Kinetics of the gas‐phase reaction of acetone with iodine: Heat of formation and stabilization energy of the acetonyl radical

Abstract

Abstract The kinetics of the gas‐phase reaction CH 3 COCH 3 + I 2 ⇄ CH 3 COCH 2 I + HI have been measured spectrophotometrically in a static system over the temperature range 340–430°. The pressure of CH 3 COCH 3 was varied from 15 to 330 torr and of I 2 from 4 to 48 torr, and the initial rate of the reaction was found to be consistent with \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm COCH}_3 + {\rm I}^{\rm .} \stackrel{1}{\rightarrow}{\rm CH}_{\rm 3} {\rm COCH} + {\rm HI} $\end{document} as the rate‐determining step. An Arrhenius plot of the variation of k 1 with temperature showed considerable scatter of the points, depending on the conditioning of the reaction vessel. After allowance for surface catalysis, the best line drawn by inspection yielded the Arrhenius equation, log [ k 1 /(M −1 sec −1 )] = (11.2 ± 0.8) – (27.7 θ 2.3)/θ, where θ = 2.303 R T in kcal/mole. This activation energy yields an acetone CH bond strength of 98 kcal/mole and δ H (CH 3 COĊH 2 ) radical = −5.7 ± 2.6 kcal/mole. As the acetone bond strength is the same as the primary CH bond strength in isopropyl alcohol, there is no resonance stabilization of the acetonyl radical due to delocalization of the radical site. By contrast, the isoelectronic allyl resonance energy is 10 kcal/mole, and reasons for the difference are discussed in terms of the π‐bond energies of acetone and propene.