Nucleophilic Addition of HCN to Carbonyl Compounds | 羰基化合物与氰化氢的亲核加成

📚 Nucleophilic Addition of HCN to Carbonyl Compounds | 羰基化合物与氰化氢的亲核加成

Nucleophilic addition is a characteristic reaction of aldehydes and ketones. When hydrogen cyanide, HCN, is added to a carbonyl compound, the product is a hydroxynitrile (cyanohydrin), which contains both an -OH group and a -CN group on the same carbon. This reaction is important because it extends the carbon chain by one carbon atom and introduces a versatile nitrile functional group.

亲核加成是醛和酮的特征反应。当氰化氢（HCN）与羰基化合物加成时，产物是羟基腈（氰醇），它同时在同一个碳原子上含有一个 -OH 基和一个 -CN 基。该反应很重要，因为它使碳链延长一个碳原子，并引入用途广泛的腈基官能团。

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1. Carbonyl Polarity and the Cyanide Nucleophile | 羰基极性与氰根亲核试剂

The carbon-oxygen double bond in aldehydes and ketones is polar because oxygen is more electronegative than carbon. The carbonyl carbon therefore carries a partial positive charge (δ⁺), making it electrophilic and susceptible to attack by nucleophiles. The cyanide ion, CN⁻, is a strong nucleophile because its carbon atom bears a lone pair and a negative charge.

醛和酮中的碳氧双键具有极性，因为氧的电负性大于碳。因此，羰基碳带部分正电荷（δ⁺），具有亲电性，容易受到亲核试剂的进攻。氰根离子 CN⁻ 是一种强亲核试剂，因为其碳原子上带有孤对电子和负电荷。

HCN + H₂O ⇌ H₃O⁺ + CN⁻

In aqueous solution, hydrogen cyanide is a weak acid and exists in equilibrium with cyanide ions, which act as the attacking species.

在水溶液中，氰化氢是一种弱酸，与氰根离子处于平衡状态，氰根离子是实际进攻的亲核物种。

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2. Reagents and Reaction Conditions | 试剂与反应条件

The reagent is hydrogen cyanide, but because HCN is a highly toxic gas, it is usually generated in the reaction mixture from sodium cyanide or potassium cyanide and dilute sulfuric acid. The reaction is carried out at room temperature in a fume cupboard with careful control of pH.

试剂是氰化氢，但由于 HCN 是剧毒气体，通常由氰化钠或氰化钾与稀硫酸在反应混合物中就地生成。反应在室温下于通风橱中进行，并严格控制 pH。

NaCN + H₂SO₄ → HCN + NaHSO₄

In the laboratory, KCN or NaCN is dissolved in water and acidified with dilute H₂SO₄ to produce HCN in situ. The in situ generation keeps the concentration of HCN low and minimises the risk of escape of toxic gas.

实验室中，将 KCN 或 NaCN 溶于水，用稀 H₂SO₄ 酸化，即就地生成 HCN。就地生成可保持 HCN 浓度较低，并最大程度地减少有毒气体逸出的风险。

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3. Why Cyanide Salts Are Used | 为何使用氰化物盐

Hydrogen cyanide boils at about 26 °C, so it is a gas at room temperature and is extremely poisonous. Therefore, it is unsafe to use directly in a school or college laboratory. Adding dilute acid to NaCN or KCN produces small amounts of HCN only as needed, and the cyanide ion required for the reaction is already present in solution.

氰化氢的沸点约为 26 °C，因此在室温下为气体，且剧毒。因此，在学校或学院实验室中直接使用是不安全的。向 NaCN 或 KCN 中加入稀酸只会在需要时产生少量 HCN，而反应所需的氰根离子已经存在于溶液中。

KCN → K⁺ + CN⁻

The cyanide salt also provides a controlled source of CN⁻ without requiring bottles of gaseous HCN.

氰化物盐还可提供可控的 CN⁻ 来源，无需使用瓶装气态 HCN。

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4. Stepwise Nucleophilic Addition Mechanism | 分步亲核加成机理

The mechanism proceeds in two stages. First, the lone pair on the carbon atom of CN⁻ forms a bond with the δ⁺ carbonyl carbon. At the same time, the π electrons of the C=O bond move onto the oxygen atom, giving the oxygen a negative charge. This produces a tetrahedral alkoxide intermediate.

该机理分两步进行。首先，CN⁻ 碳原子上的孤对电子与 δ⁺ 羰基碳成键。同时，C=O 键的 π 电子移到氧原子上，使氧带负电荷。这生成了四面体结构的烷氧负离子中间体。

R₂C=O + CN⁻ → R₂C(O⁻)CN

The intermediate is a negatively charged alkoxide joined to a nitrile group. Because the carbonyl group is planar, the cyanide ion may attack from either side of the plane.

中间体是与腈基相连的带负电的烷氧负离子。由于羰基是平面结构，氰根离子可以从平面的任意一侧进攻。

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5. Protonation and Product Formation | 质子化与产物生成

In the second stage, the alkoxide intermediate is protonated. The proton is supplied by HCN or by H₃O⁺ present in the acidic mixture. The oxygen atom gains a hydrogen atom and becomes an -OH group, forming the neutral hydroxynitrile.

在第二步中，烷氧负离子中间体被质子化。质子由 HCN 或酸性混合物中的 H₃O⁺ 提供。氧原子得到一个氢原子，变成 -OH 基团，生成中性的羟基腈。

R₂C(O⁻)CN + HCN → R₂C(OH)CN + CN⁻

R₂C(O⁻)CN + H⁺ → R₂C(OH)CN

If HCN donates the proton, a cyanide ion is regenerated, so the process can in principle continue. In practice, the overall equation is usually written with HCN as the reagent.

如果由 HCN 提供质子，则会再生一个氰根离子，因此原理上该过程可持续进行。实际上，总方程式通常以 HCN 作为试剂来书写。

R₂C=O + HCN → R₂C(OH)CN

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6. Naming Hydroxynitriles | 羟基腈的命名

The products are named as hydroxynitriles. The longest continuous carbon chain must include the nitrile carbon, and the -OH group is indicated by the prefix hydroxy with the lowest possible locant. For example, ethanal plus HCN gives 2-hydroxypropanenitrile, not 2-hydroxyethanenitrile, because the nitrile carbon counts as part of the longest chain.

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

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