Carboxylic Acids | 羧酸

📚 Carboxylic Acids | 羧酸

Carboxylic acids are a fundamental homologus series in organic chemistry, featuring the reactive –COOH group. Their chemistry is dominated by acidic behaviour, nucleophilic acyl substitution, and hydrogen bonding. Understanding the interplay between structure, physical properties, and reactivity is essential for success in both theoretical and practical examination questions.

羧酸是有机化学中一个基础的同系列，其特征官能团是 –COOH。其化学性质主要表现为酸性行为、亲核酰基取代以及氢键作用。理解结构、物理性质与反应活性之间的关联，对解答理论与实验考题至关重要。

1. Introduction to Carboxylic Acids | 羧酸简介

Carboxylic acids contain the carboxyl functional group, –COOH, which is a combination of a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. The general formula for aliphatic monocarboxylic acids is CₙH₂ₙ₊₁COOH (or CₙH₂ₙO₂). They are weak acids and can form dimers through intermolecular hydrogen bonding.

羧酸含有羧基官能团 –COOH，它由同一碳原子上的羰基 (C=O) 和羟基 (–OH) 组合而成。脂肪族一元羧酸的通式为 CₙH₂ₙ₊₁COOH（或 CₙH₂ₙO₂）。它们是弱酸，并且可以通过分子间氢键形成二聚体。

2. Nomenclature of Carboxylic Acids | 羧酸的命名

According to IUPAC rules, the parent chain must include the carboxyl carbon, and the suffix is ‘-oic acid’. The carbon of the –COOH group is always designated as position 1. Common substituents and multiple carboxyl groups give names such as 2-hydroxypropanoic acid (lactic acid) and ethanedioic acid (oxalic acid).

根据 IUPAC 命名规则，主链必须包含羧基碳，词尾使用“-酸”。–COOH 基团中的碳总是定为 1 号位。常见取代基和多个羧基会生成诸如 2-羟基丙酸（乳酸）和乙二酸（草酸）等名称。

• methanoic acid (HCOOH) – formic acid | 甲酸 (蚁酸)

• ethanoic acid (CH₃COOH) – acetic acid | 乙酸 (醋酸)

• propanoic acid (CH₃CH₂COOH) | 丙酸

• butanoic acid (CH₃CH₂CH₂COOH) | 丁酸

• benzoic acid (C₆H₅COOH) – aromatic carboxylic acid | 苯甲酸 (芳香族羧酸)

3. Physical Properties: Boiling Points and Solubility | 物理性质：沸点与溶解性

Carboxylic acids exhibit unusually high boiling points compared to alcohols of similar relative molecular mass. This is because they can form two strong hydrogen bonds per molecule, producing stable dimers in the liquid and vapour phases. Methanoic acid and ethanoic acid exist as dimers even in the gas phase.

与相对分子质量相近的醇相比，羧酸的沸点异常地高。这是因为每个分子可以形成两个强氢键，在液态和气相中产生稳定的二聚体。甲酸和乙酸甚至在气相中也以二聚体形式存在。

Short-chain carboxylic acids (up to butanoic acid) are completely miscible with water due to extensive hydrogen bonding with water molecules. As the hydrocarbon chain lengthens, solubility decreases rapidly because the hydrophobic alkyl chain dominates.

短链羧酸（丁酸及以下）可与水完全混溶，原因是能与水分子形成广泛的氢键。随着烃链增长，溶解度迅速下降，因为疏水烷基链占主导地位。

4. Acidity of Carboxylic Acids: The Carboxylate Ion Resonance | 羧酸的酸性：羧酸根离子的共振

Carboxylic acids are weak acids (pKₐ typically 3–5). They partially dissociate in water to form a carboxylate ion and a hydronium ion: RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺. The delocalisation of the negative charge over two oxygen atoms in the carboxylate ion (resonance stabilisation) makes the conjugate base more stable, thereby increasing acidity compared to alcohols.

羧酸是弱酸（pKₐ 通常在 3–5）。它们在水溶液中部分电离，生成羧酸根离子和水合氢离子：RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺。负电荷在羧酸根离子的两个氧原子上离域（共振稳定），使共轭碱更稳定，因此酸性强于醇。

Resonance in ethanoate ion: CH₃COO⁻ ↔ CH₃C(O⁻)₂ | 乙酸根离子的共振：CH₃COO⁻ ↔ CH₃C(O⁻)₂

Substituent effect | 取代基效应	Example | 示例	pKₐ trend | pKₐ 趋势
Electron-withdrawing (e.g. –Cl) | 吸电子基团（如 –Cl）	Chloroethanoic acid | 氯乙酸	Lower pKₐ (stronger acid) | pKₐ 较小（酸性更强）
Electron-donating (e.g. –CH₃) | 给电子基团（如 –CH₃）	Propanoic acid vs ethanoic acid | 丙酸与乙酸	Higher pKₐ (weaker acid) | pKₐ 较大（酸性较弱）

5. Reactions with Metals | 与金属的反应

Carboxylic acids react with reactive metals such as magnesium or zinc to form a carboxylate salt and hydrogen gas. This is a redox reaction where the acid acts as an oxidising agent. The reaction is slower than that of a strong mineral acid because of the low concentration of H⁺ ions.

