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Li-Ra Im,Sang-Eun Jeon,엄익환 대한화학회 2011 Bulletin of the Korean Chemical Society Vol.32 No.4
A kinetic study is reported for aminolysis of S-4-nitrophenyl X-substituted thiobenzoates 3a-g in 80 mol %H_2O/20 mol % DMSO at 25.0 ± 0.1 ^oC. Thiol esters 3a-g are 7.8-47.6 fold more reactive than the corresponding oxygen esters (i.e., 4-nitrophenyl X-substituted benzoates 1a-g). Such reactivity order appears to be in accordance with the expectation that 4-nitrothiophenoxide in 3a-g is a better nucleofuge than 4-nitrophenoxide in 1a-g since the former is 2.64 pK_a units less basic than the latter. Hammett plot for the reactions of 3a-g exhibit poor correlation coefficients (R2 = 0.977-0.986) with negative deviation by substrates possessing an electrondonating group (EDG), while the Yukawa-Tsuno plots result in excellent linear correlation (R^2 = 0.995-0.997)with ρ = 0.93-1.23 and r = 0.57-0.67, indicating that the negative deviation shown by substrates possessing an EDG is caused by ground-state stabilization through resonance interactions but not due to a change in ratedetermining step upon changing the nonleaving-group substituent X. The ρ value increases as the incoming amine becomes more basic and more reactive, indicating that the RSP is not operative in the current reactions.
Im, Li-Ra,Um, Ik-Hwan Korean Chemical Society 2009 Bulletin of the Korean Chemical Society Vol.30 No.10
Second-order rate constants ($k_{HOO}$‒) have been measured spectrophotometrically for nucleophilic substitution reactions of Y-substituted phenyl benzenesulfonates (1a-g) with $HOO^-$ ion in $H_2O$ at $25.0\;{\pm}\;0.1\;{^{\circ}C}$. The Br$\phi$nsted-type plot is linear with ${\beta}_{lg}$ = ‒0.73. The Hammett plot correlated with with ${\sigma}^-$ constants results in much better linearity than ${\sigma}^o$ constants, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS) either in a stepwise mechanism or in a concerted pathway. However, a stepwise mechanism in which departure of the leaving group occurs in the RDS has been excluded since $HOO^-$ ion is more basic and a poorer leaving group than the leaving Y-substituted phenoxide ions. Thus, the reactions of 1a-g with $HOO^-$ ion have been concluded to proceed through a concerted mechanism. The $\alpha$-nucleophile $HOO^-$ ion is more reactive than its reference nucleophile $OH^-$ ion although the former is ca. 4 p$K_a$ units less basic than the latter (i.e., the $\alpha$-effect). TS stabilization through intramolecular H-bonding interaction has been suggested to be irresponsible for the $\alpha$-effect shown by $HOO^-$ ion, since the magnitude of the $\alpha$-effect is independent of the electronic nature of substituent Y in the leaving group. GS destabilization through desolvation of $HOO^-$ ion has been concluded to be responsible for the $\alpha$-effect found in the this study.
Im, Li-Ra,Jeon, Sang-Eun,Um, Ik-Hwan Korean Chemical Society 2011 Bulletin of the Korean Chemical Society Vol.32 No.4
A kinetic study is reported for aminolysis of S-4-nitrophenyl X-substituted thiobenzoates 3a-g in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. Thiol esters 3a-g are 7.8-47.6 fold more reactive than the corresponding oxygen esters (i.e., 4-nitrophenyl X-substituted benzoates 1a-g). Such reactivity order appears to be in accordance with the expectation that 4-nitrothiophenoxide in 3a-g is a better nucleofuge than 4-nitrophenoxide in 1a-g since the former is 2.64 pKa units less basic than the latter. Hammett plot for the reactions of 3a-g exhibit poor correlation coefficients ($R^2$ = 0.977-0.986) with negative deviation by substrates possessing an electrondonating group (EDG), while the Yukawa-Tsuno plots result in excellent linear correlation ($R^2$ = 0.995-0.997) with ${\rho}$ = 0.93-1.23 and r = 0.57-0.67, indicating that the negative deviation shown by substrates possessing an EDG is caused by ground-state stabilization through resonance interactions but not due to a change in ratedetermining step upon changing the nonleaving-group substituent X. The ${\rho}$ value increases as the incoming amine becomes more basic and more reactive, indicating that the RSP is not operative in the current reactions.
Li-Ra Im,엄익환 대한화학회 2009 Bulletin of the Korean Chemical Society Vol.30 No.10
Second-order rate constants (kHOO‒) have been measured spectrophotometrically for nucleophilic substitution reactions of Y-substituted phenyl benzenesulfonates (1a-g) with HOO‒ ion in H2O at 25.0 ± 0.1 oC. The Brønsted-type plot is linear with βlg = ‒0.73. The Hammett plot correlated with with σ‒ constants results in much better linearity than σo constants, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS) either in a stepwise mechanism or in a concerted pathway. However, a stepwise mechanism in which departure of the leaving group occurs in the RDS has been excluded since HOO‒ ion is more basic and a poorer leaving group than the leaving Y-substituted phenoxide ions. Thus, the reactions of 1a-g with HOO‒ ion have been concluded to proceed through a concerted mechanism. The α-nucleophile HOO‒ ion is more reactive than its reference nucleophile OH‒ ion although the former is ca. 4 pKa units less basic than the latter (i.e., the α-effect). TS stabilization through intramolecular H-bonding interaction has been suggested to be irresponsible for the α-effect shown by HOO‒ ion, since the magnitude of the α-effect is independent of the electronic nature of substituent Y in the leaving group. GS destabilization through desolvation of HOO‒ ion has been concluded to be responsible for the α-effect found in the this study.
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.