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

• Published in: Chemistry : a European journal Vol. 12; no. 4; pp. 1237 - 1243
• Main Authors: Um, Ik-Hwan, Jeon, Sang-Eun, Seok, Jin-Ah
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 23.01.2006; WILEY‐VCH Verlag
• Subjects: Acetonitriles - chemistry; Amines - chemistry; aminolysis; Benzoates - chemistry; Brønsted; Hammett; Kinetics; Morpholines - chemistry; Piperidines - chemistry; solvent effects
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500647

Second‐order rate constants (kN) 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 kN values are only slightly larger in MeCN than in H2O, although the amines studied are approximately 8 pKa units more basic in the aprotic solvent than in H2O. 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 (ρY=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 βnuc 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 H2O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T±) in aprotic solvent. Consult a different medium! For the aminolysis reactions of 2,4‐dinitrophenyl X‐substituted benzoates and Y‐substituted phenyl benzoates (see equation) a change in the reaction medium from H2O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the intermediate (T±) in aprotic solvent.