The Acidity of Carboxylic Acids | 羧酸的酸性

📚 The Acidity of Carboxylic Acids | 羧酸的酸性

Carboxylic acids are one of the most important acidic functional groups in A-Level chemistry. Their acidity is stronger than that of alcohols and phenols, yet still much weaker than mineral acids such as HCl or H₂SO₄. Understanding the origin of this acidity, the factors that modify it, and the characteristic reactions that depend on it is central to the Cambridge A-Level specification.

羧酸是 A-Level 化学中最重要的酸性官能团之一。其酸性强于醇类和酚类，但仍远弱于盐酸或硫酸等无机酸。理解这种酸性的来源、影响酸性强度的因素以及依赖酸性的特征反应，是剑桥 A-Level 考试大纲的核心内容。

1. What Makes Carboxylic Acids Acidic? | 羧酸为何呈酸性

A carboxylic acid contains the carboxyl group, –COOH. In aqueous solution, it donates a proton from the hydroxyl part of the carboxyl group to a water molecule. This produces a carboxylate ion and a hydronium ion. The equilibrium lies to the left because carboxylic acids are weak acids, meaning only a small fraction of the acid molecules are ionised at any moment.

羧酸含有羧基 –COOH。在水溶液中，羧基中羟基部分的质子被转移给水分子，生成羧酸根离子和水合氢离子。由于羧酸是弱酸，该平衡偏向左侧，意味着任意时刻只有少量酸分子发生电离。

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

The acid dissociation constant, Kₐ, for a typical carboxylic acid is of the order of 10⁻⁵ mol dm⁻³, which corresponds to a pKₐ value between about 3 and 5. This is far smaller than the Kₐ of a strong acid, but much larger than the Kₐ of ethanol, which is around 10⁻¹⁶ mol dm⁻³.

典型羧酸的酸解离常数 Kₐ 约为 10⁻⁵ mol dm⁻³，对应的 pKₐ 值大约在 3 到 5 之间。这远小于强酸的 Kₐ，但远大于乙醇的 Kₐ（约为 10⁻¹⁶ mol dm⁻³）。

2. Resonance Stabilisation of the Carboxylate Ion | 羧酸根离子的共振稳定

The key reason carboxylic acids are more acidic than alcohols is the stability of the carboxylate ion formed after loss of a proton. In the carboxylate ion, RCOO⁻, the negative charge is not localised on a single oxygen atom. It is delocalised across the two oxygen atoms through a p-orbital overlap with the carbonyl π system.

羧酸比醇酸性更强的关键原因在于失去质子后生成的羧酸根离子更稳定。在羧酸根离子 RCOO⁻ 中，负电荷并不只集中在一个氧原子上，而是通过 p 轨道与羰基 π 体系的重叠，离域到两个氧原子之间。

This delocalisation is represented by two equivalent resonance structures. In reality, both carbon–oxygen bonds in the carboxylate ion have the same length and are intermediate between a C=O double bond and a C–O single bond. The negative charge is spread evenly over both oxygen atoms.

这种离域可以用两个等价的共振结构表示。实际上，羧酸根离子中两个碳氧键长度相同，介于 C=O 双键和 C–O 单键之间。负电荷均匀分布在两个氧原子上。

R–COO⁻ ⇌ R–COO⁻ (resonance forms)

Because the carboxylate ion is stabilised by resonance, the equilibrium for ionisation lies further to the right than it does for an alkoxide ion, where no such delocalisation is possible. This makes the carboxylic acid a stronger acid than an alcohol.

由于羧酸根离子因共振而稳定，其电离平衡比醇盐离子更偏向右侧，因为醇盐离子无法发生这种离域。因此羧酸的酸性强于醇。

3. Comparing Acid Strength: Carboxylic Acids vs Alcohols and Phenols | 酸性比较：羧酸、醇与酚

It is useful to compare three common organic acids: ethanol, phenol and ethanoic acid. Ethanol has a pKₐ of about 16, phenol about 10, and ethanoic acid about 4.76. A lower pKₐ means a stronger acid. The trend is explained by the stability of the conjugate base in each case.

