Gas-phase thermal unimolecular isomerizations of acetylcyclopropane. Part II. Determination of the rate constants

Abstract

The gas-phase thermal reactions of acetylcyclopropane and 2,3-dihydro-5-methylfuran have been studied in the temperature range 672–731 K. Acetylcyclopropane isomerizes by four parallel routes to yield (1) 2,3-dihydro-5-methylfuran, (2)cis-pent-3-en-2-one, (3)trans-pent-3-2-one, and (4) pent-4-en-2-one. Reaction (1) is reversible and the other reactions are essentially irreversible. The sole observable reaction of 2,3-dihydro-5-methylfuran is a reversible isomerization to acetylcyclopropane [reaction (-1)]. The experimentally determined Arrhenius relationships for the first-order rate constants are: log k₁/s^–1= 13·89 ± 0·22 –(231·2 ± 2·9 kJ mol^–1)/θ, log k_–1/s^–1= 14·85 ± 0·22 –(241·0 ± 2·9 kJ mol^–1)/θ, and log (k₂+k₃+k₄)/s^–1= 14·4 –(243·5 kJ mol^–1)/θ, where θ= 2·303 RT. These parameters can be rationalized by a diradical mechanism in which a stabilization energy of ca. 30 kJ mol^–1(7 kcal mol^–1) in the alkylacetonyl radical is operative.