A-Level Chemistry: Nucleophilic Addition of HCN to Aldehydes and Ketones | A-Level化学：醛酮与HCN的亲核加成反应

📚 A-Level Chemistry: Nucleophilic Addition of HCN to Aldehydes and Ketones | A-Level化学：醛酮与HCN的亲核加成反应

The nucleophilic addition of hydrogen cyanide (HCN) to aldehydes and ketones is a classic reaction in A-Level Chemistry, forming hydroxynitriles (cyanohydrins). This reaction is highly valued in examinations for its mechanism, practical conditions, and synthetic applications.

氰化氢（HCN）与醛酮的亲核加成反应是A-Level化学中的经典反应，生成羟基腈（氰醇）。这一反应因其机理、实验条件及合成应用而在考试中备受重视。

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1. Reaction Overview | 反应概述

HCN adds across the carbon-oxygen double bond of an aldehyde or ketone. The product is a hydroxynitrile, also called a cyanohydrin, containing both a hydroxyl group (-OH) and a nitrile group (-C≡N) on the same carbon atom.

HCN 跨越醛或酮的碳氧双键发生加成。产物为羟基腈，也称氰醇，同一碳原子上同时含有羟基（-OH）和氰基（-C≡N）。

For a general aldehyde RCHO:

对于一般的醛 RCHO：

RCHO + HCN → RCH(OH)CN

For a general ketone R₁COR₂:

对于一般的酮 R₁COR₂：

R₁COR₂ + HCN → R₁R₂C(OH)CN

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2. Why HCN is a Weak Nucleophile | 为什么HCN是弱亲核试剂

HCN is a very weak acid (pKa ≈ 9.2). In its molecular form, it is a poor nucleophile because the cyanide carbon is not strongly electron-rich. However, the cyanide ion CN⁻ is a strong nucleophile. Therefore, the reaction actually proceeds via the cyanide ion, not molecular HCN itself.

HCN 是一种极弱的酸（pKa ≈ 9.2）。在分子状态下，它是较差的亲核试剂，因为氰基碳的电子密度不够高。然而，氰离子 CN⁻ 是强亲核试剂。因此，该反应实际上是通过氰离子进行的，而非 HCN 分子本身。

The equilibrium for the dissociation of HCN in solution is critical:

HCN 在溶液中的解离平衡至关重要：

HCN ⇌ H⁺ + CN⁻

Because the equilibrium lies far to the left, the concentration of CN⁻ is very low. This makes the reaction slow and inefficient if only HCN is used alone.

由于平衡强烈偏左，CN⁻ 的浓度非常低。如果仅使用 HCN，反应会非常缓慢且效率低下。

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3. The Role of a Catalyst | 催化剂的作用

A small amount of potassium cyanide (KCN) or sodium cyanide (NaCN) is added as a source of cyanide ions. These salts dissociate fully in solution, providing a steady concentration of CN⁻, which then acts as the true nucleophile.

通常加入少量氰化钾（KCN）或氰化钠（NaCN）作为氰离子来源。这些盐在溶液中完全电离，提供稳定的 CN⁻ 浓度，CN⁻ 才是真正的亲核试剂。

The catalytic cycle can be understood as follows:

催化循环可理解如下：

• CN⁻ attacks the carbonyl carbon, forming a negatively charged intermediate.
• CN⁻ 进攻羰基碳，形成带负电的中间体。
• The intermediate abstracts a proton (H⁺) from HCN, regenerating CN⁻.
• 中间体从 HCN 夺取质子（H⁺），再生出 CN⁻。

Thus, KCN or NaCN is not consumed overall; it acts as a catalyst.

因此，KCN 或 NaCN 在总体上不被消耗，起到催化剂作用。

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4. Mechanism: Step-by-Step | 反应机理：逐步解析

The mechanism is called nucleophilic addition. Let us examine each step in detail.

该机理称为亲核加成。下面逐步详细分析。

Step 1: Nucleophilic attack — The cyanide ion CN⁻ donates its lone pair of electrons to the partially positive carbonyl carbon atom. The π bond of the C=O group breaks, and the two electrons move to the oxygen atom, forming an alkoxide ion.

第一步：亲核进攻 — 氰离子 CN⁻ 将其孤对电子提供给带部分正电荷的羰基碳原子。C=O 的 π 键断裂，两个电子转移到氧原子上，形成烷氧负离子。

δ+ C=O δ− + CN⁻ → ⁻O−C(C≡N)

Step 2: Protonation — The alkoxide oxygen then abstracts a proton from HCN (or from water in acidic work-up), forming the neutral hydroxynitrile product and regenerating the cyanide catalyst.