比较三种常见有机酸很有帮助：乙醇、苯酚和乙酸。乙醇的 pKₐ 约为 16，苯酚约为 10，乙酸约为 4.76。pKₐ 越低，酸性越强。这一趋势可由各自共轭碱的稳定性来解释。

When ethanol loses a proton, it forms an ethoxide ion, CH₃CH₂O⁻, in which the negative charge is localised on one oxygen atom. There is no delocalisation to stabilise the charge. When phenol loses a proton, the phenoxide ion is stabilised because the negative charge on oxygen can be delocalised into the aromatic ring through resonance.

乙醇失去质子后生成乙醇盐离子 CH₃CH₂O⁻，其负电荷集中在一个氧原子上，没有离域来稳定电荷。苯酚失去质子后，苯氧负离子因氧上的负电荷可通过共振离域进入芳环而得到稳定。

In the carboxylate ion, the negative charge is delocalised over two electronegative oxygen atoms, a more effective stabilisation than in phenoxide. This gives carboxylic acids the lowest pKₐ and therefore the greatest acidity among these three classes.

在羧酸根离子中，负电荷离域在两个电负性氧原子上，这种稳定作用比苯氧负离子更强。因此羧酸的 pKₐ 最低，在这三类化合物中酸性最强。

4. Inductive Effects of Substituents | 取代基的诱导效应

The strength of a carboxylic acid can be changed by substituents attached to the carbon chain. Electron-withdrawing groups, such as chlorine, fluorine and nitro groups, pull electron density away from the carboxyl group. This stabilises the negative charge of the carboxylate ion and makes the acid stronger.

羧酸的强度会受碳链上取代基的影响。吸电子基团，如氯、氟和硝基，会把电子密度从羧基拉走。这使羧酸根离子的负电荷更加稳定，从而使酸性增强。

For example, chloroethanoic acid, ClCH₂COOH, has a pKₐ of about 2.86, while ethanoic acid has a pKₐ of 4.76. Adding more chlorine atoms further increases acidity: dichloroethanoic acid has a pKₐ of about 1.29, and trichloroethanoic acid has a pKₐ of about 0.65.

例如，氯乙酸的 pKₐ 约为 2.86，而乙酸的 pKₐ 为 4.76。增加更多氯原子会进一步增强酸性：二氯乙酸的 pKₐ 约为 1.29，三氯乙酸的 pKₐ 约为 0.65。

Conversely, electron-donating alkyl groups push electron density towards the carboxylate ion, destabilising the negative charge and making the acid weaker. This is why methanoic acid, HCOOH, with pKₐ 3.75, is stronger than ethanoic acid.

相反，给电子烷基会把电子密度推向羧酸根离子，使负电荷不稳定，从而减弱酸性。这就是为什么甲酸 HCOOH 的 pKₐ 为 3.75，比乙酸更强。

The inductive effect decreases with distance: a chlorine atom on the α-carbon has a much larger effect than one on the β- or γ-carbon because the transmission of electron withdrawal falls off rapidly along a saturated chain.

诱导效应随距离增大而减弱：α 碳上的氯原子对酸性的影响远大于 β 或 γ 碳上的氯原子，因为吸电子作用沿饱和碳链迅速衰减。

5. Quantifying Acidity: pKₐ Values | 定量酸性：pKₐ 值

Acid strength is conveniently expressed using pKₐ, defined as the negative logarithm to base 10 of Kₐ. A strong acid has a large Kₐ and a low, often negative, pKₐ. A weak acid has a small Kₐ and a positive pKₐ.

