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

The vapour-phase pyrolysis of the crowded tertiary alkyl acetates AcOCMe2But(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–AcOBut 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 SN1-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.