• Bulletin of the Korean Chemical Society
• /
• v.30 no.12
• /
• pp.2909-2912
• /
• 2009

The amines studied in this study are less reactive toward ethyl propiolate (3) in MeCN than in H$_2$O although they are 7 to 9 pK$_a$ units more basic in the aprotic solvent. The reactivity of morpholine and deuterated morpholine toward 3 is found to be identical, indicating that proton transfer occurs after rate-determining step (RDS). The fact that kinetic isotope effect is absent excludes a stepwise mechanism in which proton transfer occurs in RDS as well as a concerted mechanism in which nucleophilic attack and proton transfer occur concertedly through a 4-membered cyclic transition state (TS). Thus, the reactions have been concluded to proceed through a stepwise mechanism in which proton transfer occurs after RDS. Brønsted-type plots are linear with small ${\beta}_{nuc}$ values, i.e., ${\beta}_{nuc}$ = 0.29 in H$_2$O and ${\beta}_{nuc}$ = 0.51 in MeCN, indicating that bond formation is not advanced significantly in RDS. The small ${\beta}_{nuc}$ value also supports the conclusion drawn from the study of kinetic isotope effect.

• Corrosion Science and Technology
• /
• v.5 no.6
• /
• pp.206-212
• /
• 2006

The effects of sub-ppm levels of $H_2S$ and the adsorbed water on the atmospheric corrosion of silver were studied with In situ weight balance to study the effect of the adsorbed water on the kinetic behavior and to determine the rate-controlling step, with XPS to analyze the tarnish film, and with calculation of phase equilibrium to predict the stable solid phase, the concentrations of dissolved species ($Ag^-$, $H^+$, $S^{2-}$, $HS^-$) and the equilibrium potentials ($E_{Ag^+/Ag}$, $E_{H^+/H_2}$, $E_{O_2/O^{2-}$). The results of weight measurements showed that oxygen was required for the sulfidation of silver in 100 ppb $H_2S$ and humidified environments enhanced the tarnished rate and oxidizing power. In addition, the rate determining step for tarnishing silver was shown to be changed to transport though the tarnish film.

• Bulletin of the Korean Chemical Society
• /
• v.9 no.1
• /
• pp.17-21
• /
• 1988

A mechanistic study on the chemiluminescence resulting from the reaction between bis(2,4,6-trichlorophenyl)oxalate(TCPO) and hydrogen peroxide in the presence of fluorescent polycyclic aromatic hydrocarbons in a viscous phthalate medium has been conducted. The rate determining step, decay rate constants, and relative quantum efficiencies yielded by varying the concentration of reagents generally support an existing mechanism. However, a reaction between TCPO and sodium salicylate was not observed.

• Bulletin of the Korean Chemical Society
• /
• v.10 no.3
• /
• pp.251-254
• /
• 1989

A kinetic study on the chemiluminescence resulting from the reaction between bis(2,4-dinitrophenyl) oxalate(DNPO) and hydrogen peroxide in the presence of fluorescent polycyclic aromatic hydrocarbons in a viscous phthalate medium has been conducted. The resultant data confirm that the reaction between DNPO and $H_2O_2$ is the rate determining step. Higher rate constants are obtained with DNPO than those with bis(2,4,6-trichlorophenyl) oxalate (TCPO).

• Journal of IKEEE
• /
• v.6 no.2 s.11
• /
• pp.111-118
• /
• 2002

In this paper, we present a variable block method to reduce additive computational requirements in determining step-size of variable step-size LMS (VS-LMS) algorithms. The block length is inversely proportional to the changing of step-size in VS-LMS algorithm. The technique reduces computational requirements of the conventional VS-LMS algorithms without a degradation of performance in convergence rate and steady state error. And a method for deriving initial step-size, when the input is zero mean, white Gaussian sequence, is proposed. For demonstrating the good performances of the proposed method, simulation results are compared with the conventional variable step-size algorithms in convergence speed and computational requirements.

• Applied Chemistry for Engineering
• /
• v.26 no.3
• /
• pp.372-376
• /
• 2015

6-MQCC (Cr(VI)-6-methylquinoline) complex was synthesized by the reaction of 6-methylquinoline with chromium(VI) trioxide in 6 M HCl. The structure was characterized using IR (Infrared Spectroscopy) and ICP (Inductively Coupled Plasma) analysis. The oxidation of benzyl alcohol using 6-MQCC in various solvents showed that the reactivity increased with the increase of the dielectric constant, in descending order of DMF > acetone > chloroform > cyclohexene. In the presence of DMF solvent with acidic catalyst such as sulfuric acid ($H_2SO_4$), 6-MQCC oxidized benzyl alcohol (H) and its derivatives ($p-OCH_3$, $m-CH_3$, $m-OCH_3$, m-Cl, $m-NO_2$) were effectively oxidized. Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. The Hammett reaction constant (${\rho}$) was -0.69 (308 K). The observed experimental data was used to rationalize the fact that the hydride ion transfer occurred at the rate-determining step.

