Banca de DEFESA: ANDRESSA ROCHA SOARES
Uma banca de DEFESA de MESTRADO foi cadastrada pelo programa.DISCENTE : ANDRESSA ROCHA SOARES
DATA : 29/03/2021
HORA: 09:00
LOCAL: https://meet.google.com/adc-rdag-kim
TÍTULO:
Study of the mechanism for the solvolysis and imidazolysis reactions of bis(2,4,6-trichlorophenyl)oxalate in ethanol/water and methanol/water mixtures using the Grunwald-Winstein correlation
PÁGINAS: 65
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Química
SUBÁREA: Química Orgânica
ESPECIALIDADE: Físico-Química Orgânica
RESUMO:
Solvation effects and the extent of solvolysis are major aspects regarding the decomposition of esters, particularly in polar protic media. Oxalic esters are substances that have two equivalent ester groups in the same molecular framework, but with potential different reactivities. For instance, aryl oxalic esters are commonly used in efficient organic chemiluminescent reactions (e.g., peroxyoxalate system), leading to important analytical and biological applications. The decomposition of bis(2,4,6-trichlorophenyl) oxalate (TCPO) was studied in mixtures of 100, 98, 95, 90, 80, 70, 60, 50% ethanol/water (EtOH/W) and 100, 90, 80, 70% methanol/water (MeOH/W) (% in v/v), without nucleophiles (i.e., solvolysis) and adding imidazole. In solvolysis, 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 (kobs1 and kobs2). When imidazole is added the first addition-elimination step becomes slower than the second, with only one observed rate constant. Correlating the rate constant with imidazole concentration, a second order behaviour was observed, indicating the presence of two imidazole molecules in the rate determining step (rds), providing the bimolecular (kbim) and termolecular (kter) rate constants. Using a Grunwald-Winstein (G-W) relationship between these rate constants (solvolysis, kobs1 and kobs2; and imidazolysis, kbim and kter) against the solvent ionizing power (Υ), it was possible to determine the sensibility (m) to solvation effects and charge development associated to the transition state (TS) of the rds of both transformations. In addition, for the solvolysis reaction, the Gibbs free-energy of activation (∆‡G, at 25 ºC) associated to each step was determined, 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 m values close to unity for both steps were observed (kobs1 and kobs2), indicating a full ionization in the addition TS. Larger Y values in 90–50% EtOH/W result in a more symmetrical TS, with not as many charges being generated. In 100–70% MeOH/W binary mixtures, with Y values similar to the ones for 90–50% EtOH/W, only one linear G-W plot was observed with m values similar to 90–50% EtOH/W, indicating a TS that is more symmetrical. Particularly for step 1 in MeOH/W, the observation of a solvent kinetic isotope effect that goes from inverse to normal is consistent with significant participation of the solvent in TS stabilization, resembling general acid catalysis. For imidazolysis, values for m < 1 were obtained for all media, indicating that when a nucleophile is added the system becomes less sensitive to the ionization power of the solvent.
MEMBROS DA BANCA:
Presidente - Interno ao Programa - 1939561 - FERNANDO HEERING BARTOLONI
Membro Titular - Examinador(a) Interno ao Programa - 1600860 - ALVARO TAKEO OMORI
Membro Titular - Examinador(a) Externo à Instituição - PETER WILHELM TIEDEMANN - USP
Membro Suplente - Examinador(a) Interno ao Programa - 1909951 - ARTUR FRANZ KEPPLER
Membro Suplente - Examinador(a) Externo ao Programa - 1802150 - MARCO ANTONIO BUENO FILHO
Membro Suplente - Examinador(a) Externo à Instituição - JOSEF WILHELM BAADER - USP