第二步：质子化 — 烷氧负离子随后从 HCN（或酸性后处理中的水）夺取一个质子，生成中性羟基腈产物，并再生氰化物催化剂。

⁻O−C(C≡N) + HCN → HOC(C≡N) + CN⁻

Feature | 特点	Explanation | 解释
Nucleophile | 亲核试剂	CN⁻, generated from KCN/NaCN | CN⁻，来自 KCN/NaCN
Substrate | 底物	Aldehydes or ketones with C=O | 含 C=O 的醛或酮
Intermediate | 中间体	Alkoxide ion | 烷氧负离子
Product | 产物	Hydroxynitrile (cyanohydrin) | 羟基腈（氰醇）

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5. Reactivity of Aldehydes vs Ketones | 醛与酮的反应活性比较

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions. There are two main reasons for this difference.

在亲核加成反应中，醛通常比酮更活泼。造成这种差异的原因主要有两点。

• Steric factor: In ketones, the carbonyl carbon is bonded to two alkyl or aryl groups, creating greater steric hindrance. This makes it harder for the nucleophile to approach.
• 电子因素： 酮中羰基碳与两个烷基或芳基相连，空间位阻较大，亲核试剂难以接近。
• Electronic factor: Alkyl groups are electron-donating. In ketones, two alkyl groups donate electron density to the carbonyl carbon, making it less electrophilic. Aldehydes have only one alkyl group (or hydrogen), so the carbonyl carbon is more positive and more attractive to nucleophiles.
• 电子因素： 烷基是给电子基团。酮中两个烷基向羰基碳供给电子密度，使其亲电性减弱。醛仅连接一个烷基（或氢），因此羰基碳正电性更强，对亲核试剂更具吸引力。

Methanal (HCHO) is the most reactive aldehyde because the carbonyl carbon is attached to two hydrogen atoms, offering negligible steric hindrance and no electron-donating alkyl groups.

甲醛（HCHO）是活性最高的醛，因为羰基碳连接两个氢原子，空间位阻极小且没有给电子烷基。

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6. Practice: Specific Examples | 实例练习

Let us write the products for some common reactants.

下面写出一些常见反应物的产物。

Example 1: Ethananal + HCN

示例1： 乙醛 + HCN

CH₃CHO + HCN → CH₃CH(OH)CN

Product: 2-hydroxypropanenitrile.

产物：2-羟基丙腈。

Example 2: Propanone + HCN

示例2： 丙酮 + HCN

CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN

Product: 2-hydroxy-2-methylpropanenitrile.

产物：2-羟基-2-甲基丙腈。

Example 3: Methanal + HCN

示例3： 甲醛 + HCN

HCHO + HCN → HOCH₂CN

Product: 2-hydroxyethanenitrile (hydroxyacetonitrile).

产物：2-羟基乙腈（羟基乙腈）。

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7. Synthetic Importance | 合成意义

The hydroxynitrile product is a versatile intermediate in organic synthesis. The nitrile group can be converted into a carboxylic acid or an amine, which allows chemists to lengthen carbon chains.

羟基腈产物是有机合成中的重要中间体。氰基可转化为羧酸或胺，从而实现碳链增长。

Hydrolysis to carboxylic acid:

水解生成羧酸：

CH₃CH(OH)CN + 2H₂O + H⁺ → CH₃CH(OH)COOH + NH₄⁺

This reaction is especially valuable because the carbonyl carbon of the original aldehyde/ketone becomes a new carboxylic acid carbon, increasing the chain length by one carbon atom.

该反应尤为珍贵，因为原醛/酮的羰基碳变为新的羧酸碳，使碳链增加一个碳原子。

Reduction to amine:

还原生成胺：

CH₃CH(OH)CN + 4[H] → CH₃CH(OH)CH₂NH₂

This route provides a method to synthesise β-amino alcohols, which are important in pharmaceutical chemistry.

这条路线提供了合成 β-氨基醇的方法，在药物化学中具有重要意义。

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8. Key Points for Examinations | 考试要点

Students often lose marks in CIE A-Level Chemistry exams for the following reasons:

学生在CIE A-Level化学考试中常因以下原因失分：

• Forgetting that the nucleophile is CN⁻, not HCN itself.
• 忘记真正的亲核试剂是 CN⁻ 而非 HCN 本身。
• Omitting the negative charge on the alkoxide intermediate in the mechanism diagram.
• 在机理图中遗漏烷氧负离子中间体的负电荷。
• Writing the product incorrectly, especially the position of the -OH and -CN groups.
• 产物结构书写错误，尤其是 -OH 和 -CN 的位置。
• Not stating that KCN/NaCN is a catalyst, and that the reaction requires warming.
• 未能说明 KCN/NaCN 是催化剂以及反应需要加热。
• Confusing this mechanism with nucleophilic substitution or electrophilic addition.
• 将此机理与亲核取代或亲电加成混淆。

Always draw curly arrows precisely: the arrow must start from the lone pair of CN⁻ and point toward the carbonyl carbon, and the other arrow must show the π bond pair moving to the oxygen.

