Carboxylic Acids | 羧酸

📚 Carboxylic Acids | 羧酸

Carboxylic acids are a homologous series of organic compounds containing the carboxyl functional group, –COOH. They are widely distributed in nature, from the methanoic acid in ant stings to the long-chain fatty acids in lipids. In A-Level Chemistry, understanding their structure, acidity, and characteristic reactions provides a foundation for the study of acyl chlorides, esters, amides, and polyesters. This article systematically explores the naming, physical properties, acidic behaviour, and key reactions of carboxylic acids as required by the Cambridge International A-Level specification.

羧酸是含羧基官能团（–COOH）的同系有机物。它们广泛存在于自然界中，从蚁酸（甲酸）到脂质中的长链脂肪酸。在 A-Level 化学中，理解羧酸的结构、酸性以及特征反应，是学习酰氯、酯、酰胺和聚酯的基础。本文依据剑桥国际 A-Level 考试大纲，系统地解析羧酸的命名、物理性质、酸性行为和主要反应。

1. The Carboxyl Functional Group | 羧基官能团

The carboxyl group consists of a carbonyl group (C=O) and a hydroxyl group (–OH) bonded to the same carbon atom. This arrangement gives rise to a planar structure around the carboxyl carbon, with bond angles close to 120°. The carbon atom is sp² hybridised, forming a sigma bond framework and a pi bond between carbon and oxygen. The oxygen of the hydroxyl group carries two lone pairs, which can interact with the π system of the carbonyl group, delocalising electron density and stabilising the carboxylate anion formed on deprotonation.

羧基由连接在同一个碳原子上的羰基（C=O）和羟基（–OH）组成。这种结构使得羧基碳周围呈平面构型，键角接近 120°。该碳原子为 sp² 杂化，形成 σ 键骨架以及碳氧之间的 π 键。羟基氧原子带有两对孤对电子，可与羰基的 π 体系发生相互作用，使电子密度离域，从而稳定去质子化后生成的羧酸根负离子。

2. Nomenclature of Carboxylic Acids | 羧酸的命名

According to IUPAC rules, carboxylic acids are named by identifying the longest carbon chain containing the –COOH group, replacing the terminal ‘e’ of the corresponding alkane with ‘oic acid’. The carboxyl carbon is always assigned position number 1. For example, HCOOH is methanoic acid, CH₃COOH is ethanoic acid, and CH₃CH₂COOH is propanoic acid. When substituents are present, the chain is numbered starting from the carboxyl carbon. Dicarboxylic acids are named with the suffix ‘dioic acid’, such as HOOC–COOH (ethanedioic acid) and HOOCCH₂COOH (propanedioic acid). Many carboxylic acids also retain common names, such as formic acid, acetic acid, and benzoic acid (C₆H₅COOH).

根据 IUPAC 命名规则，羧酸通过选取含 –COOH 的最长碳链，将对应烷烃名称末尾的“e”改为“oic acid”（中文为“酸”）来命名。羧基碳的位次总是定为 1 号。例如，HCOOH 为 methanoic acid（甲酸），CH₃COOH 为 ethanoic acid（乙酸），CH₃CH₂COOH 为 propanoic acid（丙酸）。当含有取代基时，从羧基碳开始给主链编号。二羧酸以后缀“dioic acid”（二酸）命名，如 HOOC–COOH（乙二酸）和 HOOCCH₂COOH（丙二酸）。许多羧酸也有俗名，如甲酸（formic acid）、乙酸（acetic acid）和苯甲酸（C₆H₅COOH，benzoic acid）。

3. Physical Properties and Solubility | 物理性质与溶解性

Lower molecular mass carboxylic acids (up to about four carbons) are colourless liquids with sharp, pungent odours, while longer-chain acids are waxy solids. The boiling points of carboxylic acids are significantly higher than those of alcohols of comparable relative molecular mass. For example, ethanoic acid (Mᵣ = 60) boils at 118 °C, whereas propan-1-ol (Mᵣ = 60) boils at only 97 °C. This arises because carboxylic acids form strong intermolecular hydrogen bonds and exist predominantly as cyclic dimers in the liquid and vapour phases. In these dimers, two carboxyl groups are held together by two hydrogen bonds, effectively doubling the molecular mass of the vapour species.

