• Bulletin of the Korean Chemical Society
• /
• 제15권7호
• /
• pp.585-589
• /
• 1994

Second-order rate constants have been measured spectrophotometrically for the nucleophilic substitution reactions of S-p-nitrophenyl thiobenzoate with various anionic nucleophiles including 6 ${\alpha}$-effect nucleophiles. A good Bronsted correlation has been observed for the reactions with 7 aryloxides. However, p-chlorothiophenoxide and hydroxide ions exhibit significantly positive and negative deviations, respectively, from the Bronsted plot. The deviations are attributed to the effect of polarizability and solvation rather than a change in the reaction mechanism. The ${\alpha}$-effect nuceophiles except highly basic ones demonstrate remarkably enhanced nucleophilicity. The effects of solvation and/or polarizability are proposed to be important for the cause of the ${\alpha}$-effect.

• Bulletin of the Korean Chemical Society
• /
• 제13권6호
• /
• pp.632-636
• /
• 1992

Second-order rate constants have been determined spectrophotometrically for nucleophilic substitution reactions of p-nitrophenyl benzoate with various anionic nucleophiles including 6 ${\alpha}$ -effect nucleophiles. The logarithmic second-order rate constants for the aryloxides give a good Bronsted correlation with the respective basicity while the ones for p-chlorothiophenoxide and hydroxide exhibit significantly positive and negative deviations, respectivity, from the Bronsted linear line. The deviations are attributed to a solvation effect rather than a change in the reaction mechanism. The ${\alpha}$-effect nucleophiles except highly basic ones demonstrate significantly higher nucleophilicity (the ${\alpha}$ -effect) than would be predicted from the respective basicity. The effect of solvation and polarizability appears to be important for the ${\alpha}$-effect as well as for the reaction rate.

• Bulletin of the Korean Chemical Society
• /
• 제34권1호
• /
• pp.201-206
• /
• 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 $H_2O$ at $25.0{\pm}0.1^{\circ}C$. A linear Br${\o}$nsted-type plot with ${\beta}_{nuc}$ = 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 Br${\o}$nsted-type plot, implying that the ${\alpha}$-effect is operative. The magnitude of the ${\alpha}$-effect ($k_{HOO^-}/k_{OH^-}$) 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 $pK_a$ units less basic than $OH^-$. Among the theories suggested as origins of the ${\alpha}$-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 ${\alpha}$-effect in this study, since the polarizable $HOO^-$ exhibits a larger ${\alpha}$-effect for the reaction of the more polarizable substrate 2.

• Bulletin of the Korean Chemical Society
• /
• 제15권10호
• /
• pp.860-864
• /
• 1994

2nd-order rate constants have been measured spectrophotometrically for the reactions of 4-nitrophenyl benzoate (1), S-4-nitrophenyl thiobenzoate (2) and 4-nitrophenyl thionbenzoate (3) with alkoxides, aryloxides and thioaryloxides in absolute ethanol at 25.0 ${\pm}0.1$${\circ}C$. The substitution of O by polarizable S in the leaving group has little affected the reactivity of 2 toward the charge localized species (eg. $EtO^-$ and $CF_3CH_2O^-$), while the effect of the similar replacement in the carbonyl group has led to a decrease in reactivity by a factor of 10. However, the reactivity of these esters toward charge delocalized aryloxides has been found to be in the order $1<3{\le}2$. The effect of replaced sulfur atom on reactivity becomes more significant for the reaction with polarizable thioaryloxides, i.e. the reactivity increases in the order $1<2{\ll}3$. The difference in reactivity for the present system is attributed to a polarizability effect.

• Bulletin of the Korean Chemical Society
• /
• 제10권6호
• /
• pp.610-611
• /
• 1989

• Bulletin of the Korean Chemical Society
• /
• 제11권5호
• /
• pp.362-364
• /
• 1990

• Bulletin of the Korean Chemical Society
• /
• 제34권8호
• /
• pp.2325-2329
• /
• 2013

