Effect of unsaturated substituents on the hydrolysis of esters

A study of the acid and alkaline hydrolysis of esters (R1CO2R2) containing unsaturated substituents in aqueous acetone confirms that where the unsaturation in R1 is conjugated with CO2R2 in the initial state, the value of the Taft steric substituent constant, Es, is considerably more negative than might be expected from normal steric interactions. When there is no conjugation, Es has values consistent with steric interactions by R1 and R2. On the other hand, the Taft polar substituent constant, σ*, has fairly large positive values regardless of the presence or absence of conjugation. This may be attributed to the electron-withdrawing power of the unsaturated substituents but the property falls off as the seat of unsaturation becomes further removed from the ester group.