较低分子量的羧酸（大约到四个碳）是无色液体，具有刺激性气味，而较长碳链的羧酸为蜡状固体。羧酸的沸点显著高于相对分子质量相近的醇。例如，乙酸（Mᵣ = 60）的沸点为 118 °C，而正丙醇（Mᵣ = 60）仅为 97 °C。这是因为羧酸分子间能形成很强的氢键，并在液态和气态中主要以环状二聚体形式存在。在这些二聚体中，两个羧基通过两个氢键结合在一起，有效使气相物种的分子质量加倍。

The first four carboxylic acids are completely miscible with water due to their ability to form hydrogen bonds with water molecules. As the non-polar hydrocarbon chain lengthens, solubility decreases rapidly. Benzoic acid, despite having only seven carbons, is sparingly soluble in cold water because of the large hydrophobic phenyl ring, but its solubility increases upon heating or in alkaline solution, where the more soluble benzoate ion is formed.

前四种羧酸与水以任意比例互溶，因为它们能与水分子形成氢键。随着非极性烃链增长，溶解性迅速下降。苯甲酸尽管只有七个碳原子，但由于疏水的苯环较大，在冷水中微溶；加热或在碱性溶液中，因生成溶解性更好的苯甲酸根离子，其溶解度增大。

4. Acidity and the Carboxylate Anion | 酸性与羧酸根离子

Carboxylic acids are weak acids with pKₐ values typically in the range 4–5. They partially dissociate in water to give a carboxylate anion and a proton: RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺. The acid strength of carboxylic acids is far greater than that of alcohols (pKₐ ≈ 16) and phenols (pKₐ ≈ 10). The enhanced acidity is attributed to the resonance stabilisation of the carboxylate ion. In the anion, the negative charge is delocalised over two oxygen atoms, making both carbon–oxygen bonds equivalent in length and strength. This delocalisation lowers the energy of the conjugate base, shifting the equilibrium further to the right. Alcohols lack this stabilisation because the alkoxide ion cannot delocalise its negative charge.

羧酸是弱酸，pKₐ 通常处于 4–5 范围内。它们在水溶液中部分电离，生成羧酸根离子和质子：RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺。羧酸的酸性远强于醇（pKₐ ≈ 16）和酚（pKₐ ≈ 10）。酸性增强的原因在于羧酸根离子的共振稳定作用。在该阴离子中，负电荷离域分布于两个氧原子上，使两个碳氧键键长和键能变得等同。这种离域降低了共轭碱的能量，使平衡进一步向右移动。醇不能享受这种稳定作用，因为烷氧负离子无法离域其负电荷。

5. Factors Influencing Acid Strength | 影响酸性强度的因素

The acid strength of a carboxylic acid is significantly affected by the inductive effect of substituents on the carbon chain. Electron-withdrawing groups (such as –Cl, –NO₂, –CN) increase acidity by stabilising the carboxylate anion through withdrawal of electron density, thereby spreading the negative charge more effectively. For instance, chloroethanoic acid (ClCH₂COOH, pKₐ = 2.86) is a stronger acid than ethanoic acid (pKₐ = 4.76). The effect intensifies with the number of electronegative substituents and with their proximity to the carboxyl group; trichloroethanoic acid is a much stronger acid than monochloroethanoic acid. Electron-donating alkyl groups, conversely, slightly decrease acidity by intensifying the negative charge on the carboxylate ion, as observed in the trend: methanoic acid (pKₐ = 3.75) > ethanoic acid > propanoic acid.

