Carboxylic Acids for Cambridge A-Level Chemistry

📚 Carboxylic Acids for Cambridge A-Level Chemistry | 羧酸

Carboxylic acids are an important homologous series in Cambridge A-Level Chemistry. They contain the carboxyl functional group, -COOH, which gives the series its characteristic acidic properties and its many reactions with metals, bases, carbonates, alcohols and reducing agents.

羧酸是剑桥 A-Level 化学中一个重要的同系物。它们含有羧基官能团 -COOH，这一官能团决定了羧酸的酸性特征，以及它们与金属、碱、碳酸盐、醇和还原剂发生的多种反应。

1. General Formula and Functional Group | 通式与官能团

The carboxyl group is written as -COOH. It consists of a carbonyl group C=O bonded directly to a hydroxyl group O-H on the same carbon atom. For saturated aliphatic monocarboxylic acids, the general formula is CₙH₂ₙO₂ or CₙH₂ₙ₊₁COOH.

羧基写作 -COOH。它由直接连接在同一碳原子上的羰基 C=O 和羟基 O-H 组成。对于饱和脂肪族一元羧酸，通式为 CₙH₂ₙO₂ 或 CₙH₂ₙ₊₁COOH。

Examples include methanoic acid HCOOH, ethanoic acid CH₃COOH and propanoic acid CH₃CH₂COOH. Benzoic acid C₆H₅COOH is the simplest aromatic carboxylic acid.

常见的例子包括甲酸 HCOOH、乙酸 CH₃COOH 和丙酸 CH₃CH₂COOH。苯甲酸 C₆H₅COOH 是最简单的芳香族羧酸。

2. IUPAC Nomenclature | IUPAC 命名法

To name a carboxylic acid, identify the longest continuous carbon chain containing the -COOH group. Replace the final -e of the parent alkane with -oic acid. The carboxyl carbon is always numbered as carbon 1, so no locant is needed for the acid group.

命名羧酸时，先找出含有 -COOH 的最长连续碳链。将母体烷烃词尾的 -e 改为 -oic acid。羧基碳总是编号为 1 号碳，因此酸基不需要再标注位次。

For example, HCOOH is methanoic acid, CH₃COOH is ethanoic acid, CH₃CH₂COOH is propanoic acid and CH₃CH₂CH₂COOH is butanoic acid. With substituents, CH₃CH(CH₃)COOH is 2-methylpropanoic acid and CH₃CH(OH)COOH is 2-hydroxypropanoic acid.

例如，HCOOH 为甲酸，CH₃COOH 为乙酸，CH₃CH₂COOH 为丙酸，CH₃CH₂CH₂COOH 为丁酸。带有取代基时，CH₃CH(CH₃)COOH 为 2-甲基丙酸，CH₃CH(OH)COOH 为 2-羟基丙酸。

3. Physical Properties and Hydrogen Bonding | 物理性质与氢键

Carboxylic acids are polar molecules because the carboxyl group contains both a highly polar C=O bond and an O-H bond. They can form hydrogen bonds with each other and with water molecules.

羧酸是极性分子，因为羧基中既含有强极性的 C=O 键，也含有 O-H 键。它们既能与彼此形成氢键，也能与水分子形成氢键。

In the liquid and solid states, many carboxylic acids exist as hydrogen-bonded dimers, in which two carboxyl groups form two intermolecular hydrogen bonds. This gives carboxylic acids higher boiling points than alcohols of similar relative molecular mass.

在液态和固态下，许多羧酸以氢键二聚体形式存在，两个羧基之间形成两条分子间氢键。这使得羧酸的沸点高于相对分子质量相近的醇。

Short-chain carboxylic acids such as methanoic, ethanoic and propanoic acids are completely miscible with water because they can hydrogen-bond to water. Solubility decreases as the non-polar hydrocarbon chain becomes longer.

