Thermolytic reactions of esters. Part 12. Steric versus polar effects in pyrolytic β-elimination of acetic acid from (tertiary) alkyl acetates

Abstract

The vapour-phase pyrolysis of the crowded tertiary alkyl acetates AcOCMe₂Bu^t(II) and AcOCMePrⁱ₂(III) to give acetic acid and the appropriate alkene(s) has been studied. Rates of formation of alk-1-ene and of isobutene from t-butyl acetate were compared. On this basis steric acceleration in pyrolysis of AcOR, employing known steric parameters E′_s from groups R, is quantitatively analysed. Although the steric effect can be quite substantial, the remarkable rate increase in the series AcOEt–AcOPrⁱ–AcOBu^t is largely due to a polar effect. A rationale is presented for the apparent inconsistency: the rate effect of α-alkylation in R is even larger than that in S_N1-type solvolysis of RBr, suggesting a large charge separation AcO^–, R⁺ in the transition state for concerted β-elimination, whereas the Hammett ρ value observed for pyrolysis of e.g. AcOCH(Me)C₆H₄Z is only a few percent of that associated with formation of benzylic carbenium ions.