Experimental and theoretical studies of proton removal from propene

Abstract

The kinetics and energetics of proton removal from propene, which contains several sites of different acidities, were investigated both theoretically and experimentally. Rate and equilibrium constants were measured for the proton-transfer reaction [Formula: see text]at 296 ± 2 K using the flowing afterglow technique. The rate constants were determined to be kforward = (1.1 ± 0.3) × 10−9 cm3 molecule−1 s−1 and kreverse = (5.4 ± 1.9) × 10−10 cm3 molecule−1 s−1. The ratio of rate constants, kf/kr = 2.1 ± 0.7, was found to be in agreement with the equilibrium constant, K = 2.2 ± 0.8, determined from equilibrium concentrations. Abinitio molecular orbital calculations predicted the removal of a methyl proton from propene to yield the allyl anion to be energetically favoured. This prediction was supported by measurements of deuteron removal from CD3CHCH2. The measured value of K corresponds to a standard free energy change, ΔG0298, of −0.44 ± 0.14 kcal mol−1 which provided values for the standard enthalpy change ΔH0298 = +0.5 ± 0.4 kcal mol−1, the proton affinity, PA298(C3H5−) = 391 ± 1 kcal mol−1, the heat of formation, ΔH0f,298(C3H5−) = 29.0 ± 0.8 kcal mol−1, and the electron affinity EA(CH2CHCH2) = 12.4 ± 1.9 kcal mol−1. The experimentally established value for the proton affinity of the allyl anion was in reasonable accord with the value of 422.3 kcal mol−1 determined by calculation. The electron affinity of the allyl radical derived in this study is supported by previous calculations and several limiting values obtained experimentally.