The mechanism of the gas-phase pyrolysis of esters. Part 7. The effects of substituents at the acyl carbon

Some t-butyl esters of substituted acetates (RCH2CO2But) have been prepared and pyrolysed at temperatures between 620 and 519 K. The rate data give an approximate correlation with σI values (ρ=ca. 1.4 at 600 K), deviations being most probably due to steric acceleration of elimination when the substituent at the acyl carbon is bulky. This is also indicated by the fact that isopropyl privalate and t-butyl pivalate each pyrolyse more rapidly than their corresponding acetates whereas on electronic grounds alone the reverse reactivity order should obtain. An alternative explanation of these anomalies is that hyperconjugation leads to increased nucleophilicity of the carbonyl oxygen, and this may in particular account for the high reactivity of t-butyl trimethylsilylacetate.