羧酸的酸性强弱显著受到碳链上取代基诱导效应的影响。吸电子基团（如 –Cl、–NO₂、–CN）通过吸引电子密度来稳定羧酸根负离子，使负电荷更有效地分散，从而增强酸性。例如，氯乙酸（ClCH₂COOH，pKₐ = 2.86）是比乙酸（pKₐ = 4.76）更强的酸。这种效应随吸电子取代基数目增多以及距羧基越近而增强；三氯乙酸是远强于一氯乙酸的酸。相反，给电子烷基会略微降低酸性，因为它们增强了羧酸根离子上的负电荷，这在以下趋势中很明显：甲酸（pKₐ = 3.75）> 乙酸 > 丙酸。

6. Reactions as Acids: Salts and Carbonates | 作为酸的反应：盐与碳酸盐

Carboxylic acids exhibit the typical reactions of acids, though they react more slowly than strong mineral acids due to their weak acidity. They react with reactive metals such as magnesium to liberate hydrogen gas and form a carboxylate salt: 2RCOOH + Mg → (RCOO)₂Mg + H₂. With bases such as sodium hydroxide, a neutralisation reaction occurs producing a soluble carboxylate salt and water: RCOOH + NaOH → RCOONa + H₂O. The carboxylate salts are ionic and generally soluble in water.

羧酸表现出酸的通性，但因酸性较弱，反应速率比强无机酸慢。它们与活泼金属如镁反应，放出氢气并生成羧酸盐：2RCOOH + Mg → (RCOO)₂Mg + H₂。与氢氧化钠等碱发生中和反应，生成可溶性羧酸盐和水：RCOOH + NaOH → RCOONa + H₂O。羧酸盐属于离子化合物，通常可溶于水。

A particularly useful test for the carboxyl group is the reaction with sodium carbonate or sodium hydrogencarbonate. Carboxylic acids react with carbonates to give a carboxylate salt, carbon dioxide gas, and water: 2RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O. With hydrogencarbonates, effervescence of CO₂ is observed even with weak carboxylic acids: RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O. This reaction can be used to distinguish carboxylic acids from phenols, which are not acidic enough to liberate CO₂ from hydrogencarbonates.

一个鉴定羧基尤为有用的检验是与碳酸钠或碳酸氢钠的反应。羧酸与碳酸盐反应生成羧酸盐、二氧化碳气体和水：2RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O。与碳酸氢盐反应时，即便是弱羧酸也能观察到 CO₂ 的冒泡现象：RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O。这一反应可用于区分羧酸和酚，因为酚的酸性不足以从碳酸氢盐中释放 CO₂。

7. Esterification | 酯化反应

Carboxylic acids react with alcohols in the presence of a strong acid catalyst (commonly concentrated H₂SO₄) to form esters and water. This reversible condensation reaction is known as Fischer esterification. The equilibrium can be driven to the right by using an excess of one reactant or by removing water as it is formed. The reaction proceeds via nucleophilic addition–elimination at the acyl carbon. The alcohol oxygen attacks the protonated carbonyl carbon; after proton transfers and elimination of water, the ester is produced. For example, ethanoic acid reacts with ethanol to give ethyl ethanoate, a sweet-smelling liquid used as a solvent and in flavourings: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.

羧酸在强酸催化剂（通常为浓硫酸）存在下与醇反应生成酯和水，这一可逆缩合反应称为费歇尔酯化。可通过使用过量某一种反应物或及时移除生成的水来使平衡向右移动。该反应按酰基碳上的亲核加成–消除机理进行。醇氧原子进攻质子化的羰基碳，经过质子转移和水分子的消除后生成酯。例如，乙酸与乙醇反应生成乙酸乙酯，这是一种具有果香味的液体，常作溶剂和食用香料：CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O。

The formation of polyesters from dicarboxylic acids and diols is an industrially important extension of this reaction. For instance, terephthalic acid (benzene-1,4-dicarboxylic acid) reacts with ethane-1,2-diol to form the polyester PET (polyethylene terephthalate), widely used in plastic bottles and fibres.

由二羧酸和二醇生成聚酯的反应是这一反应在工业上的重要延伸。例如，对苯二甲酸（1,4-苯二甲酸）与乙二醇反应生成聚酯 PET（聚对苯二甲酸乙二酯），广泛用于塑料瓶和纤维。

8. Reduction to Primary Alcohols | 还原为伯醇

Carboxylic acids are at the highest oxidation level among the common oxygen-containing organic compounds. They can be reduced to primary alcohols using a powerful reducing agent, lithium aluminium hydride (LiAlH₄), in dry ether. Sodium borohydride (NaBH₄) is not sufficiently reactive to reduce the carboxyl group. The overall transformation replaces the –OH of the carboxyl group with two hydrogen atoms across the C=O bond: RCOOH + 4[H] → RCH₂OH + H₂O. The reaction proceeds through an aldehyde intermediate, which is itself rapidly reduced to the primary alcohol under the reaction conditions. This reduction is synthetically important for converting fatty acids into fatty alcohols.