• Bulletin of the Korean Chemical Society
• /
• v.31 no.7
• /
• pp.1915-1919
• /
• 2010

Second-order rate constants $(k_N)$ have been measured for reactions of 2,4-dinitrophenyl phenyl carbonate (2) with a series of pyridines in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$ and compared with the $k_N$ values reported for the corresponding reactions of 2,4-dinitrophenyl benzoate (1) to investigate the effect of nonleaving group on reactivity and mechanism. The reactions of 2 result in larger $k_N$ values than those of 1. The Br${\o}$nsted-type plot for the reactions of 2 exhibits a downward curvature (i.e., ${\beta}2$ = 0.84 and ${\beta}1$ = 0.16), which is typical for reactions reported to proceed through a stepwise mechanism with a change in rate-determining step. The $pK_a$ at the center of the Br${\o}$nsted curvature, defined as $pK_a{^{\circ}}$, has been found to be 8.5 and 9.5 for the reactions of 2 and 1, respectively. Dissection of $k_N$ into the microscopic rate constants (e.g., $k_1$ and $k_2/k_{-1}$ ratio) has revealed that the reactions of 2 result in larger k1 values than those of 1, indicating that PhO behaves as a stronger electron-withdrawing group than Ph. However, the $k_2/k_{-1}$ ratio has been found to be independent of the electronic nature of Ph and PhO.

• Applied Chemistry for Engineering
• /
• v.27 no.1
• /
• pp.110-114
• /
• 2016

Cr(VI)-pyrazine complex (PZCC) was synthesized by the reaction of pyrazine with chromium (VI) trioxide in 6 M HCl. The structure was characterized using IR spectroscopy and inductively coupled plasma (ICP). The oxidation of benzyl alcohol using PZCC in various solvents showed that the reactivity increased with the increase of the dielectric constant, in the order: N,N'-dimethylform-amide > acetone > chloroform > cyclohexene. In the presence of N,N'-dimethylformamide solvent with an acidic catalyst such as sulfuric acid ($H_2SO_4$ solution), PZCC oxidized benzyl alcohol (H) and its derivatives ($p-OCH_3$, $m-CH_3$, $m-OCH_3$, m-Cl, $m-NO_2$). Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. Hammett reaction constant (${\rho}$) was -0.70 (308 K). The observed experimental data were used to rationalize the hydride ion transfer in the rate-determining step.

• Bulletin of the Korean Chemical Society
• /
• v.32 no.4
• /
• pp.1165-1169
• /
• 2011

Second-order rate constants ($k_N$) have been measured spectrophotometrically for nucleophilic substitution reactions of 2,4-dinitrophenyl phenyl thionocarbonate 4 with a series of Z-substituted pyridines in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The Br${\o}$nsted-type plot for the reactions of 4 exhibits downward curvature (i.e., ${\beta}_1$ = 0.21 and ${\beta}_2$ = 1.04), indicating that the reactions proceed through a stepwise mechanism with a change in rate-determining step. It has been found that 4 is less reactive than its oxygen analogue, 2,4-dinitrophenyl phenyl carbonate 3, although the thionocarbonate is expected to be more electrophilic than its oxygen analogue. The $pK_a$ at the center of the Br${\o}$nsted curvature, defined as $pK_a^o$, has been analyzed to be 6.6 for the reactions of 4 and 8.5 for those of 3. Dissection of $k_N$ into the microscopic rate constants $k_1$ and $k_2/k_{-1}$ ratio has revealed that the reactions of 4 result in smaller $k_1$ values but larger $k_2/k_{-1}$ ratios than the corresponding reactions of 3. The larger $k_2/k_{-1}$ ratios have been concluded to be responsible for the smaller $pK_a^o$ found for the reactions of 4.

• Bulletin of the Korean Chemical Society
• /
• v.31 no.9
• /
• pp.2593-2597
• /
• 2010

Second-order rate constants ($k_N$) have been measured spectrophotometrically for reactions of 2,4-dinitrophenyl X-substituted benzoates (X = 4-MeO, H and 4-$NO_2$) with a series of Z-substituted pyridines in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The Br${\o}$nsted-type plots exhibit downward curvature (e.g., $\beta_2$ = 0.89 ~ 0.96 when $pK_a$ < 9.5 while $\beta_1$ = 0.38 ~ 0.46 when $pK_a$ > 9.5), indicating that the reaction proceeds through a stepwise mechanism with a change in rate-determining step (RDS). The ${pK_a}^o$, defined as the $pK_a$ at the center of Br${\o}$nsted curvature, has been analyzed to be 9.5 regardless of the electronic nature of the substituent X in the benzoyl moiety. Dissection of $k_N$ into the microscopic rate constants $k_1$ and $k_2/k_{-1}$ ratio has revealed that $k_1$ is governed by the electronic nature of the substituent X but the $k_2/k_{-1}$ 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 $k_2/k_{-1}$ ratio.