• Title/Summary/Keyword: Acetates

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Kinetics and Mechanism of the Aminolysis of Thiophenyl Acetates in Acetonitrile

  • 오혁근;양진희;이해황;이익춘
    • Bulletin of the Korean Chemical Society
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    • v.20 no.12
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    • pp.1418-1420
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    • 1999

  • Kinetics and mechanism of the aminolysis of Z-thiophenyl acetates with X-benzylamines are investigated in acetonitrile at 45.0 ℃. The magnitudes of Bronsted coefficients β$_x$ (=1.3~-1.6) and β$_z$ (= -2.1~-2.4) are all large and cross-interaction constant ρxz is relatively large and positive (0.90). These trends are consistent with the rate-limiting breakdown of a tetrahedral intermediate, $T^±$. The proposed mechanism is also supported by adherence of the rate data to the reactivity-selectivity principle (RSP). The kinetic isotope effects, $k_H/k_D$, are greater than unity (1.3-1.4) suggesting a possibility of hydrogen-bonded four-centered transition state. The activation parameters, ΔH$^≠$ and ΔS$^≠$, are consistent with this transition-state structure.

  • https://doi.org/10.5012/bkcs.1999.20.12.1418 인용 PDF

The Effect of Solvent on the $\alpha$-Effect(3): Nucleophilic Substitution Reactions of Aryl Acetates in $MeCN-H_2O$ Mixtures of Varying Compositions

  • Um Ik-Hwan;Hahn Gee-Jung;Lee Gwang-Ju;Kwon Dong-Song
    • Bulletin of the Korean Chemical Society
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    • v.13 no.6
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    • pp.642-647
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    • 1992

  • Second-order rate constants have been measured spectrophotometrically for the reactions of substituted phenyl acetates with butane-2,3-dione monoximate and p-chlorophenoxide anions in MeCN-H$_2$O mixtures of varying compositions. The reaction rate, unexpectedly, decreased remarkably upon initial additions of MeCN to H$_2$O up to 30-40 mole ${\%}$ MeCN, and followed by a gradual increase upon further additions of MeCN. The change in solvent composition also influenced the magnitude of the ${\alpha}$-effect, i.e., the ${\alpha}$-effect increased as the mole ${\%}$ MeCN increased. The solvent dependent ${\alpha}$-effect for the present system appears to indicate that the differential solvation between the ${\alpha}$-effect nucleophile and the corresponding normal nucleophile is not solely responsible but the difference in the transition-state stabilization is also responsible for the ${\alpha}$ -effect in organic solvent-rich region.

  • https://doi.org/10.5012/bkcs.1992.13.6.642 인용 PDF

Comparison of Physicochemical Properties of Starch Acetates Prepared by Conventional, Preheat Treatment and Extrusion Process (습식법과 예열처리법 및 Extrusion 공정에 의해 제조한 초산전분의 이화학적 성질비교)

  • Kim, Chong-Tai;Ryu, Gi-Hyung;Kim, Dong-Chul;Kim, Chul-Jin
    • Korean Journal of Food Science and Technology
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    • v.22 no.6
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    • pp.659-667
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    • 1990

  • Starch acetates were prepared by conventional method, preheat treatment, and extrusion process through acetylation of corn starch with acetic anhydride and their physicochemical properties were investigated. The optimal conditions of the acetylation of starch by conventional method(CSA) was found that starch concentration was 30%, reaction temperature $35^{\circ}C$ and pH 8.5. With increasing the molar ratio of acetic anhydride to anhydrous glucose unit from 0.03 to 0.20, DS(Degree of substitution) value of corn starch acetate prepared at the optimum condition was increased from 0.019 to 0.080, while the acetylation efficiency was decreased from 31.6% to 20.5%. In the case of the preheated (gelatinized), then acetylated starch(PSA), DS value was increased from 0.027 to 0.04 at the fixed molar ratio of the acetic anhydride with increasing preheating temperature from $60^{\circ}C\;to\;90^{\circ}C$. The DS was low as 0.02 in the case of starch acetate prepared by extrusion process(WESA). The CSA and PSA showed lowering gelatinization temperature and enthalpy than raw corn starch with increasing DS. All of starch acetates showed the increased degree of transparency, the decreased lightness and the increased yellowness as compared to the raw corn starch. WESA showed lower apparent viscosity and more close to the characteristic of the Newtonian fluid than CSA and PSA. Intrinsic viscosity was reduced in CSA and WESA, although PSA has a slightly higher one than raw corn starch. The rate of retrogradation of the gels was retarded in all starch acetates.

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Biotransformation of Exogenous Monoterpenoids by Plant Cell Culture

  • Shin, Seung-Won
    • Korean Journal of Pharmacognosy
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    • v.26 no.3
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    • pp.227-238
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    • 1995

  • Recent reports on biotransformation of monoterpene alchols, aldehydes, acetates and epoxides are summerized. The studies have focused on stereospecific reaction of the functional groups of exogenous foreign substrates by foreign plant cells and micro-organisms. An another important aspect of research is the development of the immobilization technique for cells or related enzymes.

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