羧酸在常见含氧有机物中处于最高氧化态。它们可被强还原剂氢化铝锂（LiAlH₄）在干燥乙醚中还原为伯醇。硼氢化钠（NaBH₄）活性不够，无法还原羧基。总反应是将羧基中的 –OH 替换为两个氢原子，加在 C=O 键上：RCOOH + 4[H] → RCH₂OH + H₂O。反应经过醛中间体，而醛在该条件下会迅速被进一步还原为伯醇。这一还原在将脂肪酸转化为脂肪醇的合成中具有重要意义。

9. Reaction with Phosphorus Halides to Form Acyl Chlorides | 与卤化磷反应生成酰氯

Carboxylic acids react with phosphorus pentachloride (PCl₅) or phosphorus trichloride (PCl₃) at room temperature, and with thionyl chloride (SOCl₂) under gentle heating, to replace the –OH group with a chlorine atom, yielding acyl chlorides (acid chlorides). With PCl₅, steamy fumes of hydrogen chloride are observed and phosphorus oxychloride (POCl₃) is formed as a by‑product: RCOOH + PCl₅ → RCOCl + POCl₃ + HCl. Thionyl chloride is particularly convenient because the by‑products are gaseous (SO₂ and HCl), leaving a pure acyl chloride after removal of excess reagent. Acyl chlorides are highly reactive derivatives used in the synthesis of esters, amides, and other carbonyl compounds.

羧酸在室温下与五氯化磷（PCl₅）或三氯化磷（PCl₃）反应，在微热下与氯化亚砜（SOCl₂）反应，可将 –OH 基团替换为氯原子，生成酰氯。与 PCl₅ 反应时，可观察到氯化氢的白雾，并生成副产物磷酰氯（POCl₃）：RCOOH + PCl₅ → RCOCl + POCl₃ + HCl。使用氯化亚砜尤其方便，因为副产物 SO₂ 和 HCl 均为气体，除去过量试剂后即可得到纯净的酰氯。酰氯是高活性衍生物，用于合成酯、酰胺及其他羰基化合物。

10. Decarboxylation Reactions | 脱羧反应

Decarboxylation is the loss of carbon dioxide from a carboxyl group. Simple carboxylic acids are generally resistant to decarboxylation, but sodium salts of carboxylic acids undergo decarboxylation when heated with soda lime (a mixture of NaOH and CaO). This reaction produces an alkane with one fewer carbon atom. For example, heating sodium ethanoate with soda lime yields methane: CH₃COONa + NaOH (CaO, heat) → CH₄ + Na₂CO₃. This is a useful laboratory method for preparing small alkanes. In the reaction, the strongly basic conditions generate a carbanion-like transition state, which abstracts a proton from the alkali to give the hydrocarbon.

脱羧是指从羧基中失去二氧化碳的反应。简单羧酸通常不易脱羧，但羧酸的钠盐与碱石灰（NaOH 和 CaO 的混合物）共热时可发生脱羧反应，生成少一个碳原子的烷烃。例如，将乙酸钠与碱石灰共热可得到甲烷：CH₃COONa + NaOH (CaO, 加热) → CH₄ + Na₂CO₃。这是实验室制备小分子烷烃的常用方法。反应中，强碱性条件生成类似碳负离子的过渡态，该过渡态从碱中夺取一个质子，最终得到烃。

Certain carboxylic acids with electron-withdrawing groups on the α-carbon, or β-keto acids, decarboxylate much more readily. For example, propanedioic acid and its derivatives undergo easy thermal decarboxylation through a cyclic transition state because the resulting enol intermediate tautomerises to a more stable carbonyl compound.

