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Derivatives of Carboxylic Acids
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Derivatives of Carboxylic Acids 羧酸衍生物 Esters are neutral compounds derived from reaction between a carboxylic acid and an alcohol. The esters of any individual carboxylic acid form a homologous series whose lower members are volatile liquids with pleasant, often fruit-like odours. As the relative molecular mass increases, volatility decreases and the esters gradually become viscous, oily liquids or waxy solids. They are immiscible with water and ethyl ethanoate is widely used as an organic solvent. They are polar compounds with boiling points well below those of carboxylic acids of corresponding relative molecular mass, since there is no intermolecular hydrogen bonding: 酯类是羧酸和醇反应产生的中性化合物。羧酸形成的酯组成一个同系列，其中低分子量的酯类是一种带有令人愉悦气味如水果香味的易挥发性液体。随着相对分子质量的增加，其挥发性降低，酯逐渐变粘而成油状液体或蜡状固体。它们不溶于水，而且乙酸乙酯是一种广泛使用的有机溶剂。它们是一类极性化合物，沸点远低于相对应相同分子量的羧酸，因为它们没有分子间氢键。 Esters occur naturally. Those derived from certain long-chain carboxylic acids (fatty acids) and the alcohol propane-l,2,3-triol (glycerol) are important constituents of vegetable oils and animal fats. Ester polymers, called polyesters, are very important synthetic materials; an example is the synthetic polyester fibre Terylene.The following reactions are typical of esters. 酯是自然产生的。酯从某些长链羧酸（脂肪酸）中获得，丙三醇（甘油）是植物油和动物脂肪的重要成分。酯的聚合物，又叫聚酯，是很重要的合成材料。例如合成聚酯纤维涤纶。以下是酯的典型反应。 1.1 Hydrolysis The hydrolysis of an ester to carboxylic acid and alcohol is the reverse of the esterification process and involves an equilibrium: RCOOR'+H2O == RCOOH + R'OH The composition of the equilibrium mixture is the same, no matter whether one has started from an equimolar mixture of acid and alcohol, or ester and water." Like esterification, the rate of hydrolysis is very slow but hydrolysis is catalysed by acid or alkali. Alkali hydrolysis is usually the more effective method because the salt of the carboxylic acid, rather than the free acid, is obtained, so that the equilibrium position is pushed so much towards the products that reaction is virtually complete. The hydrolysis is normally effected by heating the ester with sodium hydroxide solution under reflux. As hydrolysis proceeds, the separate ester and aqueous layers gradually merge into one. If the alcohol is wanted it can be distilled out of the alkaline mixture; acidification of the residue with sulphuric acid will liberate the free carboxylic acid, which may then be distilled from the solution. The alkaline hydrolysis of esters is sometimes referred to as saponification. because the reaction is used in soap-making. 1.1 水解 酯水解生成羧酸和醇是酯化过程的逆反应，这涉及到一个平衡： RCOOR'+H2O == RCOOH + R'OH 起始的等摩尔混合物无论是羧酸和醇还是酯和水，平衡混合物的组成都是相同的。像酯化反应，水解速率很慢，但可以通过酸或碱来催化。碱水解通常是更有效的方法，因为得到的是羧酸盐而不是游离酸，以致平衡向生成完整产品的方向移动。通常用在回流条件下，加热酯和NaOH溶液的方法来影响水解。当水解进行时，分开的酯和含水层逐步开始混融成一体。如果希望醇从碱性混合物中被分离出来，残渣用硫酸酸化作用将释放出游离羧酸，然后可以从溶液中蒸馏出来。酯的碱性水解有时也称为皂化反应，因为通常用这个反应来制作肥皂。 1.2 Reduction Esters are reduced by lithium tetrahydridoaluminate to primary alcohols corresponding to the acid from which the ester is derived. The other organic product is the alcohol corresponding to the alcohol component of the ester: 1.2还原 酯被氢化锂铝还原为相应于形成酯的羧酸的伯醇。其他有机产品是对应于形成酯的醇部分的醇。 |
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