• Bulletin of the Korean Chemical Society
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• v.17 no.6
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• pp.520-524
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• 1996

Solvolyses of phenyl chloroformate in water, D2O, CH3OD, 50% D2O-CH3OD, and in aqueous binary mixtures of acetone, ethanol and methanol are investigated at 25.0 ℃. Product selectivities are reported at 25 ℃ for a wide range of ethanol-water and methanol-water solvent compositions. The Grunwald-Winstein plots of first-order rate constants for phenyl chloroformate with YCl (based on 2-adamantyl chloride) show marked dispersions into three separate lines for the three aqueous mixtures with a small m value (m< 0.2) and a rate maximum for aqueous alcohol solvents. Third-order rate constants, kww, kaw, kwa and kaa were calculated from the observed kww and kaa values together with kaw and kwa calculated from the intercept and slope of the plot of 1/S vs. [alcohol]/[water]. The calculated rate constants, kcalc and mol % of ester agree satisfactorily with those of the observed rate constants, kobs and mol % of ester, supporting the stoichiometric solvation effect analysis. The kinetic solvent isotope effects determined in water and methanol are consistent with the proposed mechanism of the general base catalyzed and/or carbonyl addition for phenyl chloroformate solvolyses based on mass law and stoichiometric solvation effect studies.

• Journal of Life Science
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• v.8 no.4
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• pp.410-415
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• 1998

Procine pepsin hydrolysis of hexapeptide L-S-pNF-Nle-A-OMe in the presence of dipeptide L-L generates a new peak on HPLC analysis of reaction mixtures that is not seen when enzyme is incubated with either peptide alone. The peaks can be detected spectroscopically at either 214 or 254 nm, the latter consistent with a new peptide containing the p-nitro-F residue. The data suggest acyl transpeptidation between E(L-S-pNF) and L-L to form L-S-pNF-L-L. Consistent with this inference are (1) the ability of L-L-NH$_{2}$ and inability of Boc-L-L to undergo a similar transpeptidation reaction, and (2) the data from electrospray mass spectrum. This synthesis requires that Nle-A-L-OMe be released before L-S-pNF, an order opposite to that proposed on the basis of product inhibition kinetics. Consistent with this inference are reciprocal solvent isotope effects ; normal isotope effects of 1.736$\pm$0.121 on the formation of Nle-A-L-OMe and 2.281$\pm$0.184 in the formation of L-S-pNF, coupled to an inverse isotope effects of 0.576$\pm$0.045 on the formation of L-S-pNF-L-L. Because transpeptidation precedes faster in D$_{2}$O, the isotopically-sensitive step must occur after release of Nle-A-L-OMe. Isotopically-enhanced transpeptidation is consistent with the Uni-Bi iso memchanism postulated on the basis of an isotope effects on Vmax but not on Vmax/Km$^{1)}$ and confirmed by isotope effects on the onset of inhibition by pepstatin$^{2)}$.

• Bulletin of the Korean Chemical Society
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• v.28 no.12
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• pp.2377-2381
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• 2007

Solvolyses of 3,4-dimethoxybenzenesulfonyl chloride (DSC) in water, D2O, CH3OD, and in aqueous binary mixtures of acetone, acetonitrile, 1,4-dioxane, ethanol, methanol, and 2,2,2-trifluoroethanol (TFE) have been investigated at 25.0 oC. Kinetic solvent isotope effects (KSIE) in water and in methanol and product selectivities in alcohol-water mixtures are also reported. The Grunwald-Winstein plot of first-order rate constants for the solvolyic reaction of DSC with YCl shows marked dispersions into separated lines for various aqueous mixtures. With use of the extended Grunwald-Winstein equation, the l and m values obtained are 1.12 and 0.58 respectively for the solvolyses of DSC. The relatively large magnitude of l is consistent with substantial nucleophilic solvent assistance. From Grunwald-Winstein plots the rate data are dissected approximately into contributions from two competing reaction channels. This interpretation is supported for alcohol-water mixtures by the trends of product selectivities, which show a maximum for ethanol-water mixtures. From the KSIE of 1.45 in methanol, it is proposed that the reaction channel favored in methanolwater mixtures and in all less polar media is general-base catalysed and/or is possibly (but less likely) an addition-elimination pathway. Also, the KISE value of 1.35 for DSC in water is expected for SN2-SN1 processes, with minimal general base catalysis, and this mechanism is proposed for solvolyses in the most polar media.

