Dissociation of 3-hydroxybut-1-ene and the resonance energy of the hydroxyallyl radical

Abstract

The pyrolysis of 3-hydroxybut-1-ene has been investigated at temperatures over the range 773–834 K and pressures between 5 and 100 Torr. Under these conditions, methane, butadiene and water are primary reaction products, the latter two being formed at similar initial rates. From this evidence and by analogy with the modes of dissociation of 3-chlorobut-1-ene, the decomposition of the hydroxybutene is concluded to involve the two distinct primary steps CH₃CH(OH)CH₂→C₄H₆+ H₂O (1), and CH₃CH(OH)CH₂→CH₃·+·CH(OH)CHCH₂. (2) Under the experimental conditions both reactions (1) and (2) were found to be essentially homogeneous and first order. Measurements of the initial rates of formation of water yielded the rate expression log(k₁/s^–1)=(12.9 ± 0.5)–(55 700 ± 1 600)/θ and assuming that methyl radicals add to and abstract from 3-hydroxybut-1-ene at similar rates, measurements of the initial rates of formation of methane yielded the rate expression log(k₂/s^–1)=(16.26 ± 0.50)–(69 200 ± 600)/θ where θ= 2.303 RT/cal mol^–1(1 cal = 4.18 J).

The latter activation energy yields a value of 80.1 kcal mol^–1 for D(H—CH(OH)CH:CH₂) and assuming D(H—CH(OH)C₂H₅)= 92.5 ± kcal mol^–1, an estimate of 12.4 ± 1.4 kcal mol^–1 for the resonance energy of the hydroxyallyl radical. This value is identical to that obtained previously for the allyl radical itself and agrees within the experimental limits with those found for the chloroallyl, methylallyl and dimethylallyl radicals. On this evidence it is concluded that the resonance energy of the allyl radical and of substituted allyl radicals can be taken as 12.6 kcal mol^–1 and that the effects of substituents on this value are less than the probable error limits of ±(1.4–2.0) kcal mol^–1.