某些 α-碳上带有吸电子基团的羧酸或 β-酮酸更容易脱羧。例如，丙二酸及其衍生物可通过环状过渡态发生热脱羧，因为生成的烯醇中间体会互变异构成更稳定的羰基化合物。

11. Dicarboxylic Acids and Their Behaviour | 二羧酸及其性质

Dicarboxylic acids contain two carboxyl groups. Common examples include ethanedioic acid (oxalic acid, HOOC–COOH), propanedioic acid (malonic acid, HOOCCH₂COOH), butanedioic acid (succinic acid), and the aromatic benzene-1,4-dicarboxylic acid (terephthalic acid). These acids ionise in two steps, with the first ionisation being significantly stronger than the second because removing a proton from a negatively charged species is more difficult. For ethanedioic acid, pKₐ₁ ≈ 1.23 and pKₐ₂ ≈ 4.19. Dicarboxylic acids can form two series of salts (hydrogen salts and neutral salts) and undergo many of the same reactions as monocarboxylic acids, including esterification and reduction. They also form cyclic anhydrides when heated, if a five- or six-membered ring can be produced.

二羧酸含有两个羧基。常见的例子有乙二酸（草酸，HOOC–COOH）、丙二酸（HOOCCH₂COOH）、丁二酸（琥珀酸）以及芳香族的 1,4-苯二甲酸（对苯二甲酸）。这些酸分两步电离，第一步电离的强度明显大于第二步，因为从带负电的物种中再移除一个质子较为困难。以乙二酸为例，pKₐ₁ ≈ 1.23，pKₐ₂ ≈ 4.19。二羧酸能形成两类盐（酸式盐和正盐），并发生与一元羧酸类似的许多反应，包括酯化和还原。加热时，如果能够形成五元或六元环，它们还能生成环状酸酐。

12. Spectroscopic Identification | 光谱鉴定

Infrared spectroscopy provides strong evidence for the presence of a carboxyl group. Carboxylic acids show a very broad O–H stretching absorption centred around 2500–3300 cm⁻¹, which overlaps with the C–H stretching region. The carbonyl C=O stretch appears as an intense peak at approximately 1700–1725 cm⁻¹ for saturated aliphatic acids; conjugation or hydrogen bonding can shift this to slightly lower wavenumbers. The C–O stretching band near 1210–1320 cm⁻¹ also supports identification. In proton NMR, the acidic proton of a carboxylic acid appears as a broad singlet, typically in the region δ 10–13 ppm, and is exchangeable with D₂O. The α-hydrogens adjacent to the carbonyl are deshielded and usually resonate between δ 2.0–2.5 ppm.

红外光谱能为羧基的存在提供有力证据。羧酸的特征是中心在 2500–3300 cm⁻¹ 的极宽 O–H 伸缩吸收峰，与 C–H 伸缩区域重叠。饱和脂肪酸的羰基 C=O 伸缩振动表现为约 1700–1725 cm⁻¹ 的强吸收峰；共轭或氢键作用可使其略向低波数移动。位于 1210–1320 cm⁻¹ 附近的 C–O 伸缩带也能辅助鉴定。在质子核磁共振谱中，羧酸的酸性氢呈现为一个宽单峰，通常位于 δ 10–13 ppm 区间，且可以通过 D₂O 交换确证。与羰基相邻的 α-氢因去屏蔽效应，通常在 δ 2.0–2.5 ppm 范围内出峰。

In mass spectrometry, aliphatic carboxylic acids often show a molecular ion peak, though it may be weak. A characteristic fragmentation is the loss of OH (M − 17) and loss of COOH (M − 45). The McLafferty rearrangement, if the acid has a γ-hydrogen, gives a prominent peak at m/z = 60 for ethanoic acid and appropriately higher masses for substituted acids.

在质谱中，脂肪族羧酸通常显示分子离子峰，但可能较弱。特征性的碎裂包括丢失 OH（M − 17）以及丢失 COOH（M − 45）。若酸分子含有 γ-氢，发生麦氏重排（McLafferty rearrangement），在乙酸中产生突出的 m/z = 60 峰，对于取代酸则出现在相应更高的质荷比。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Carboxylic Acids | 羧酸