• Bulletin of the Korean Chemical Society
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• v.19 no.9
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• pp.968-973
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• 1998

Solvolyses of para-substituted phenyl chloroformates in water, $D_2O,\;CH_3OD$, 50% $D_2O-CH_3OD$, and in aqueous binary mixtures of acetone, ethanol, methanol are investigated at 25.0 ℃. Product selectivities are reported for a wide range of ethanol-water and methanol-water solvent compositions. These data are interpreted using the Grunwald-Winstein relationship, Hammett equation, and quantum mechanical model. Grunwald-Winstein plots of the first-order rate constants for phenyl chloroformates with $Y_{Cl}$ (based on 1-adamantyl chloride) show marked dispersions into three separate curves for the three aqueous mixtures with a small m value and a rate maximum for aqueous alcohol solvents. To account for these results, third-order rate constants, $k_{ww},\;k_{aw},\;k_{wa}$, and $k_{aa}$ were calculated from the observed $k_{ww}\;and\;k_{aa}$ values together with $k_{aw}\;and\;k_{wa}$ calculated from the computer fit. The kinetic solvent isotope effects determined in water and methanol are consistent with the proposed mechanism of the general base catalyzed carbonyl addition-elimination for para-substituted phenyl chloroformates solvolyses based on mass law and stoichiometric salvation effect studies. This study has shown that the quantum mechanical model predicts transition state variation correctly for $S_N2\;like\;S_AN$ reaction mechanism of para-substituted phenyl chloroformates.

• Bulletin of the Korean Chemical Society
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• v.29 no.10
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• pp.1911-1914
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• 2008

Second-order rate constants (kN) have been measured for Michael-type addition reactions of a series of alicyclic secondary amines to 1-phenyl-2-propyn-1-one (2) in MeCN at 25.0 ${\pm}$ 0.1 ${^{\circ}C}$. All the amines studied are less reactive in MeCN than in $H_2O$ although they are more basic in the aprotic solvent by 7-9 p$K_a$ units. The Bronsted-type plot is linear with $\beta_{nuc}$ = 0.40, which is slightly larger than that reported previously for the corresponding reactions in $H_2O$ ($\beta_{nuc}$ = 0.27). Product analysis has shown that only E-isomer is produced. Kinetic isotope effect is absent for the reactions of 2 with morpholine and deuterated morpholine (i.e., $k^H/k^D$ = 1.0). Thus, the reaction has been concluded to proceed through a stepwise mechanism, in which proton transfer occurs after the rate-determining step. The reaction has been suggested to proceed through a tighter transition state in MeCN than in H2O on the basis of the larger $\beta_{nuc}$ in the aprotic solvent. The nature of the transition state has been proposed to be responsible for the decreased reactivity in the aprotic solvent.

• Bulletin of the Korean Chemical Society
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• v.26 no.11
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• pp.1761-1766
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• 2005

Solvolysis rates of isopropenyl chloroformate (3) in water, $D_2O$, $CH_3OD$ and in aqueous methanol, ethanol, 2,2,2-trifluoroethanol (TFE), acetone, 1,4-dioxane as well as TFE-ethanol at 10 ${^{\circ}C}$ are reported. Additional kinetic data for pure water, pure ethanol and 80%(w/w) 2,2,2-trifuoroethanol (T)-water (W) at various temperatures are also reported. These rates show the phenomena of maximum rates in specific solvents (30% (v/v) methanol-water and 20% (v/v) ethanol-water) and, variations in relative rates are small in aqueous alcohols. The kinetic data are analyzed in terms of GW correlations, steric effect, kinetic solvent isotope effects (KSIE), and a third order model based on general base catalysis (GBC). Solvolyses based on predominately stoichiometric solvation effect relative to medium solvation are proceeding in 3 and the results are remarkably similar to those for p-nitrobenzoyl chloride (4) in mechanism and reactivity.

• Applied Chemistry for Engineering
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• v.8 no.5
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• pp.777-783
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• 1997

$TiO_2$ powders were synthesized via hydrolysis reaction of titanium n-propoxide in n-propanol solvent and the reaction rates were studied by use of UV-vis spectroscopic method. Concentration of water, reaction temperature, reaction time and acid-base effects of the solution were investigated to determine the optimum conditions for $TiO_2$ powder synthesis. The reaction were controlled to proceed to pseudo-first orders reaction in the presence of excess water in n-propanol solvent. The rate constants which varied with temperature and concentration of water were calculated by Guggenheim method. Reaction using $D_2O$ was also carried out to determine the catalytic character of water. $TiO_2$ powders were synthesized only in the neutral and basic solution and those were almost spheric forms having average particle size of $0.4-0.7{\mu}m$ diameter. Particle size decreased with increasing concentration of water and reaction temperature, however, increased with increasing reaction time. Associative $S_N2$ mechanism for the hydrolysis was proposed from the data of n-value in the transition state and thermodynamic parameter. $D_2O$ solvent isotope effect showed that $H_2O$ molecules reacted as nucleophilic catalysis.

