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A Mechanistic Study on Alkaline Hydrolysis of Y-Substituted Phenyl Benzenesulfonates
엄익환,Li-Ra Im,Youn-Min Park 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.12
Second-order rate constants (kOH−) have been measured spectrophotometrically for reactions of Y-substituted phenyl benzenesulfonates (1a-h) with OH– in H2O containing 20 mol % DMSO at 25.0 ± 0.1 oC. The Brønstedtype plot is linear with βlg = –0.55 including the points for the reactions of 2,4-dinitrophenyl benzenesulfonate (1a) and 4-chloro-2-nitrophenyl benzenesulfonate (1c), indicating that the ortho-nitro group on the leaving aryloxide does not exert steric hindrance in the current reactions. The Hammett plot correlated with σo constants exhibits highly scattered points, while the Hammett correlation with σ– constants results in a slightly better correlation but still many points deviate from the linearity. In contrast, the Yukawa-Tsuno plot shows an excellent linear correlation with r = 0.52, implying that leaving-group departure occurs at the RDS either in a stepwise mechanism or in a concerted pathway. However, the stepwise mechanism in which the leaving group departs in the RDS is excluded since the incoming OH– is much more basic and a poorer nucleofuge than the leaving aryloxide. Thus, the alkaline hydrolysis of 1a-h has been concluded to proceed through a concerted mechanism.
Kinetics and Mechanism of Azidolysis of Y-Substituted Phenyl Benzoates
엄익환,김은희,Hyun-Joo Hana 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.3
Second-order rate constants (kN) have been measured spectrophotometrically for reactions of Y-substituted phenyl benzoates (1a-h) with azide ion (N3) in 80 mol % H2O/20 mol % DMSO at 25.0 0.1 oC. The Brnsted-type plot for the azidolysis exhibits a downward curvature, i.e., the slope (b lg) changes from 0.97 to 0.20 as the basicity of the leaving group decreases. The pKao (defined as the pKa at the center of the Brnsted curvature) is 4.8, which is practically identical to the pKa of the conjugate acid of N3 ion (4.73). Hammett plots correlated with s o and s constants exhibit highly scattered points for the azidolysis. On the contrary, the corresponding Yukawa-Tsuno plot results in an excellent linear correlation with r = 2.45 and r = 0.40, indicating that the leaving group departs in the rate-determining step. The curved Brnsted-type plot has been interpreted as a change in the rate-determining step in a stepwise mechanism. The microscopic rate constants (k1 and k2/k1 ratio) have been calculated for the azidolysis and found to be consistent with the proposed mechanism.
엄익환,Se-Won Min 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.3
Second-order rate constants (kN) have been measured for reactions of Y-substituted phenyl 2-thiophenecarboxylates (6a-h) with morpholine and piperidine in 80 mol % H2O/20 mol % DMSO at 25.0 0.1 oC. The Brnsted-type plot for the reactions of 6a-h with morpholine is linear with b lg = 1.29, indicating that the reactions proceed through a tetrahedral zwitterionic intermediate (T?). On the other hand, the Brnsted-type plot for the reactions of 6a-h with piperidine exhibits a downward curvature, implying that a change in the rate-determining step occurs on changing the substituent Y in the leaving group. Dissection of kN into microscopic rate constants (i.e., k1 and k2/k1 ratio) has revealed that k1 is smaller for the reactions of 6a-h than for those of Y-substituted phenyl 2-furoates (5a-h), while the k2/k1 ratio is almost the same for the reactions of 5a-h and 6a-h. It is also reported that modification of the nonleaving group from the furoyl (5a-h) to the thiophenecarbonyl (6a-h) does not influence pKao (defined as the pKa at the center of the Brnsted curvature) as well as the k2/k1 ratio.
엄익환,Jin-A Seo, 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.8
2-thiophenecarboxylate (2) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % dimethyl sulfoxide at 25.0 oC. The Brønsted-type plot exhibits a downward curvature for the aminolysis of 2. The curved Brønsted-type plot is similar to that reported for the corresponding reactions of 2,4-dinitrophenyl 2- thiophenecarboxylate (1). The reactions of 1 and 2 have been suggested to proceed through the same mechanism, i.e., through a zwitterionic tetrahedral intermediate (T±) with a change in the rate-determining step. Substrate 2 is less reactive than 1 toward weakly basic amines (e.g., pKa < 10.4) but becomes more reactive as the basicity of amines increases further. Dissection of kN into the microscopic rate constants has revealed that the reaction of 2 results in a smaller k2/k–1 ratio but larger k1 than the corresponding reaction of 1. Steric hindrance exerted by the ortho-nitro group has been suggested to be responsible for the smaller k1 value found for the reactions of 1.
엄익환,Seung-Eun Lee,Yeon-Ju Hong,Jee Eun Park 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.1
Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for nucleophilic substitution reactions of 5-nitro-8-quinolyl benzoate (5) with alkali metal ethoxides, EtO– M+ (M+ = Li+, Na+ and K+) in anhydrous ethanol (EtOH) at 25.0 ± 0.1 °C. The plots of kobsd vs. [EtO– M+] exhibit upward curvatures, while the corresponding plots for the reactions of 5 with EtO– Na+ and EtO– K+ in the presence of complexing agents, 15-crown-5-ether and 18-crown-6-ether are linear with rate retardation. The reactions of 5 with EtO– Na+ and EtO– Li+ result in significant rate enhancements on additions of Na+ClO4–, indicating that the M+ ions behave as a catalyst. The dissociated EtO– and ion-paired EtO–M+ have been proposed to react with 5. The second-order rate constants for the reactions with EtO– (kEtO–) and EtO–M+ (kEtO–M+) have been calculated from ion-pairing treatments. The kEtO– and kEtO–M+ values decrease in the order kEtO–Na+ > kEtO–K+ > kEtO–Li+ > kEtO–, indicating that ion-paired EtO–M+ species are more reactive than the dissociated EtO– ion, and Na+ ion exhibits the largest catalytic effect. The M+ ions in this study form stronger complex with the transition state than with the ground state. Coordination of the M+ ions with the O and N atoms in the leaving group of 5 has been suggested to be responsible for the catalytic effect shown by the alkali metal ions in this study.
엄익환,Eun-Hee Kim,Li-Ra Im,Masaaki Mishima 대한화학회 2010 Bulletin of the Korean Chemical Society Vol.31 No.9
Second-order rate constants (kN) have been measured spectrophotometrically for reactions of 2,4-dinitrophenyl X-substituted benzoates (X = 4-MeO, H and 4-NO2) with a series of Z-substituted pyridines in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. The Brønsted-type plots exhibit downward curvature (e.g., β2 = 0.89 ~ 0.96 when pKa <9.5 while β1 = 0.38 ~ 0.46 when pKa > 9.5), indicating that the reaction proceeds through a stepwise mechanism with a change in rate-determining step (RDS). The pKa o, defined as the pKa at the center of Brønsted curvature, has been analyzed to be 9.5 regardless of the electronic nature of the substituent X in the benzoyl moiety. Dissection of kN into the microscopic rate constants k1 and k2/k–1 ratio has revealed that k1 is governed by the electronic nature of the substituent X but the k2/k–1 ratio is not. Theoretical calculations also support the argument that the electronic nature of the substituent X in the benzoyl moiety does not influence the k2/k–1 ratio.