双语化学辞典Chem Lexicon
反应 · 有机化学 · 羰基化学 Organic chemistry · Carbonyl chemistry

碘仿反应Haloform reaction

拼音Pinyin diǎn fǎng fǎn yìng　别名Also 卤仿反应；iodoform test； Lieben haloform reaction

定义Definition

卤仿反应是甲基酮（RCOCH₃）或能被氧化成甲基酮的醇（RCH(OH)CH₃）在碱性卤素溶液中，甲基被完全卤代后断裂，生成卤仿（CHX₃）和少一个碳的羧酸盐的反应。用碘时生成黄色有特殊气味的碘仿沉淀，称碘仿试验。

In the haloform reaction a methyl ketone (RCOCH₃), or an alcohol RCH(OH)CH₃ that can be oxidised to one, is fully halogenated at the methyl group in basic halogen solution and then cleaved, giving a haloform (CHX₃) and a carboxylate with one carbon fewer. With iodine the yellow, sharp-smelling precipitate of iodoform forms — the iodoform test.

机理Mechanism

碱夺取甲基 α-H 生成烯醇负离子，与 X₂ 反应生成 α-卤代酮。卤素吸电子使剩余 α-H 酸性增大，所以第二、第三次卤代更快，得到 RCOCX₃。OH⁻ 加成到羰基，CX₃⁻ 作为离去基团离去（被三个卤素稳定），质子交换后得到 CHX₃ 和 RCOO⁻。

Base removes an α-H of the methyl group; the enolate reacts with X₂ to give an α-halo ketone. Each halogen acidifies the remaining α-H, so the second and third halogenations are faster, giving RCOCX₃. Hydroxide adds to the carbonyl and CX₃⁻, stabilised by three halogens, leaves; proton exchange gives CHX₃ and RCOO⁻.

由来Origin

1822 年塞吕拉斯由乙醇、碘和碱得到碘仿；1870 年奥地利化学家阿道夫·利本把它发展为检验乙醇和含 CH₃CO– 化合物的方法。

Sérullas obtained iodoform from ethanol, iodine and alkali in 1822; in 1870 the Austrian chemist Adolf Lieben developed it into a test for ethanol and CH₃CO– compounds.

鉴定与合成Diagnosis and synthesis

阳性物质：乙醛、甲基酮、乙醇和仲醇 RCH(OH)CH₃；甲醇、丙醛、3-戊酮阴性。合成上用于把甲基酮转化为羧酸（如次氯酸钠氧化）。

Positive: acetaldehyde, methyl ketones, ethanol and secondary alcohols RCH(OH)CH₃; negative: methanol, propanal, 3-pentanone. Synthetically it converts methyl ketones into carboxylic acids (e.g. with bleach).

反应Reactions

RCOCH3 + 3 X2 + 4 OH− ⟶ RCOO− + CHX3↓ + 3 X− + 3 H2O

I₂/KI 与 NaOH 水溶液，温热

I₂/KI with aqueous NaOH, warm

  1. 甲基逐次卤代为 –CX₃。

    Successive halogenation of the methyl group to –CX₃.

  2. OH⁻ 加成到羰基形成四面体中间体。

    Hydroxide adds to the carbonyl.

  3. CX₃⁻ 离去，质子交换得卤仿和羧酸根。

    CX₃⁻ departs; proton exchange gives haloform and carboxylate.

CH3COCH3 + 3 I2 + 4 NaOH ⟶ CH3COONa + CHI3↓ + 3 NaI + 3 H2O

丙酮生成黄色碘仿沉淀。

Acetone gives a yellow iodoform precipitate.

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

OH⁻ 夺取甲基酮的 α-H

OH⁻ removes an α-H of the methyl ketone

2

烯醇负离子被卤化；每引入一个 X，剩下的 H 更酸，直到 CX₃

The enolate is halogenated; each X makes the remaining H more acidic, up to CX₃

再重复两次twice more
3

OH⁻ 进攻羰基

OH⁻ attacks the carbonyl

4

CBr₃⁻ 作为离去基团离开

CBr₃⁻ leaves

质子交换H⁺ transfer
5

溴仿 + 乙酸根；用 I₂ 得黄色碘仿沉淀（碘仿试验）

Bromoform + acetate; with I₂, yellow iodoform precipitates (iodoform test)

年表Timeline

  • 1822

    塞吕拉斯首次制得碘仿

    Sérullas first prepares iodoform

  • 1870

    利本把碘仿反应用于鉴定

    Lieben uses the iodoform reaction as a test