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Kim M.-Y.,Ewha Womans University | Kim T.-E.,Ewha Womans University | Lee J.,Gocheok High School | Um I.-H.,Ewha Womans University
Bulletin of the Korean Chemical Society | Year: 2014

Second-order rate constants (kN) have been measured spectrophotometrically for the reaction of 4-chloro-2- nitrophenyl X-substituted-benzoates (6a-6h) with a series of primary amines including hydrazine in 80 mol % H2O/20 mol % DMSO at 25.0°C. The Brønsted-type plot for the reaction of 4-chloro-2-nitrophenyl benzoate (6d) is linear with βnuc = 0.74 when hydrazine is excluded from the correlation. Such a linear Brønsted-type plot is typical for reactions reported previously to proceed through a stepwise mechanism in which expulsion of the leaving group occurs in the rate-determining step (RDS). The Hammett plots for the reactions of 6a-6h with hydrazine and glycylglycine are nonlinear. In contrast, the Yukawa-Tsuno plots exhibit excellent linear correlations with ?X = 1.29-1.45 and r = 0.53-0.56, indicating that the nonlinear Hammett plots are not due to a change in RDS but are caused by resonance stabilization of the substrates possessing an electron-donating group (EDG). Hydrazine is ca. 47-93 times more reactive than similarly basic glycylglycine toward 6a-6h (e.g., the α-effect). The α-effect increases as the substituent X in the benzoyl moiety becomes a stronger electronwithdrawing group (EWG), indicating that destabilization of the ground state (GS) of hydrazine through the repulsion between the nonbonding electron pairs on the two N atoms is not solely responsible for the substituent-dependent α-effect. Stabilization of transition state (TS) through five-membered cyclic TSs, which would increase the electrophilicity of the reaction center or the nucleofugality of the leaving group, contributes to the α-effect observed in this study.


Im H.-J.,Ewha Womans University | Lee J.,Gocheok High School | Kim M.-Y.,Duksung Womens University | Um I.-H.,Ewha Womans University
Bulletin of the Korean Chemical Society | Year: 2014

Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for the nucleophilic substitution reaction of Y-substituted-phenyl picolinates (7a-f) with potassium ethoxide (EtOK) in anhydrous ethanol at 25.0 ± 0.1 °C. The plot of kobsd vs. [EtOK] curves upward while the plot of kobsd/[EtO-] eq vs. [EtO-]eq is linear with a positive intercept in all cases. Dissection of kobsd into kEtO- and kEtOK (i.e., the second-order rate constants for the reactions with the dissociated EtO- ion and ion-paired EtOK, respectively) has revealed that the ion-paired EtOK is more reactive than the dissociated EtO -. The σ° constants result in a much better Hammett correlation than σ- constants, indicating that the reaction proceeds through a stepwise mechanism in which departure of the leaving group occurs after the rate-determining step (RDS). K+ ion catalyzes the reaction by increasing the electrophilicity of the reaction center through formation of a cyclic transition state (TS). The catalytic effect decreases as the substituent Y becomes a stronger electron-withdrawing group (EWG). Development of a positive charge on the N atom of the picolinyl moiety through resonance interactions is responsible for the decreasing K+ ion catalysis.


Hong H.-J.,Duksung Womens University | Lee J.,Gocheok High School | Bae A.R.,Ewha Womans University | Um I.-H.,Ewha Womans University
Bulletin of the Korean Chemical Society | Year: 2013

The second-order rate constants (kOH-) for the reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with OH- in H 2O at 25.0 ± 0.1 °C have been measured spectrophotometrically. Comparison of kOH- with kEtO- (the second-order rate constants for the corresponding reactions with E tO- in ethanol) has revealed that EtO- is less reactive than OH- although the former is ca. 3.4 pKa units more basic than the latter, indicating that the reactivity of these nucleophiles is not governed by their basicity alone. The Brønsted-type plot for the reactions of 4a-4i with OH- is linear with βlg = -0.36. The Hammett plot correlated with σ- constants results in a slightly better correlation than that correlated with σo constants but exhibits many scattered points. In contrast, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation with ρ = 0.95 and r = 0.55. The r value of 0.55 implies that a negative charge develops partially on the O atom of the leaving group. Thus, the reactions of 4a-4i with OH- have been concluded to proceed through a concerted mechanism. Copyright © 2005 KCSNET.


Bae A.-R.,Ewha Womans University | Lee J.,Gocheok High School | Um I.-H.,Ewha Womans University
Bulletin of the Korean Chemical Society | Year: 2013

The second-order rate constants have been measured spectrophotometrically for the reactions of paraoxon 1 and parathion 2 with a series of alicyclic secondary amines, OH- and HOO- ions in H2O at 25.0 ± 0.1 °C. A linear Brønsted-type plot with βnuc = 0.40 was obtained for the reactions of 1 with amines and OH-. The reaction has been concluded to proceed through a concerted mechanism. HOO- deviates positively from the linear Brønsted-type plot, implying that the α-effect is operative. The magnitude of the α-effect (kHOO-/kOH-) was found to be ca. 55 for the reaction of 1 and 290 for that of parathion 2, indicating that HOO- is highly effective in decomposition of the toxic phosphorus compounds although it is over 4 pKa units less basic than OH-. Among the theories suggested as origins of the α-effect (e.g., TS stabilization through an intramolecular Hbonding interaction, solvent effect, and polarizability effect), polarizability effect appears to be the most important factor for the α-effect in this study, since the polarizable HOO- exhibits a larger α-effect for the reaction of the more polarizable substrate 2.


Lee J.,Ewha Womans University | Kim M.-Y.,Gocheok High School | Um I.-H.,Ewha Womans University
Bulletin of the Korean Chemical Society | Year: 2014

Pseudo-first-order rate constants (kobsd) have been measured spectrophotometrically for the reactions of 5-nitro-8-quinolyl nicotinate (4) and 5-nitro-8-quinolyl isonicotinate (5) with alkali metal ethoxides (EtOM; M = K, Na and Li) in anhydrous ethanol at 25.0 ± 0.1 °C. The plots of kobsd vs. [EtOM] curve slightly upward for the reactions with EtOK and EtONa but are linear for the reactions with EtOLi and for those with EtOK in the presence of 18-crown-6-ether. Dissection of kobsd into k EtO- and kEtOM (i.e., the second-order rate constants for the reactions with the dissociated EtO- and ion-paired EtOM, respectively) has revealed that the reactivity increases in the order EtO - ≈ EtOLi < EtOK < EtONa for the reactions of 4 and EtOLi < EtO- < EtOK < EtONa for the reactions of 5. Comparison of the kinetic results for the reactions of 4 and 5 with those reported previously for the corresponding reactions of 5-nitro-8-quinolyl benzoate (2) and picolinate (3) has revealed that the esters possessing a pyridine ring (i.e., 3-5) are significantly more reactive than the benzoate ester 2 due to the presence of the electronegative N atom (e.g., 2 ≪ 3 < 4 < 5). It has been concluded that M+ ion catalyzes the reactions of 3-5 by increasing the electrophilicity of the reaction center through a five-membered cyclic transition state (TS) for the reaction of 3 and via a four-membered cyclic TS for the reactions of 4 and 5.

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