利用縮水的方法可以將羧酸轉(zhuǎn)變?yōu)楦鞣N羧酸的衍生物，如酰氯、酸酐、酯、酰胺和腈等，反過來通過水解反應(yīng)各種羧酸衍生物也可以轉(zhuǎn)變?yōu)轸人幔Ｂ群退狒?，酯?酰胺和腈的水解一般都可以在酸或堿的催化下進(jìn)行。
羧酸衍生物水解的速度與羰基碳原子的正電性有關(guān)，一般來說正電性越大，水解速度越快。
酰氯的水解
把羧酸制成酰氯，再通過酰氯去進(jìn)行各種反應(yīng)是有機(jī)合成中常用的方法，較少有用酰氯來制備羧酸的例子，但在有些條件下利用酰氯來制羧酸也是較為有效的。例如芳環(huán)通過佛克反應(yīng)與草酰氯反應(yīng)得到相應(yīng)的酰氯。一般來講脂肪酰氯的水解是相當(dāng)快的，不用加任何催化劑即可反應(yīng)。但芳香酰氯的反應(yīng)要相對(duì)慢了很多，有時(shí)為了促進(jìn)其水解需要加入堿和酸。
酰氯的水解示例
Aluminum chloride (160 g, 1.2 mol) was added to a stirred solution of the 4-n-pentylbiphenyl (224 g, 1 mol) in methylene chloride (1000 mL) at approximately -20°C. Then Oxalyl chloride (96 mL) was added to the solution at a temperature below -10°C. After addition, the reaction temperature was allowed to rise to approximately 0°C. Stirring continued at approximately 0°C untill the evolution of hydrogen chloride was ceased. The resulting reaction mixture was poured into ice water forming two nonmiscible fluid layers. The organic layer was seperated from the aqueous layer, washed with water, and dried over anhydrous sodium sulfate. The solvent was removed by distillation to yield a crude 4-n-pentylbiphenyl-4’-carboxy chloride (213 g, 75%) which was used directly in the next step.
The 4-n-pentylbiphenyl-4’-carboxy chloride (142 g, 0.5 mol) was reacted with a hot, stirred solution of sodium hydroxide (60 g, 0.75 mol) in water (50 mL) and acetone (500 mL) for about three hours. The cold reaction mixture was then acidified with concentrated hydrochloric acid solution. The acid was filtered and washed with water untill the filtrate was neutralized. The product of the reaction was purified from ethanol to yield 4-n-pentylbiphenyl-4’-carboxylic acid (102.4 g, 80%).