绘制弯箭头时务必精确：箭头必须从 CN⁻ 的孤对电子出发指向羰基碳，另一箭头必须表示 π 键电子对移动到氧原子上。

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9. Reaction Conditions | 反应条件

In practice, the reaction is carried out by mixing the aldehyde or ketone with a solution of sodium or potassium cyanide, followed by addition of a slight excess of dilute hydrochloric acid or sulfuric acid. The acid is needed to protonate the alkoxide intermediate but must not be too strong, as it would convert all CN⁻ into HCN and stop the reaction.

实际操作中，将醛或酮与氰化钠或氰化钾溶液混合，然后加入稍过量的稀盐酸或稀硫酸。酸用于质子化烷氧负离子中间体，但不宜过强，否则所有 CN⁻ 会转化为 HCN，反应停止。

Alternatively, the reaction can be performed with HCN gas in the presence of a basic catalyst such as KCN.

另一种方法是在 KCN 等碱性催化剂存在下使 HCN 气体参与反应。

Reagent | 试剂	Condition | 条件	Purpose | 目的
KCN or NaCN | 氰化钾或氰化钠	Aqueous solution | 水溶液	Provide CN⁻ | 提供 CN⁻
Dilute H₂SO₄ | 稀硫酸	Slight excess | 稍过量	Protonate intermediate | 质子化中间体
Heat | 加热	Gently warm | 微热	Increase reaction rate | 提高反应速率

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10. Comparison with Other Nucleophilic Additions | 与其他亲核加成反应的比较

HCN addition shares the same fundamental mechanism as the addition of sodium hydrogensulfite (NaHSO₃) and Grignard reagents, but there are notable differences.

HCN 加成与亚硫酸氢钠（NaHSO₃）和格氏试剂的加成具有相同的基本机理，但也存在显著差异。

• HCN addition is reversible; the product can revert to the carbonyl compound under basic conditions.
• HCN 加成是可逆的；产物在碱性条件下可复原为羰基化合物。
• NaHSO₃ addition is also reversible and useful for purifying aldehydes.
• NaHSO₃ 加成同样可逆，常用于提纯醛。
• Grignard reagent addition is irreversible and forms alcohols, not hydroxynitriles.
• 格氏试剂加成不可逆，生成醇而非羟基腈。

Understanding these comparisons helps in exam questions that ask you to distinguish reaction types.

理解这些比较有助于回答要求区分反应类型的考试问题。

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11. Curly Arrow Notation in the Mechanism | 机理中的弯箭头表示法

In CIE exams, the mechanism is often tested as a drawn diagram. Ensure your diagram includes:

在CIE考试中，该机理常以图示形式考查。确保你的图示包含以下几点：

• The correct structure of the aldehyde or ketone, showing the δ⁺ and δ− on the C=O bond.
• 醛或酮的正确结构，标明 C=O 的 δ⁺ 和 δ−。
• The CN⁻ ion with a complete octet and a negative charge.
• 具有完整八隅体和负电荷的 CN⁻ 离子。
• A curly arrow from the lone pair of CN⁻ to the carbonyl carbon.
• 一条从 CN⁻ 孤对电子指向羰基碳的弯箭头。
• A curly arrow from the C=O π bond to the oxygen atom.
• 一条从 C=O π 键指向氧原子的弯箭头。
• The alkoxide intermediate with a negative charge on oxygen.
• 氧上带有负电荷的烷氧负离子中间体。
• A second curly arrow from the alkoxide oxygen to H⁺ of HCN.
• 第二条从烷氧负离子氧指向 HCN 中 H⁺ 的弯箭头。

Do not forget one arrow or you will lose marks even if the overall equation is correct.

千万不能遗漏任何一条箭头，即使总反应式正确也会因此失分。

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12. Summary | 总结

The addition of HCN to aldehydes and ketones is a nucleophilic addition reaction producing hydroxynitriles. The cyanide ion CN⁻ is the active nucleophile, supplied by KCN or NaCN. Aldehydes react faster than ketones due to steric and electronic factors. The products are valuable synthetic intermediates; hydrolysis converts them to α-hydroxy carboxylic acids and reduction converts them to amino alcohols.

HCN 与醛酮的加成是生成羟基腈的亲核加成反应。氰离子 CN⁻ 是实际亲核试剂，由 KCN 或 NaCN 提供。醛由于空间和电子因素比酮反应更快。产物是重要的合成中间体；水解得到 α-羟基羧酸，还原得到氨基醇。

Mastering the mechanism, conditions, and reactivity trends will help you answer both multiple-choice and structured questions confidently in your CIE A-Level Chemistry examination.

掌握机理、条件和反应活性趋势，将帮助你在CIE A-Level化学考试中自信地应对选择题和结构化问题。

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