Better late than never! Transition state character involved in the neutral solvolysis of an oxalic ester determined by the ionizing power of ethanol/water and methanol/water mixtures

Author(s)

Soares, Andressa R. ^[1] ; Reis, Roberta A. ^[1] ; Melo, Diego U. ^[1] ; Boaro, Andreia ^[1] ; Bartoloni, Fernando H. ^[1]

Affiliation

^[1] Univ Fed ABC, Ctr Ciencias Nat & Humanas, Santo Andre, SP - Brazil

• Document type: Journal article
• Source: JOURNAL OF MOLECULAR LIQUIDS; v. 316, OCT 10 2020.
• Web of Science Citations: 0

Abstract

Solvation effects and solvolysis extent are major aspects of the decomposition of esters, particularly in polar protic media. Among the many different ester derivatives, oxalic esters are substances that have two ester groups in the same molecular framework, but with potentially different reactivities. Aryl oxalic esters are commonly used in efficient organic chemiluminescent reactions, leading to important analytical and biological applications. We studied the decomposition of bis(2.4.6-trichlorophenyl) oxalate (TCPO) in 100,98,95, 90, 80, 70, 60, and 50% ethanol/water (EtOH/W) and 100, 90, 80, and 70% methanol/water (MeOH/W) mixtures (% in v/v). In these media, one phenolic residue is generated in a fast addition-elimination (step 1), followed by a second slower addition-elimination (step 2), thus, two rate constants are observed. Using a Grunwald-Winstein (G-W) relationship between these rate constants and the solvent ionizing power (gamma), we determined the sensibility (m) to solvation effects and charge development associated with the transition state (TS) of the rate-determining step (rds). We also determined the Gibbs free energy of activation (Delta(double dagger)G, at 25 degrees C) associated with each step, and used it to rationalize the TS character of the rds, considering that in both steps a zwitterionic intermediate is generated and that the addition step is rate limiting. In EtOH/W binary mixtures a non-linear G-W plot was observed, and for 100-95% EtOH/W mixtures the sensibility was m- 1.0 (step 1, Delta(double dagger)G = 21.6 to 20.6 kcal mol(-1)) and 1.1 (step 2, Delta(double dagger)G = 23.9 to 23.0 kcal mol(-1)), indicating full ionization in the TS of the addition step. Smaller Y values in 90-50% EtOH/W reflect a more symmetrical TS, with not as many charges being generated (step 1, m = 0.196, Delta(double dagger)G = 20.3 to 19.4 kcal mol(-1); step 2, m = 0.30, Delta(double dagger)G = 22.6 to 21.7 kcal mol(-1)). In 100%-70% MeOHAV binary mixtures, with Y values similar to those for 90-50% EtOH/W, m = 0.52 (step 1, Delta(double dagger)G = 20.3 to 18.8 kcal mol(-1)) and 0.27 (step 2, Delta(double dagger)G = 21.5 to 20.9 kcal mol(-1)) were observed. Particularly for step 1 in MeOH/W, the observation of a solvent kinetic isotope effect that varies from inverse (0.63 +/- 0.09 in pure MeOH) to normal (1.9 +/- 0.3 in 70% MeOH/W) is consistent with the significant participation of the solvent in TS stabilization, resembling general acid catalysis. (C) 2020 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 12/13807-7 - New tools for the study of electron transfer reactions in PS II model systems
• Grantee: Fernando Heering Bartoloni
• Support Opportunities: Regular Research Grants
• FAPESP's process: 16/10585-4 - Mechanistic aspects regarding the induced decomposition of hydroperoxides and silylperoxides derived from lophine
• Grantee: Andréia Boaro
• Support Opportunities: Scholarships in Brazil - Doctorate