羧酸能与活泼金属（如镁或锌）反应，生成羧酸盐和氢气。这是一个氧化还原反应，酸在此充当氧化剂。由于 H⁺ 离子浓度低，反应速度比强无机酸慢。

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ ↑

Observations include effervescence and the metal dissolving. The resulting solution can be crystallised to obtain the carboxylate salt. This reaction can be used to distinguish carboxylic acids from alcohols, which do not react with magnesium.

观察到的现象包括产生气泡和金属溶解。所得溶液可经结晶获得羧酸盐。该反应可用于区分羧酸与醇，因为醇不与镁反应。

6. Reactions with Bases and Carbonates | 与碱和碳酸盐的反应

As Brønsted–Lowry acids, carboxylic acids undergo neutralisation with strong bases such as sodium hydroxide to produce a carboxylate salt and water. The reaction is exothermic and essentially irreversible.

作为 Brønsted–Lowry 酸，羧酸能与强碱（如氢氧化钠）发生中和反应，生成羧酸盐和水。反应放热且基本不可逆。

CH₃COOH + NaOH → CH₃COONa + H₂O

With carbonates and hydrogencarbonates, carboxylic acids liberate carbon dioxide gas. This provides a simple test to differentiate carboxylic acids from weaker acids such as phenol, which does not react with carbonate solutions.

与碳酸盐和碳酸氢盐反应时，羧酸会释放出二氧化碳气体。这是一个区分羧酸与较弱酸（如不能与碳酸盐溶液反应的苯酚）的简便方法。

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O

CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ + H₂O

7. Esterification (Fischer–Speier) | 酯化反应（费歇尔酯化）

When a carboxylic acid is heated with an alcohol in the presence of a concentrated acid catalyst (typically H₂SO₄), an ester and water are formed. This reversible condensation reaction reaches an equilibrium. The ester link is –COO–, and the odour of the product often shifts from pungent to fruity.

羧酸与醇在浓酸催化剂（通常为浓硫酸）存在下加热，生成酯和水。这是一个可逆的缩合反应，达到平衡。酯键为 –COO–，产物的气味常由刺鼻转变为果香。

RCOOH + R’OH ⇌ RCOOR’ + H₂O (catalyst H⁺, heat)

Common examples include the formation of ethyl ethanoate (a nail-varnish-remover smell) and pentyl ethanoate (pear drops). Removing water or using an excess of one reactant can drive the equilibrium to favour ester formation.

常见例子包括乙酸乙酯（指甲油去除剂气味）和乙酸戊酯（梨味）的生成。移除水或使用过量反应物可使平衡向生成酯的方向移动。

• RCOOH + R’OH ⇌ RCOOR’ + H₂O | 羧酸 + 醇 ⇌ 酯 + 水

8. Reduction to Primary Alcohols | 还原成伯醇

Carboxylic acids are relatively resistant to reduction, but powerful reducing agents such as lithium aluminium hydride (LiAlH₄) in dry ether can reduce them directly to primary alcohols. The intermediate aldehyde is not isolated because LiAlH₄ is too strong a reducing agent and reduces the aldehyde further.

羧酸相对难以还原，但强还原剂如四氢铝锂 (LiAlH₄) 在干燥乙醚中可将它们直接还原为伯醇。中间产物醛无法分离，因为 LiAlH₄ 是过强的还原剂，会进一步将醛还原。

RCOOH + 4[H] → RCH₂OH + H₂O (using LiAlH₄)

Sodium borohydride (NaBH₄) is not strong enough to reduce carboxylic acids. This selectivity allows chemists to reduce aldehydes or ketones in the presence of a carboxyl group, which is a useful synthetic distinction.

硼氢化钠 (NaBH₄) 不足以还原羧酸。这种选择性使化学家能够在羧基存在的情况下还原醛或酮，这是一个有用的合成区别。

9. Formation of Acyl Chlorides | 酰氯的生成

Carboxylic acids react with phosphorus(V) chloride (PCl₅), phosphorus trichloride (PCl₃), or thionyl chloride (SOCl₂) to form acyl chlorides (RCOCl). The hydroxyl group is replaced by a chlorine atom, and the mixture gives off steamy fumes of HCl.

