📚 Reactions of Phenol | 酚的反应
Phenol is one of the most versatile molecules in A-Level organic chemistry. Its structure, featuring a hydroxyl group directly bonded to an aromatic ring, gives rise to a distinctive set of reactions that set it apart from both alcohols and carboxylic acids. This article will explore the key reactions of phenol, including its acidic behaviour, electrophilic substitution on the activated ring, esterification with acyl derivatives, and the classic colour test with iron(III) chloride. Understanding these reactions is essential for mastering Cambridge International A-Level Chemistry and for appreciating how the interplay between the –OH group and the benzene π system governs reactivity.
酚是A-Level有机化学中用途最广的分子之一。其结构特点是羟基直接连接在芳环上，这赋予它一组独特的反应，使其既不同于醇也不同于羧酸。本文将探讨酚的关键反应，包括其酸性表现、活化苯环上的亲电取代、与酰基衍生物的酯化，以及与氯化铁(III)的经典显色检验。理解这些反应对于掌握剑桥国际A-Level化学以及理解–OH基团与苯π体系之间如何调控反应性至关重要。
1. Structure and Reactivity of Phenol | 酚的结构与反应性
Phenol consists of a benzene ring carrying a single hydroxyl substituent, C₆H₅OH. One of the oxygen lone pairs overlaps with the delocalised π electrons of the ring, forming an extended p–π conjugated system. This electron-donating resonance effect increases the electron density on the ortho and para positions of the ring, making phenol far more susceptible to electrophilic attack than benzene itself. At the same time, the withdrawal of electron density from the oxygen atom towards the ring weakens the O–H bond and stabilises the phenoxide ion (C₆H₅O⁻) after deprotonation. As a result, phenol exhibits both enhanced acidity compared with aliphatic alcohols and heightened reactivity towards electrophilic aromatic substitution.
酚由一个携带单一羟基取代基的苯环构成，化学式为C₆H₅OH。氧原子的一个孤对电子与苯环的离域π电子重叠，形成扩展的p–π共轭体系。这种给电子的共振效应增大了环上邻位和对位的电子密度，使酚比苯本身更容易受到亲电试剂的进攻。同时，氧原子上的电子云向环移动削弱了O–H键，并稳定了去质子化后生成的苯氧基负离子(C₆H₅O⁻)。因此，与脂肪醇相比，酚既表现出增强的酸性，也表现出对亲电芳族取代更高的反应活性。
2. Acidity of Phenol: Reaction with Sodium | 酸性：与钠反应
Like alcohols, phenol reacts with metallic sodium to produce the corresponding phenoxide salt and hydrogen gas. However, the reaction occurs more readily with phenol because its acidic proton is held less tightly owing to resonance stabilisation of the conjugate base. When a small piece of sodium is added to molten phenol or a phenol solution in an inert solvent, effervescence is observed as hydrogen is evolved.
2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂↑
The product, sodium phenoxide, is an ionic solid that contains the resonance-stabilised phenoxide ion. This reaction demonstrates that phenol is a stronger acid than ethanol, where similar reaction with sodium is less vigorous.
与醇类似，酚与金属钠反应生成相应的苯酚盐和氢气。不过，由于共振稳定了共轭碱，酚的酸性质子结合得更弱，因此该反应更容易进行。将一小块钠加入熔融酚或酚的惰性溶剂溶液中，会观察到因氢气放出而产生的泡腾现象。产物苯酚钠是一种含有共振稳定苯氧基负离子的离子型固体。该反应表明酚是比乙醇更强的酸，因为乙醇与钠的类似反应较不剧烈。
3. Reaction with Sodium Hydroxide | 与氢氧化钠反应
Phenol dissolves in aqueous sodium hydroxide to form a clear solution of sodium phenoxide and water. This is a simple neutralisation that confirms phenol is acidic enough to react with a strong base.
C₆H₅OH + NaOH → C₆H₅ONa + H₂O
In contrast, simple alcohols such as ethanol do not react with sodium hydroxide under comparable conditions. The ability to deprotonate phenol with NaOH, but not ethanol, highlights the significant difference in acid strength: the pKₐ of phenol is around 10, whereas that of ethanol is approximately 16. The phenoxide ion is stabilised by delocalisation of the negative charge into the aromatic ring, which shifts the equilibrium towards the products.