酸的强度通常用 pKₐ 表示，其定义为 Kₐ 以 10 为底的负对数。强酸具有较大的 Kₐ 和较低、通常为负的 pKₐ。弱酸具有较小的 Kₐ 和正的 pKₐ。

pKₐ = –log₁₀ Kₐ

The table below compares some common carboxylic acids and other organic acids. Notice that each electron-withdrawing chlorine atom lowers the pKₐ significantly.

下表比较了一些常见羧酸和其他有机酸。注意每个吸电子氯原子都会显著降低 pKₐ。

Acid	Formula	pKₐ
Ethanol	CH₃CH₂OH	≈ 16
Phenol	C₆H₅OH	≈ 10
Ethanoic acid	CH₃COOH	4.76
Methanoic acid	HCOOH	3.75
Chloroethanoic acid	ClCH₂COOH	2.86
Dichloroethanoic acid	Cl₂CHCOOH	1.29
Trichloroethanoic acid	Cl₃CCOOH	0.65

These numerical trends provide strong evidence for the inductive effect and help predict the relative acidity of substituted carboxylic acids in exam questions.

这些数值趋势为诱导效应提供了有力证据，有助于在考试题中预测取代羧酸的相对酸性。

6. Reactions with Carbonates and Hydrogencarbonates | 与碳酸盐和碳酸氢盐反应

Carboxylic acids are strong enough to liberate carbon dioxide from carbonates and hydrogencarbonates. This reaction is often used as a chemical test to distinguish carboxylic acids from weaker acids such as phenol, which does not react with sodium carbonate solution.

羧酸的酸性足以使碳酸盐和碳酸氢盐释放出二氧化碳。该反应常被用作化学检验，以区分羧酸和苯酚等较弱酸，因为苯酚不与碳酸钠溶液反应。

With sodium carbonate, the reaction produces a carboxylate salt, carbon dioxide and water. With sodium hydrogencarbonate, effervescence of CO₂ is also observed at room temperature.

与碳酸钠反应生成羧酸盐、二氧化碳和水。与碳酸氢钠反应在室温下也会观察到 CO₂ 气泡。

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

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

The production of carbon dioxide can be confirmed by bubbling the gas through limewater, which turns milky due to the formation of insoluble calcium carbonate.

生成的二氧化碳可通过将气体通入石灰水来验证，石灰水因生成不溶性碳酸钙而变浑浊。

7. Reaction with Metals and Alkalis | 与金属和碱反应

Like other acids, carboxylic acids react with reactive metals such as magnesium and zinc to produce hydrogen gas and a salt. The reaction with magnesium is usually slow at room temperature but becomes more rapid on warming.

与其他酸一样，羧酸与镁、锌等活泼金属反应生成氢气和盐。与镁的反应在室温下通常较慢，加热后加快。

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

Carboxylic acids are also neutralised by aqueous alkalis such as sodium hydroxide. The reaction forms a carboxylate salt and water. Unlike the carbonate reaction, no gas is produced, so neutralisation with an indicator may be used in titrations.

羧酸也可被氢氧化钠等碱溶液中和，生成羧酸盐和水。与碳酸盐反应不同，该反应不产生气体，因此滴定中可使用指示剂进行中和。

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

The salts formed are named by replacing the ‘ic acid’ ending with ‘ate’. For example, ethanoic acid forms ethanoate salts, and propanoic acid forms propanoate salts.

生成的盐命名时把 ‘ic acid’ 结尾改为 ‘ate’。例如，乙酸生成乙酸盐，丙酸生成丙酸盐。

8. Solubility and Acid-Base Behaviour in Water | 水溶性与酸碱行为

Short-chain carboxylic acids are highly soluble in water because the polar carboxyl group can form hydrogen bonds with water molecules. Methanoic acid, ethanoic acid and propanoic acid are completely miscible with water.

短链羧酸极易溶于水，因为极性羧基能与水分子形成氢键。甲酸、乙酸和丙酸可与水完全混溶。

As the non-polar hydrocarbon chain lengthens, solubility decreases. Butanoic acid is moderately soluble, while long-chain acids such as decanoic acid are almost insoluble in water. This reflects the increasing importance of the hydrophobic alkyl portion of the molecule.