羧酸与五氯化磷 (PCl₅)、三氯化磷 (PCl₃) 或氯化亚砜 (SOCl₂) 反应生成酰氯 (RCOCl)。羟基被氯原子取代，混合物释放出 HCl 的白色烟雾。

CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

The reaction with SOCl₂ is particularly elegant because the by-products are gases (HCl and SO₂), leaving the acyl chloride in a purer form. Acyl chlorides are highly reactive and are used in the synthesis of esters, amides, and ketones.

与 SOCl₂ 的反应特别精巧，因为副产物为气体（HCl 和 SO₂），留下的酰氯纯度较高。酰氯反应活性极高，用于合成酯、酰胺和酮。

10. Decarboxylation Reactions | 脱羧反应

Decarboxylation is the loss of carbon dioxide from a carboxylate group. Simple monocarboxylic acids resist thermal decarboxylation, but β‑keto acids and certain dicarboxylic acids decarboxylate readily on heating. Sodium salts of carboxylic acids react with soda lime (NaOH/CaO) upon strong heating to form alkanes with one carbon fewer.

脱羧是指从羧酸根基团失去二氧化碳。简单一元羧酸难以发生热脱羧，但 β-酮酸和某些二元羧酸在加热时容易脱羧。羧酸的钠盐与碱石灰（NaOH/CaO）强热反应，生成少一个碳的烷烃。

CH₃COONa + NaOH (CaO, heat) → CH₄ + Na₂CO₃

This reaction is used to shorten carbon chains and is a classic laboratory preparation of methane. In biochemical systems, decarboxylation of β‑keto acids occurs during metabolism in the citric acid cycle.

该反应用于缩短碳链，是实验室制备甲烷的经典方法。在生物化学体系中，β-酮酸的脱羧发生在新陈代谢中，如三羧酸循环。

11. Tests for the Carboxyl Group | 羧基的检验

The carboxyl group can be identified through a combination of simple chemical tests. Carboxylic acids turn blue litmus paper red, but this is not definitive, as other acids show the same behaviour. A more specific test is the effervescence with sodium hydrogencarbonate solution, which indicates the presence of a carboxylic acid (phenol and alcohols give no reaction).

通过组合简单的化学检验可鉴定羧基。羧酸能使蓝色石蕊试纸变红，但这并非确证，因为其他酸也表现出相同行为。更专一的检验是与碳酸氢钠溶液反应产生气体，这指示羧酸的存在（苯酚和醇不反应）。

Esterification with a known alcohol and a fruity or distinct sweet smell also suggests a carboxylic acid. Additionally, the formation of a neutral iron(III) carboxylate complex (no strong colour change compared to the violet phenol complex) can be used. For solid acids, checking solubility in sodium hydrogencarbonate solution is effective.

与已知醇类酯化，产生果香或独特的甜味，同样提示羧酸存在。此外，形成中性的铁(III)羧酸根配合物（与苯酚的紫色配合物相比无明显颜色变化）也可利用。对于固体酸，检查其在碳酸氢钠溶液中的溶解性是一种有效方法。

12. Summary and Exam Tips | 总结与备考技巧

Carboxylic acids are planar around the carbonyl carbon, exhibit strong hydrogen bonding, and act as weak acids with pKₐ values between 3 and 5. Key reactions include neutralisation, esterification, reduction with LiAlH₄, and formation of acyl chlorides. The carboxylate ion is resonance-stabilised, which explains the enhanced acidity compared to alcohols.

羧酸在羰基碳周围呈平面结构，表现出强氢键作用，是 pKₐ 值在 3–5 之间的弱酸。关键反应包括中和、酯化、LiAlH₄ 还原以及酰氯的生成。羧酸根离子因共振而稳定，这解释了其酸性强于醇的原因。

Examiners often ask candidates to compare acidity using inductive effects, to draw mechanisms for esterification (nucleophilic addition–elimination), and to plan a synthetic route from a carboxylic acid. Practise writing balanced equations and identifying the limiting reagent in aspirin preparation (esterification of salicylic acid). Always specify concentrated sulfuric acid as the esterification catalyst and remember that NaHCO₃ is the reagent used to test the –COOH group in qualitative analysis.

考官常要求考生运用诱导效应比较酸性强弱，绘制酯化反应机理（亲核加成–消除），以及设计从羧酸出发的合成路线。练习书写配平方程式，并识别阿司匹林制备（水杨酸的酯化）中的限量试剂。始终指定浓硫酸作为酯化催化剂，并记住 NaHCO₃ 是定性分析中用于检验 –COOH 基团的试剂。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）