Second-order rate constants $k_N$ have been measured for reactions of benzyl 2-pyridyl thionocarbonate (4b) and t-butyl 2-pyridyl thionocarbonate (5b) with a series of cyclic secondary amines in MeCN at $25.0{\pm}0.1^{\circ}C$. The $k_N$ values for the reactions of 4b and 5b have been compared with those reported previously for the corresponding reactions of benzyl 2-pyridyl carbonate (4a) and t-butyl 2-pyridyl carbonate (5a) to investigate the effect of changing the electrophilic center from C=O to C=S on reactivity and reaction mechanism. The thiono compound 4b is more reactive than its oxygen analogue 4a. The Br${\o}$nsted-type plots for the reactions of 4a and 4b are linear with ${\beta}_{nuc}=0.57$ and 0.37, respectively. The reactions of 4a were previously reported to proceed through a concerted mechanism, while those of 4b in this study have been concluded to proceed through a stepwise mechanism with formation of an intermediate being the rate-determining step on the basis of the ${\beta}_{nuc}$ value of 0.37. Enhanced polarizability upon changing the C=O in 4a by C=S has been suggested to be responsible for the reactivity order and the contrasting reaction mechanisms. In contrast, the reactivity of 5a and 5b is similar, but they are much less reactive than 4a and 4b. Furthermore, the reactions of 5a and 5b have been concluded to proceed through the same mechanism (i.e., a concerted mechanism) on the basis of linear Bronsted-type plots with ${\beta}_{nuc}=0.45$ or 0.47. It has been concluded that the strong steric hindrance exerted by the t-Bu in 5a and 5b causes a decrease in their reactivity and forces the reactions to proceed through a concerted mechanism.

• Bulletin of the Korean Chemical Society
• /
• 제30권1호
• /
• pp.214-218
• /
• 2009

Second-order rate constants ($k_N$) have been measured for nucleophilic substitution reactions of S-4-nitrophenyl thiobenzoate with a series of alicyclic secondary amines in $H_2O$ containing 20 mol % DMSO at 25.0 ${\pm}$ 0.1 ${^{\circ}C}$. The Br$\phi$nsted-type plot exhibits a downward curvature, i.e., $\beta_{nuc}$ decreases from 0.94 to 0.34 as the amine basicity increases. The reactions in the aqueous DMSO have also been suggested to proceed through a zwitterionic tetrahedral intermediate (T${\pm}$) with change in the RDS on the basis of the curved Br$\phi$nsted-type plot. The reactions in the aqueous DMSO exhibit larger $k_N$ values than those in the aqueous EtOH. The macroscopic rate constants ($k_N$) for the reactions in the two solvent systems have been dissected into the microscopic rate constants ($k_1\;and\;k_2/k_{-1}$ ratio) to investigate effect of medium on reactivity in the microscopic level. It has been found that the $k_2/k_{-1}$ ratios are similar for the reactions in the two solvent systems, while $k_1$ values are larger for the reactions in 20 mol % DMSO than for those in 44 wt % EtOH, indicating that the larger $k_1$ is mainly responsible for the larger $k_N$. It has been suggested that the transition state is more stabilized in 20 mol % DMSO through mutual polarizability interaction than in 44 wt % EtOH through H-bonding interaction.

• KSTLE International Journal
• /
• 제7권1호
• /
• pp.18-21
• /
• 2006

The electrorheology of hollow PANI derivative suspensions in silicone oil was investigated by varying the electric fields and shear rates, respectively. The hollow PANI derivative susepnsions showed a typical electrorheological (ER) response caused by the polarizability of an amide polar group and shear yield stress due to the formation of chains upon application of an electric field. The shear stress for the hollow PANI succinate suspension exhibited an electric field power of 0.67. On the basis of the experimental results, the newly synthesized hollow PANI derivative suspensions were found to be an anhydrous ER fluid.

• Bulletin of the Korean Chemical Society
• /
• 제19권5호
• /
• pp.561-564
• /
• 1998

Solvent effects on the reactions of DANSYL and BANSYL chlorides with substituted pyridines have been investigated using two parameters of Taft's solvatochromic correlation and four parameters of Kirkwood-Onsager, Parker, Marcus, Hildebrand equation. The acetonitrile molecules accelerate charge separation of the reactants and stabilize the transition state. The coefficient of the solvent parameters provide a good information to predict and to analyze the reaction mechanism. The nucleophilic substitution reaction of DANSYL and BANSYL chlorides with substituted pyridines are ruled by the contribution of the change in dipole moment term and polarity-polarizability term.