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Relevant academic research and scientific papers

Effect of changing electrophilic center C=O to C=S on rates and mechanism: Pyridinolyses of O-2,4-dinitrophenyl thionobenzoate and its oxygen analogue

Second-order rate constants have been measured spectrophotometrically for the reactions of O-2,4-dinitrophenyl thionobenzoate (1) and 2,4-dinitrophenyl benzoate (2) with a series of substituted pyridines in 80 mol % H 2O/20 mol % DMSO at 25.0 ± 0.1 °C. The Bronsted-type plots obtained are nonlinear with β1 = 0.26, β2 = 1.07, and pKa° = 7.5 for the reactions of 1 and β2 = 0.40, β2 = 0.90, and pKa° = 9.5 for the reactions of 2, suggesting that the pyridinolyses of 1 and 2 proceed through a zwiterionic tetrahedral intermediate T± with a change in the rate-determining step at pKa° = 7.5 and 9.5, respectively. The thiono ester 1 is more reactive than its oxygen analogue 2 except for the reaction with the strongest basic pyridine studied (pK a = 11.30). The k1 value is larger for the reactions of 1 than for those of 2 in the low pKa region, but the difference in the k1 value becomes negligible with increasing the basicity of pyridines. On the other hand, 1 exhibits slightly larger k2/k -1 ratio than 2 in the low pKa region but the difference in the K2/k-1 ratio becomes more significant with increasing the basicity of pyridines. Pyridines are more reactive than alicyclic secondary amines of similar basicity toward 2 in the pKa above ca. 7.2 but less reactive in the pKa below ca. 7.2. The k1 value is slightly larger, but the k2/k-1 ratio is much smaller for the reactions of 2 with pyridines than with isobasic secondary amines in the low pKa region, which is responsible for the fact that the weakly basic pyridines are less reactive than isobasic secondary amines.

Modification of both the electrophilic center and substituents on the nonleaving group in pyridinolysis of O-4-nitrophenyl benzoate and thionobenzoates

(Chemical Equation Presented) A kinetic study is reported for reactions of 4-nitrophenyl benzoate (1c) and O-4-nitrophenyl X-substituted thionobenzoates (2a-e) with a series of pyridines in 80 mol % H2O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 ± 0.1°C. O-4-Nitrophenyl thionobenzoate (2c) is more reactive than its oxygen analogue 1c toward all the pyridines studied. The Bronsted-type plot is linear with βnuc = 1.06 for reactions of 1c but curved for the corresponding reactions of 2c with βnuc decreasing from 1.38 to 0.38 as the pyridine basicity increases, indicating that the reaction mechanism is also influenced on changing the electrophilic center from C=O to C=S. The curvature center of the curved Bronsted-type plots (defined as pKao) occurs at pKa = 9.3 regardless of the electronic nature of the substituent X in the nonleaving group. The Hammett plot for reactions of 2a-e with 4-aminopyridine is nonlinear, i.e., the substrates having an electron-donating substituent exhibit negative deviations from the Hammett plot. However, the Yukawa-Tsuno plot for the same reactions exhibits good linear correlation, indicating that the negative deviations shown by these substrates arise from stabilization of the ground state through resonance interaction between the electron-donating substituent X and the C=S bond.