短链羧酸如甲酸、乙酸和丙酸可与水完全互溶，因为它们能与水形成氢键。随着非极性烃链变长，羧酸在水中的溶解度逐渐下降。

4. Acidity and Dissociation in Water | 酸性及在水中的解离

Carboxylic acids are weak acids. In aqueous solution they only partially dissociate, establishing an equilibrium with the carboxylate ion and the hydronium ion.

羧酸是弱酸。在水溶液中它们只发生部分解离，并与羧酸根离子和水合氢离子建立平衡。

RCOOH(aq) + H₂O(l) ⇌ RCOO⁻(aq) + H₃O⁺(aq)

The acid dissociation constant Kₐ is given by the expression below. Ethanoic acid has pKₐ of approximately 4.76, which is typical of aliphatic carboxylic acids.

酸解离常数 Kₐ 的表达式如下。乙酸的 pKₐ 约为 4.76，这是脂肪族羧酸的典型值。

Kₐ = [RCOO⁻][H₃O⁺] / [RCOOH]

The conjugate base, the carboxylate ion RCOO⁻, is stabilised by delocalisation of the negative charge over two oxygen atoms. This resonance stabilisation makes carboxylic acids much stronger acids than alcohols or water, but they are still much weaker than strong mineral acids such as HCl.

共轭碱羧酸根离子 RCOO⁻ 因负电荷离域在两个氧原子上而得到稳定。这种共振稳定作用使羧酸的酸性远强于醇或水，但仍远弱于盐酸等强无机酸。

The relative acid strength is: carboxylic acid > phenol > ethanol. Typical pKₐ values are about 4.8 for ethanoic acid, about 10 for phenol and about 16 for ethanol.

相对酸性强弱为：羧酸 > 苯酚 > 乙醇。乙酸的 pKₐ 约为 4.8，苯酚约为 10，乙醇约为 16。

5. Factors Affecting Acidity | 影响酸性的因素

Electron-withdrawing groups near the carboxyl group increase acid strength by stabilising the carboxylate ion through the negative inductive effect. Electron-donating alkyl groups decrease acid strength by intensifying the negative charge on the carboxylate ion.

羧基附近的吸电子基团通过负诱导效应稳定羧酸根离子，从而增强酸性。给电子烷基会增强羧酸根上的负电荷，从而减弱酸性。

Chlorine substitution shows a clear trend: as more chlorine atoms are introduced on the α-carbon, acidity increases.

氯取代表现出明显趋势：随着 α-碳上引入的氯原子增多，酸性逐渐增强。

Acid	Formula	Approximate pKₐ
Ethanoic acid	CH₃COOH	4.76
Chloroethanoic acid	ClCH₂COOH	2.86
Dichloroethanoic acid	Cl₂CHCOOH	1.29
Trichloroethanoic acid	Cl₃CCOOH	0.65

Methanoic acid, pKₐ about 3.75, is a stronger acid than ethanoic acid because the methyl group in ethanoic acid is electron-donating and destabilises the ethanoate ion relative to the methanoate ion.

甲酸的 pKₐ 约为 3.75，其酸性强于乙酸，因为乙酸中的甲基具有给电子性，使乙酸根离子相对于甲酸根离子更不稳定。

6. Reactions with Metals, Bases and Carbonates | 与金属、碱和碳酸盐的反应

Carboxylic acids react with reactive metals such as magnesium to form carboxylate salts and hydrogen gas. The reaction is similar to the reaction of dilute mineral acids with metals, but is generally slower because carboxylic acids are weak acids.

羧酸可与镁等活泼金属反应，生成羧酸盐和氢气。该反应类似于稀无机酸与金属的反应，但由于羧酸是弱酸，反应通常较慢。

2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g)

Carboxylic acids are neutralised by aqueous sodium hydroxide or potassium hydroxide to give the soluble carboxylate salt and water.

羧酸可被氢氧化钠或氢氧化钾水溶液中和，生成可溶性羧酸盐和水。

CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)

With sodium carbonate or sodium hydrogencarbonate, carboxylic acids produce a carboxylate salt, water and carbon dioxide gas. This effervescence is an important simple test for carboxylic acids.

