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首页> 外文期刊>Indian Journal of Chemistry, Section A. Inorganic, Physical, Theoretical & Analytical >Solvent effects on ester linkage of 4-nitrophenyl acetate in aqueous and ethanol solutions with imidazole and hydroxide ion as nucleophiles
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Solvent effects on ester linkage of 4-nitrophenyl acetate in aqueous and ethanol solutions with imidazole and hydroxide ion as nucleophiles

机译：以咪唑和氢氧离子为亲核试剂对乙酸4-硝基苯酯在水溶液和乙醇溶液中酯键的影响

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摘要

The reaction of 4-nitrophenyl acetate in aqueous and ethanol solutions with imidazole as a nucleophile has been monitored spectrophotometrically. The second order rate constants of these reactions are 10 times higher in water than in alcoholic solutions. This is attributed to better solvation of the initial state and less solvation of the excited state in the alcohol medium. The entropy of activation in water is more negative indicating the greater structuredness of the excited state in water. In addition to the above, the base hydrolysis reaction of the ester using OH~- as a nucleophile in buffered aqueous solutions has been followed spectrophotometrically as a function of pH. The observed pseudo first order rate constant obeys the relationship k_(obs)=k_o+k_(OH) [OH~-] where k_o represents the water reactions and the buffer dependent rate constant, and k_(OH) is the rate constant for the OH~- catalyzed (specific base) reaction. For both cases, a mechanism involving a tetrahedral intermediate and in which the nucleophile attacks at the electrophillic carbon of the ester C=O is proposed.
机译：分光光度法已监测了乙酸4-硝基苯酯在水溶液和乙醇溶液中与咪唑作为亲核试剂的反应。这些反应的二级常数在水中比在酒精溶液中高10倍。这归因于在醇介质中更好的初始状态的溶剂化和更少的激发态的溶剂化。水中的活化熵更负，表明水中的激发态具有更大的结构性。除上述以外，还使用分光光度法根据pH值，对在缓冲水溶液中使用OH-作为亲核试剂的酯进行了碱水解反应。观测到的伪一级速率常数服从关系式k_（obs）= k_o + k_（OH）[OH〜-]，其中k_o代表水反应和依赖于缓冲液的速率常数，而k_（OH）是OH〜-催化（特定的碱）反应。对于这两种情况，提出了一种涉及四面体中间体并且亲核试剂攻击酯C ＝ O的亲电子碳的机理。

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