双语化学辞典Chem Lexicon
技术 · 有机化学 · 官能团检验 Organic chemistry · Functional-group tests

卢卡斯试剂Lucas' reagent

拼音Pinyin lú kǎ sī shì jì　别名Also 卢卡斯试验； Lucas test

定义Definition

卢卡斯试剂是无水氯化锌的浓盐酸溶液，用来区别伯、仲、叔醇（限于能溶于试剂的低级醇，约 6 个碳以下）：醇转化为不溶的氯代烷，使溶液变浑浊或分层；叔醇立即浑浊，仲醇约 3–5 分钟，伯醇室温下长时间不变。

Lucas' reagent is anhydrous zinc chloride dissolved in concentrated hydrochloric acid, used to distinguish primary, secondary and tertiary alcohols that dissolve in it (up to about six carbons). The alcohol is converted to an insoluble alkyl chloride that clouds the solution: tertiary alcohols immediately, secondary within about 3–5 minutes, primary not appreciably at room temperature.

原理Principle

ZnCl₂ 作路易斯酸与羟基氧配位，使 –OH 变为好的离去基团。叔醇、仲醇经 SN1 机理生成碳正离子，速率由碳正离子稳定性决定（叔 > 仲 ≫ 伯），因此浑浊出现的快慢反映醇的级数。

ZnCl₂, a Lewis acid, coordinates the hydroxyl oxygen and turns –OH into a good leaving group. Tertiary and secondary alcohols react by SN1 via carbocations, so the rate follows carbocation stability (3° > 2° ≫ 1°) and the time to turbidity reveals the alcohol class.

由来Origin

美国化学家霍华德·卢卡斯于 1930 年提出。

Introduced by the American chemist Howard Lucas in 1930.

局限Limitations

只适用于在试剂中可溶的醇；烯丙醇、苄醇虽是伯醇也会迅速反应（碳正离子稳定）。现代结构鉴定多用光谱方法。

Only for alcohols soluble in the reagent; allylic and benzylic primary alcohols also react quickly (stabilised cations). Spectroscopy has largely replaced the test.

反应Reactions

R3C–OH + HCl ZnCl2⟶ R3C–Cl不溶，浑浊+ H2OR3C–OH + HCl ZnCl2⟶ R3C–Cl(, )+ H2O

室温，样品少量加入试剂中振荡观察

Room temperature; shake a little sample with the reagent and watch

  1. ZnCl₂ 与羟基配位活化。

    ZnCl₂ coordinates and activates the hydroxyl.

  2. C–O 键断裂生成碳正离子（SN1）。

    C–O heterolysis gives a carbocation (SN1).

  3. Cl⁻ 与碳正离子结合得到不溶的氯代烷。

    Chloride captures the cation, giving an insoluble alkyl chloride.

(CH3)3COH ⟶ (CH3)3CCl立即浑浊(CH3)3COH ⟶ (CH3)3CCl

叔丁醇立即反应。

tert-Butanol reacts at once.

反应机理图Mechanism

红色弯箭头表示一对电子的移动方向：从电子来源（孤对电子或化学键）指向电子去向。Red curly arrows show where an electron pair moves: from its source (lone pair or bond) to its destination.

1

ZnCl₂/HCl 使羟基质子化（ZnCl₂ 也可与 O 配位）

HCl/ZnCl₂ protonates (or Zn coordinates) the OH

2

失水生成碳正离子：叔 > 仲 ≫ 伯，决定变浑浊的快慢

Water leaves to give a carbocation: 3° > 2° ≫ 1°, which sets how fast it turns cloudy

慢slow
3

Cl⁻ 与碳正离子结合

Cl⁻ captures the carbocation

快fast
4

氯代烃不溶于试剂，溶液变浑浊

The alkyl chloride is insoluble, so the solution turns cloudy

年表Timeline

  • 1930

    卢卡斯发表该试验

    Lucas publishes the test