羧酸与碳酸钠或碳酸氢钠反应，生成羧酸盐、水和二氧化碳气体。这种冒泡现象是检验羧酸的一种重要简单方法。

2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g)

CH₃COOH(aq) + NaHCO₃(aq) → CH₃COONa(aq) + H₂O(l) + CO₂(g)

7. Esterification with Alcohols | 与醇的酯化反应

Carboxylic acids react with alcohols in the presence of a strong acid catalyst, usually concentrated sulfuric acid, under reflux to form an ester and water. The reaction is reversible and is called esterification.

在浓硫酸等强酸催化剂存在下，羧酸与醇在回流条件下反应，生成酯和水。该反应可逆，称为酯化反应。

RCOOH + R’OH ⇌ RCOOR’ + H₂O

For example, ethanoic acid reacts with ethanol to form ethyl ethanoate, an ester with a sweet, fruity smell. The ester is named as alkyl alkanoate, where the alkyl part comes from the alcohol and the alkanoate part comes from the carboxylic acid.

例如，乙酸与乙醇反应生成乙酸乙酯，这是一种具有甜果香味的酯。酯命名为某酸某酯，其中烷基部分来自醇，酸根部分来自羧酸。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Concentrated H₂SO₄ acts as a catalyst and also helps to shift the equilibrium to the right by absorbing water. In the esterification reaction, the water molecule is formed from the O-H group of the carboxylic acid and the H atom of the alcohol.

浓硫酸既作催化剂，又通过吸水使平衡向右移动。在酯化反应中，水分子由羧酸的 O-H 基团和醇的氢原子形成。

8. Reduction to Primary Alcohols | 还原为伯醇

Carboxylic acids are difficult to reduce and require a powerful reducing agent such as lithium tetrahydridoaluminate, LiAlH₄, in dry ether. Sodium borohydride, NaBH₄, is not strong enough to reduce the carboxyl group.

羧酸难以被还原，需要使用强还原剂，如干燥乙醚中的四氢合铝酸锂 LiAlH₄。硼氢化钠 NaBH₄ 的还原能力不足以还原羧基。

Reduction of a carboxylic acid produces the corresponding primary alcohol. The reaction uses four reducing equivalents per carboxyl group.

羧酸被还原后生成相应的伯醇。每个羧基需要消耗四个还原当量。

RCOOH + 4[H] → RCH₂OH + H₂O

For example, propanoic acid is reduced to propan-1-ol. This is a useful way to obtain a primary alcohol from a carboxylic acid, although LiAlH₄ must be handled carefully because it reacts violently with water.

例如，丙酸被还原为丙-1-醇。这是由羧酸制备伯醇的有用方法，但 LiAlH₄ 必须小心使用，因为它遇水会发生剧烈反应。

CH₃CH₂COOH + 4[H] → CH₃CH₂CH₂OH + H₂O

9. Conversion to Acyl Chlorides | 转化为酰氯

Carboxylic acids react with phosphorus(V) chloride, PCl₅, at room temperature to form an acyl chloride, phosphoryl chloride and hydrogen chloride gas. Steamy white fumes of HCl are observed.

羧酸在室温下与五氯化磷 PCl₅ 反应，生成酰氯、磷酰氯和氯化氢气体。可观察到 HCl 的白色酸雾。

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

Alternatively, sulfur dichloride oxide, SOCl₂, also converts carboxylic acids into acyl chlorides. The by-products are sulfur dioxide and hydrogen chloride.

另外，二氯亚砜 SOCl₂ 也能将羧酸转化为酰氯。副产物为二氧化硫和氯化氢。

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

Acyl chlorides are much more reactive than carboxylic acids because the chlorine atom withdraws electron density strongly and the chloride ion is a good leaving group. They are important intermediates in organic synthesis.

酰氯比羧酸活泼得多，因为氯原子强烈吸引电子，且氯离子是良好的

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Carboxylic Acids for Cambridge A-Level Chemistry | 羧酸