Banca de QUALIFICAÇÃO: ANDRESSA ROCHA SOARES

Uma banca de QUALIFICAÇÃO de MESTRADO foi cadastrada pelo programa.
DISCENTE : ANDRESSA ROCHA SOARES
DATA : 16/12/2019
HORA: 09:30
LOCAL: sala 303, 3º andar, Bloco B, Campus SA da Fundação Universidade Federal do ABC, localizada na Avenida dos Estados, 5001, Santa Terezinha, Santo André, SP
TÍTULO:

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

PÁGINAS: 16
RESUMO:

The peroxyoxalate (PO) system, a base- or non-catalyzed transformation between an aryl oxalic ester and hydrogen peroxide in the presence of an activator, is one of the most efficient organic chemiluminescent reactions, leading to important analytical and biological applications. Since solvolysis is a major factor in reducing light emission yield, we studied the decomposition of bis(2,4,6-trichlorophenyl) oxalate (TCPO), the most common ester used in the PO system, in 100, 98, 95, 90, 80, 70, 60, 50% ethanol/water (EtOH/W) and 100, 90, 80, 70% methanol/water (MeOH/W) mixtures (% in v/v). In this media, one phenolic residue is generated in a fast addition-elimination (step 1), followed by a second slower addition-elimination (step 2), thus, with the observation of two rate constants. Using a Grunwald-Winstein (G-W) relationship between these rate constants and the solvent ionizing power (Υ), we determined the sensibility (m) to solvation effects andcharge development associated to the transition state (TS) of the rate-determining step (rds). We also determined the Gibbs free-energy of activation (∆G^≠, at 25 ºC) associated to 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 m= 1.0 (step 1, ∆G^≠ = 21.6 to 20.6 kcal mol^–1) and 1.1 (step 2, ∆G^≠ = 23.9 to 23.0 kcal mol^–1), indicating a full ionization in the addition TS. Smaller Y values in 90-50% EtOH/W result in a more symmetrical TS, with not as many charges being generated (step 1, m= 0.196, ∆G^≠ = 20.3 to 19.4 kcal mol^–1; step 2, m = 0.30, ∆G^≠ = 22.6 to 21.7 kcal mol^–1). In 100-70% MeOH/W binary mixtures, with Y values similar to the ones for 90-50% EtOH/W, m = 0.52 (step 1, ∆G^≠= 20.3 to 18.8 kcal mol^–1) and 0.27 (step 2, ∆G^≠ = 21.5 to 20.9 kcal mol^–1). Particularly for step 1 in MeOH/W, the observation of a solvent kinetic isotope effect that goes from inverse (0.63 ± 0.09 in pure MeOH) to normal (1.9 ± 0.3 in 70% MeOH/W) is consistent with significant participation of the solvent in TS stabilization, resembling general acid catalysis.

MEMBROS DA BANCA:
Presidente - Interno ao Programa - 1939561 - FERNANDO HEERING BARTOLONI
Membro Titular - Examinador(a) Interno ao Programa - 250.887.078-19 - ERICK LEITE BASTOS - USP
Membro Titular - Examinador(a) Interno ao Programa - 1600860 - ALVARO TAKEO OMORI
Membro Suplente - Examinador(a) Externo ao Programa - 1909951 - ARTUR FRANZ KEPPLER