Hydrolysis of aryl and alkyl isothiocyanates in aqueous perchloric acid
Hydrolysis of aryl and alkyl isothiocyanates in aqueous perchloric acid
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异硫氰酸芳基酯和异硫氰酸烷基酯在高氯酸水溶液中的水解
DOI：
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发表时间：
1992
期刊：
影响因子：
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通讯作者：
W. N. Wassef
中科院分区：
文献类型：
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作者：
V. Joseph;D. Satchell;R. S. Satchell;W. N. Wassef
添加高氯酸可促进芳香族和脂肪族异硫氰酸酯在水中的缓慢水解。水解首先导致硫代氨基甲酸，但这种物质迅速分解为（质子化的）胺，并且通常不被检测到。当[HClO 4] ≤ 6.0 mol dm-3时，在50 °C下获得方便的水解速率。取代基，温度和酸浓度对所观察到的速率常数的影响进行了研究。脂肪族异硫氰酸酯比芳香族衍生物具有更高的反应活性，但取代基变化的影响通常较小，电子释放有利于反应。靠近氮原子的取代基阻碍反应。ΔS ε的值通常为-120至-220 J K-1 mol-1，通过过量酸度方法分析酸度依赖性显示m ε = 0.8。提出了水对异硫氰酸酯NC双键的加成反应是通过同时进行质子转移到氮原子上和水对具有环状过渡态的碳原子进行亲核攻击的机理进行的。当[HClO 4] > 9.0 mol dm-3时，由脂肪族异硫氰酸酯形成的氨基甲酸具有足够的碱性，以使它们以其质子化形式被越来越多地捕获。
The slow hydrolysis of aromatic and aliphatic isothiocyanates in water is promoted by added perchloric acid. The hydrolysis leads first to the thiocarbamic acid, but this species decomposes rapidly to the (protonated) amine, and is not normally detected. Convenient rates of hydrolysis are obtained at 50 °C when [HClO4]≳ 6.0 mol dm–3. The effects of substituents, temperature, and acid concentration on the observed rate constant have been studied. Aliphatic isothiocyanates are somewaht more reactive than aromatic derivatives, but the effect of substituent changes is generally small, with electron release favouring reaction. Substituents close to the nitrogen atom hinder reaction. The value of ΔS‡ is typically –120 to –220 J K–1 mol–1, and analysis of the acidity dependence by the excess acidity approach shows m‡≃ 0.8. Addition of water to the isothiocyanate NC double bond via a mechanism invloving simultaneous proton transfer to nitrogen and nucleophilic attack by water at carbon with a cyclic transition state is proposed. The carbamic acids formed by the aliphatic isothiocyanates are sufficiently basic for them to be increasingly trapped as their protonated forms when [HClO4] > 9.0 mol dm–3.