Aminolysis of 2,4‐Dinitrophenyl X‐Substituted Benzoates and Y‐Substituted Phenyl Benzoates in MeCN: Effect of the Reaction Medium on Rate and Mechanism

Second‐order rate constants ( k N ) have been determined spectrophotometrically for the reactions of 2,4‐dinitrophenyl X‐substituted benzoates ( 1 a – f ) and Y‐substituted phenyl benzoates ( 2 a – h ) with a series of alicyclic secondary amines in MeCN at 25.0±0.1 °C. The k N values are only slightly larger in MeCN than in H 2 O, although the amines studied are approximately 8 p K a units more basic in the aprotic solvent than in H 2 O. The Yukawa–Tsuno plot for the aminolysis of 1 a – f is linear, indicating that the electronic nature of the substituent X in the nonleaving group does not affect the rate‐determining step (RDS) or reaction mechanism. The Hammett correlation with σ − constants also exhibits good linearity with a large slope ( ρ Y =3.54) for the reactions of 2 a – h with piperidine, implying that the leaving‐group departure occurs at the rate‐determining step. Aminolysis of 2,4‐dinitrophenyl benzoate ( 1 c ) results in a linear Brønsted‐type plot with a β nuc value of 0.40, suggesting that bond formation between the attacking amine and the carbonyl carbon atom of 1 c is little advanced in the transition state (TS). A concerted mechanism is proposed for the aminolysis of 1 a – f in MeCN. The medium change from H 2 O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T ± ) in aprotic solvent.