(D-D) kcal mol-1. These similarities and the postulation of atomic mechanisms for two of these exchange studies indicate that the molecular complexity of the exchanging partners is not the determining factor with respect to the mechanistic pathway in contrast to the earlier suggestion of BL.6 An investigation of the CD* + CH* and CH* + D2 exchange systems is currently underway in this laboratory with particular attention focussed on purification procedures, impurity levels, and determination of product time dependence.In summary it can be stated that there are other diagnostic features that are more definite than an interpretation of the activation energy. In the CD*-HC1 system, the changing time dependence from quadratic to linear for product formation, the changing CHS: CH* ratio during extensive decomposition, and the presence of pyrolytic products whose formation most certainly involves radicals are facts which support an atomic mechanism for the exchange.A cknowledgments. The authors gratefully recognize the assistance of Mr. D. Olivier and Mr. T. Dupuy in the collection of data. Mr. Dupuy and Mrs. . E. Bopp are thanked for their role in the tedious task of data reduction. The authors also thank the referees for several helpful suggestions.
The operational parameters of an improved flow method for direct determination of the kinetics of gas-phase thermolysis reactions have been evaluated. A scheme has been devised for rapidly estimating the rates of first-order decompositions in a single series of simple measurements at an established reactor temperature. The activation parameters for decomposition of ethyl acetate and dicyclopentadiene have been determined in this fashion and compared with the corresponding values reported by earlier authors applying a variety of kinetic techniques. The use of a gold-surfaced reactor and a highly diluted reaction medium has simplified (or eliminated) the problem of residual wall reaction.
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