酚可溶于氢氧化钠水溶液，生成澄清的苯酚钠溶液和水。这是一步简单的中和反应，证实酚的酸性足以与强碱反应。相比之下，简单的醇如乙醇在类似条件下不与氢氧化钠反应。能用NaOH使酚去质子化而不能使乙醇去质子化，这凸显了酸强度的显著差异：酚的pKₐ约为10，而乙醇的pKₐ约为16。苯氧基负离子因负电荷离域到芳环中而得到稳定，从而使平衡移向产物方向。
4. Why Phenol Does Not React with Sodium Carbonate | 为何酚不与碳酸钠反应
Although phenol is distinctly acidic, it is not a strong enough acid to decompose sodium carbonate or sodium hydrogencarbonate. When phenol is shaken with aqueous Na₂CO₃, no effervescence of CO₂ occurs. The reason becomes clear by comparing pKₐ values: phenol (pKₐ ~10) is a weaker acid than carbonic acid (pKₐ₁ ~6.4). Therefore, the equilibrium C₆H₅OH + NaHCO₃ ⇌ C₆H₅ONa + H₂CO₃ lies far to the left, and CO₂ is not liberated. In practical laboratory tests, this allows a simple distinction between phenol and carboxylic acids: carboxylic acids (pKₐ ~5) readily produce CO₂ with Na₂CO₃, while phenol does not. This places phenol's acid strength neatly between that of alcohols and carboxylic acids.
尽管酚有明显的酸性，但其酸性还不足以分解碳酸钠或碳酸氢钠。将酚与Na₂CO₃水溶液混合振荡，不会出现CO₂气泡。通过比较pKₐ值可以清楚地解释：酚(pKₐ ~10)是比碳酸(pKₐ₁ ~6.4)更弱的酸。因此，平衡C₆H₅OH + NaHCO₃ ⇌ C₆H₅ONa + H₂CO₃强烈偏向左边，不会释放二氧化碳。在实验室测试中，这提供了一个区分酚与羧酸的简单方法：羧酸(pKₐ ~5)极易与Na₂CO₃反应产生CO₂，而酚则不能。这使得酚的酸强度恰好介于醇和羧酸之间。
5. Esterification of Phenol: Using Acyl Chlorides and Acid Anhydrides | 酚的酯化：使用酰氯和酸酐
Direct esterification of phenol with carboxylic acids is not a practical route; phenol is both a weak acid and a weak nucleophile, making the acid-catalysed equilibrium with a carboxylic acid highly unfavourable. Instead, phenolic esters are prepared by reaction with more reactive acylating agents such as acyl chlorides and acid anhydrides. For example, phenol reacts vigorously with ethanoyl chloride (acetyl chloride) at room temperature to give phenyl ethanoate and hydrogen chloride gas.
C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ + HCl
With ethanoic anhydride, gentle warming is required; the products are phenyl ethanoate and ethanoic acid. Both reactions proceed via nucleophilic addition–elimination, where the phenolic oxygen attacks the electrophilic carbonyl carbon. These methods are widely used in the synthesis of aspirin and other pharmaceutical intermediates. It is important to note that phenol itself is corrosive and should be handled with care, but the resulting esters are often far less hazardous and are used as fragrances and precursors.
酚与羧酸直接酯化并非实用路径；酚既是弱酸又是弱亲核试剂，使得与羧酸的酸催化平衡极为不利。酚酯通常是利用活性更高的酰化试剂如酰氯和酸酐来制备。例如，酚在室温下与乙酰氯（乙酰氯）剧烈反应，生成乙酸苯酯和氯化氢气体。与乙酸酐反应则需要温和加热，产物为乙酸苯酯和乙酸。这两类反应均通过亲核加成–消除机理进行，其中酚氧原子进攻亲电的羰基碳。这些方法广泛用于阿司匹林及其他药物中间体的合成。需要注意的是酚本身有腐蚀性，操作应小心，但生成的酯往往危害小得多，并被用作香料和前体。
6. Electrophilic Substitution: Bromination of Phenol | 亲电取代：溴化
The electron-rich aromatic ring in phenol undergoes electrophilic substitution extremely readily. When phenol is treated with bromine water at room temperature, the red-brown colour of bromine disappears instantly, and a white precipitate of 2,4,6-tribromophenol forms. No catalyst is needed, in stark contrast to the bromination of benzene which requires a halogen carrier such as FeBr₃.
C₆H₅OH + 3Br₂ → C₆H₂Br₃OH↓ + 3HBr
The hydroxyl group strongly activates the ortho and para positions, so substitution occurs at all three available sites. The reaction is so sensitive that it can be used as a qualitative test for phenol and even for detecting trace amounts in water. Mechanistically, the polarised Br–Br molecule is attacked by the π system of the ring; the electron-donating effect of the –OH group stabilises the cationic Wheland intermediate, especially at the 2, 4, and 6 positions. The final product, 2,4,6-tribromophenol, is a bulky, insoluble molecule that precipitates cleanly from the aqueous mixture.