随着非极性烃链增长，溶解度下降。丁酸有一定的溶解度，而癸酸等长链酸几乎不溶于水。这反映出分子中疏水性烷基部分的影响越来越大。

The acidity of carboxylic acids in water arises because water acts as a Brønsted–Lowry base, accepting the proton from the carboxyl group. Without water or another base, dry carboxylic acids exist mainly as hydrogen-bonded dimers, not as free carboxylate ions.

羧酸在水中的酸性来源于水作为 Brønsted–Lowry 碱接受羧基的质子。如果没有水或其他碱，干燥的羧酸主要以氢键二聚体形式存在，而不是游离的羧酸根离子。

9. Dicarboxylic Acids and Stepwise Ionisation | 二元羧酸与分步电离

Dicarboxylic acids contain two carboxyl groups. They can ionise in two steps, each with its own pKₐ value. The first ionisation is usually more favourable than the second because the first produces a singly charged anion, while the second requires removing a proton from an already negatively charged species.

二元羧酸含有两个羧基，可分两步电离，每一步都有各自的 pKₐ 值。第一步电离通常比第二步更容易，因为第一步生成带一个负电荷的阴离子，而第二步需要从已经带负电的物种中再移去一个质子。

For ethanedioic acid, HOOC–COOH, the first pKₐ is about 1.23 and the second is about 4.19. The second ionisation is less favourable because of the additional electrostatic repulsion between the proton and the negative charge on the monoanion.

对于乙二酸 HOOC–COOH，第一 pKₐ 约为 1.23，第二 pKₐ 约为 4.19。第二步电离较不利，因为质子与单负离子上的负电荷之间存在额外的静电排斥。

HOOC–COOH(aq) ⇌ HOOC–COO⁻(aq) + H⁺(aq)

HOOC–COO⁻(aq) ⇌ ⁻OOC–COO⁻(aq) + H⁺(aq)

Dicarboxylic acids therefore show two equivalence points in titration curves when titrated against a strong base, provided the pKₐ values differ sufficiently. This can be a useful way to identify them experimentally.

因此，用强碱滴定时，如果两个 pKₐ 值相差足够大，二元羧酸的滴定曲线会显示两个等当点。这在实验中可用来鉴别二元羧酸。

10. Buffer Relevance and Biological Context | 缓冲相关性与生物背景

Because carboxylic acids are weak acids, mixtures of a carboxylic acid and its conjugate base can act as buffer solutions. A common example is the ethanoic acid / sodium ethanoate buffer, which resists changes in pH when small amounts of acid or alkali are added.

由于羧酸是弱酸，羧酸与其共轭碱的混合物可以起到缓冲溶液的作用。一个常见例子是乙酸/乙酸钠缓冲液，它在加入少量酸或碱时能抵抗 pH 的变化。

The buffer action can be explained using the weak acid equilibrium. Added H⁺ shifts the equilibrium to the left, while added OH⁻ reacts with H₃O⁺ and is replaced by further ionisation of the weak acid, so the pH remains nearly constant.

缓冲作用可用弱酸平衡解释。加入 H⁺ 使平衡向左移动，而加入 OH⁻ 会与 H₃O⁺ 反应，并通过弱酸的进一步电离来补充，因此 pH 几乎保持不变。

Carboxylic acids are also biologically important. Fatty acids are long-chain carboxylic acids found in lipids, while amino acids contain both a carboxyl group and an amino group. The ionisation of the carboxyl group is essential for protein structure and enzyme activity.

羧酸在生物中也十分重要。脂肪酸是脂质中的长链羧酸，而氨基酸同时含有羧基和氨基。羧基的电离对蛋白质结构和酶活性至关重要。

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The Acidity of Carboxylic Acids | 羧酸的酸性