A Comparison of the Bimolecular and Intramolecular Nucleophilic Catalysis of the Hydrolysis of Substituted Phenyl Acylates by the Dimethylamino Group

The rate constants and activation parameters have been determined for the nucleophilic displacement of substituted phenols from phenyl acylates (solv. H₂O) in the systems I [trimethylamine + m- and p-substituted phenyl acetates], II [m- and p-substituted phenyl γ-(N,N-dimethylamino)-butyrates] and III [m- and p-substituted phenyl δ-(N,N-dimethylamino)-valerates]. The values of ΔH^‡ were found to be, within experimental error, invariant for all the esters of system I, II and III. Therefore, all electronic and steric factors—both being large—are reflected in ΔS^‡, providing linear free energy relationships (unusually) dependent only on ΔS^‡. The OH⊖-catalyzed hydrolysis of m- and p-substituted O-acetylphenols was also found to be characterized by a constant ΔH^‡ value and a substituent controlled ΔS‡. The unimolecular processes of II and III involving five-and six-membered transition states had TΔS‡ higher by 3.6 ± 0.6 and 4.7 ± 1.1 kcal. mole⁻¹, respectively, than the bimolecular process of I. The Hammett ρ-values for I, II and III are very nearly identical (+2.2, +2.5, +2.5, respectively). The constancy of ΔH‡ and ρ-for systems I, II and III as well as the uniform dependence of ΔΔS‡ on the nature of the substituent group provides convincing evidence that the bimolecular and unimolecular processes take place by the same mechanism. The ratio of rate constants for the similarly substituted esters of II vs. those of III was found to be 2.3. This value is but 1/100 that previously determined as the ratio for the formation of glutaric and succinic anhydride in the intramolecular carboxyl anion nucleophilic catalysis of the hydrolysis of monophenyl glutarates and succinates. This difference as well as the apparent change in mechanism previously noted for the conversion of carboxyl anion catalysis of substituted phenyl acetates to an intramolecular process are discussed.