• Bulletin of the Korean Chemical Society
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• v.21 no.10
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• pp.955-956
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• 2000

Solvolyses of methanesulfonyl chloride in water, $D^2O$, $CH^3OD$, and in aqueous binary mixtures of acetone, eth-anol and methanol are investigated at 25, 35 and $45^{\circ}C.$ The Grunwald-Winstein plot of first-order rate con-stants for the solvolytic react ion of methanesulfonyl chloride with YCl (based on 2-adamantyl chloride) shows marked dispersions into three separate lines for three aqueous mixtures with a small m value (m < 0.30), and shows a rate maximum for aqueous alcoholic solvents. Stoichiometric third-order rate constants, kww and kaa were calculated from the observed first-order rate constants and (kaw + kwa) was calculated from the kww and kaa values. The kinetic solvent isotope effects determined in water and methanol are consistent with the proposed mechanism of the general base catalyzed and/or SAN/SN2 reaction mechanism for methanesulfonyl chloride solvolyses based on mass law and stoichiometric solvation effect studies.

• Bulletin of the Korean Chemical Society
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• v.33 no.11
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• pp.3544-3550
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• 2012

Cetrizine dihydrochloride (CTZH) is widely used as an anti-allergic drug. Sodium N-bromo-p-toluenesulfonamide or bromamine-T (BAT) is the bromine analogue of chloramine-T (CAT) and is found to be a better oxidizing agent than CAT. In the present research, the kinetics of oxidation of CTZH with BAT in acid and alkaline media was studied at 313 K. The experimental rate laws obtained are: -d[BAT]/dt=$k[BAT][CTZH]^{0.80}[H^+]^{-0.48}$ in acid medium and -d[BAT]/dt=$k[BAT][CTZH]^{0.48}[OH^-]^{0.52}[PTS]^{-0.40}$ in alkaline medium where PTS is p-toluenesulfonamide. Activation parameters and reaction constants were evaluated. The solvent isotope effect was studied using $D_2O$. The dielectric effect is positive. The stoichiometry of the reaction was found to be 1:1 and the oxidation products were identified as 4-chlorobenzophenone and (2-piperazin-1-yl-ethoxy)-acetic acid in both media. The rate of oxidation of CTZH is faster in acid medium. Suitable mechanisms and related rate laws have been worked out.

• Bulletin of the Korean Chemical Society
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• v.30 no.9
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• pp.1939-1945
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• 2009

Kinetic investigations on the oxidation of pyrazine and four 2-substituted pyrazines viz., 2-methylpyrazine, 2-ethylpyrazine, 2-methoxypyrazine and 2-aminopyrazine by bromamine-B (BAB) to the respective N-oxides have been studied in HCl$O_4$ medium at 303 K. The reactions show identical kinetics being first-order each in $[BAB]_o\;and\;[pyrazine]_o$, and a fractional- order dependence on $[H^+]$. Effect of ionic strength of the medium and addition of benzenesulfonamide or halide ions showed no significant effect on the reaction rate. The dielectric effect is positive. The solvent isotope effect was studied using $D_2$O. The reaction has been studied at different temperatures and activation parameters for the composite reaction have been evaluated from the Arrhenius plots. The reaction showed 1:1 stoichiometry and the oxidation products of pyrazines were characterized as their respective N-oxides. Under comparable experimental conditions, the oxidation rate of pyrazines increased in the order: 2-aminopyrazine > 2-methoxypyrazine > 2-ethylpyrazine > 2-methylpyrazine > pyrazine. The rates correlate with the Hammett $\sigma$ relationship and the reaction constant $\rho$ was found to be -0.8, indicating that electron donating centres enhance the rate of reaction. An isokinetic temperature of $\beta$ = 333 K, indicated that the reaction was enthalpy controlled. A mechanism consistent with the experimental results has been proposed in which the rate determining step is the formation of an intermediate complex between the substrate and the diprotonated species of the oxidant. The related rate law in consistent with observed results has been deduced.