酚中富电子的芳环极易发生亲电取代。室温下将酚用溴水处理，溴的红棕色立即消失，并生成2,4,6-三溴苯酚的白色沉淀。无需催化剂，这与苯的溴化（需要FeBr₃等卤素载体）形成鲜明对比。羟基强烈地活化邻、对位，因此三个可用位置均发生取代。该反应灵敏度极高，可用作酚的定性检验，甚至可检测水中的痕量酚。机理上，极化的Br–Br分子被环的π体系进攻；–OH基团的给电子效应稳定了阳离子Wheland中间体，特别是在2、4、6位。最终产物2,4,6-三溴苯酚是一个体积较大、不溶于水的分子，能从水混合物中干净地沉淀出来。
7. Electrophilic Substitution: Nitration of Phenol | 亲电取代：硝化
Nitration of phenol requires milder conditions than those used for benzene because the ring is highly activated and phenol is easily oxidised. Using a mixture of concentrated nitric and sulfuric acids leads to extensive oxidation and tar formation. Instead, phenol is treated with dilute nitric acid at low temperature (around 0–5 °C). This yields a mixture of ortho-nitrophenol and para-nitrophenol.
C₆H₅OH + HNO₃ → o-HOC₆H₄NO₂ + p-HOC₆H₄NO₂ + H₂O
The ortho isomer can be separated from the para isomer by steam distillation because ortho-nitrophenol exhibits intramolecular hydrogen bonding, making it more volatile, whereas para-nitrophenol forms intermolecular hydrogen bonds and has a higher boiling point. This separation technique is often discussed in A-Level practical assessments. The nitro group, being electron-withdrawing, deactivates the ring towards further substitution, so careful control of stoichiometry and temperature prevents dinitration. Phenolic nitration illustrates how the balance between activation by –OH and deactivation by –NO₂ can be exploited in synthesis.
酚的硝化需要比苯更温和的条件，因为环被高度活化且酚易氧化。使用浓硝酸与浓硫酸的混合物会导致剧烈氧化并生成焦油状物。因此，酚用稀硝酸在低温（约0–5 °C）下处理，得到邻硝基苯酚和对硝基苯酚的混合物。邻位异构体可通过水蒸气蒸馏与对位异构体分离，因为邻硝基苯酚存在分子内氢键，挥发性更高，而对硝基苯酚形成分子间氢键，沸点更高。这一分离技术常在A-Level实验考试中讨论。由于硝基是吸电子基团，它会使环对进一步取代变得钝化，因此需小心控制化学计量和温度以避免二硝化。酚的硝化展示了如何利用–OH的活化作用与–NO₂的钝化作用之间的平衡进行合成。
8. Reaction with Iron(III) Chloride: Test for Phenol | 与氯化铁反应：酚的检验
One of the most characteristic reactions of phenol is the formation of an intense violet-purple colour when a few drops of neutral iron(III) chloride solution are added to an aqueous or alcoholic solution of phenol. The colour arises from the formation of a soluble iron(III)–phenol complex, in which the phenolic oxygen atoms coordinate to the iron centre. This colour test is not given by most simple aliphatic alcohols, making it an excellent qualitative test for the phenolic –OH group.
将几滴中性氯化铁(III)溶液加入酚的水溶液或醇溶液中，会立即产生深紫罗兰色，这是酚最具特征性的反应之一。该颜色来源于可溶的铁(III)–酚配合物的生成，其中酚氧原子与铁中心配位。大多数简单的脂肪醇不会产生此显色反应，因此这是一个极好的酚羟基定性检验。需要注意的是，不同取代酚可能产生蓝、绿或红色，但原理相同。这一试验常在A-Level定性分析中出现，应记住它不是氧化还原反应，而是配合物的形成。
9. Summary of Key Points | 要点总结
The reactions of phenol are governed by two principal electronic effects: the electron-donating resonance of the –OH group that activates the aromatic ring towards electrophilic substitution, and the acid-strengthening effect of the phenoxide ion's resonance stabilisation. Phenol reacts with sodium and sodium hydroxide but not with sodium carbonate, placing its acidity between alcohols and carboxylic acids. Esterification requires reactive acyl donors such as acid chlorides. Electrophilic substitutions, notably bromination and nitration, occur under much milder conditions than with benzene, and the tri-bromo derivative serves as a characteristic test. The purple colour with iron(III) chloride provides a fast, reliable detection of phenolic groups. Together, these reactions not only illustrate fundamental concepts of organic chemistry but also underpin numerous practical applications from pharmaceuticals to dyes.
酚的反应由两个主要电子效应控制：–OH基团的给电子共振活化芳环进行亲电取代，以及苯氧基负离子共振稳定所带来的酸性增强效应。酚与钠和氢氧化钠反应，但不与碳酸钠反应，这使其酸性介于醇和羧酸之间。酯化需要使用活性酰基供体如酰氯。亲电取代，特别是溴化和硝化，在比苯温和得多的条件下发生，三溴衍生物可用作特征检验。与氯化铁(III)产生的紫色可快速、可靠地检测酚羟基。这些反应不仅展示了有机化学的基本概念，也支撑着从药物到染料的众多实际应用。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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