The activation volume for thermal decomposition of tert-butyl perpivalate in cumene ranges from ca. $0.3 to + 1.9 ml/mol over a 5000 atm pressure range, while that for tert-butyl perisobutyrate in the same solvent changes from cu. +1.6 to +3.2 ml/mol over the same range. It is proposed that these results indicate that twobond aliphatic perester thermolyses are characterized by polar contributions in their decomposition transition states. olar effects in free-radical reactions are a subject of continuing interest and debate. They have been proposed to account for rate correlations with the Hammett equation, particular selectivity patterns in competitive hydrogen abstractions, and solvent effects, among others. Classic examples have included the effect of substituents on hydrogen abstraction from substituted toluenes3-j and on the decomposition rates of tert-butyl phenylperacetates.6-8
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