36665-06-2Relevant 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
Um, Ik-Hwan,Han, Hyun-Joo,Baek, Mi-Hwa,Bae, Sun-Young
, p. 6365 - 6370 (2007/10/03)
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.
