📚 Organic Synthesis: Multi-Step Routes and Techniques | 有机合成：多步路线与实验技术
Organic synthesis is the discipline of constructing complex organic molecules from simpler starting materials through a sequence of functional-group transformations. In AQA A-Level Chemistry, this topic demands that you recall the entire map of organic reactions learned across both years of study, plan logical multi-step routes, and apply practical techniques such as reflux, distillation, and recrystallisation with confidence.
有机合成是一门通过一系列官能团转化反应，从简单起始原料构建复杂有机分子的学科。在 AQA A-Level 化学中，本专题要求你回忆两年课程中学习过的全部有机反应图景，规划合理的多步合成路线，并熟练运用回流、蒸馏、重结晶等实验技术。
1. The Organic Synthesis Map | 有机反应总览
At A-level, you are not expected to invent new reactions; you must be able to select from the known pool of transformations the most efficient sequence to convert a starting molecule into a target product. A clear mental map linking alkanes, alkenes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, esters, nitriles, amines and aromatic compounds is essential.
在 A-Level 阶段，你不需要发明新反应，而是要从已知的反应库中选出最高效的序列，将起始分子转化为目标产物。你必须在脑中形成一张清晰的反应网络图，将烷烃、烯烃、卤代烷烃、醇、醛、酮、羧酸、酯、腈、胺和芳香族化合物联系起来。
The table below summarises the core aliphatic interconversions you must know:
下表总结了必须掌握的核心脂肪族官能团转化：
| Starting Functional Group | Reagent and Conditions | Product Functional Group |
|---|---|---|
| Alkene | Steam, H₃PO₄ catalyst, 300 °C, 60 atm | Alcohol |
| Alkene | HX (e.g. HBr) at room temperature | Halogenoalkane |
| Halogenoalkane | NaOH (aq), reflux | Alcohol |
| Halogenoalkane | KCN in ethanol, reflux | Nitrile |
| Halogenoalkane | Excess NH₃ in ethanol, reflux | Amine |
| Halogenoalkane | NaOH in ethanol, reflux | Alkene |
| Primary alcohol | K₂Cr₂O₇ / H₂SO₄, distil | Aldehyde |
| Primary alcohol | K₂Cr₂O₇ / H₂SO₄, reflux | Carboxylic acid |
| Secondary alcohol | K₂Cr₂O₇ / H₂SO₄, reflux | Ketone |
| Aldehyde or ketone | NaBH₄ in water | Alcohol |
| Aldehyde or ketone | KCN / H⁺ (i.e. HCN) | Hydroxynitrile |
| Carboxylic acid | Alcohol, conc H₂SO₄, reflux | Ester |
2. Alkanes to Halogenoalkanes | 烷烃到卤代烷烃
Alkanes are relatively unreactive, but under ultraviolet light they undergo free-radical substitution with halogens. For example, methane reacts with chlorine to produce chloromethane:
烷烃相对不活泼，但在紫外光照射下可与卤素发生自由基取代反应。例如，甲烷与氯气反应生成氯甲烷：
CH₄ + Cl₂ → CH₃Cl + HCl (UV light)
The reaction conditions are essential: UV light provides the energy to break the Cl-Cl bond homolytically, producing chlorine radicals. In a synthesis context, this route is rarely selective for a single product, so it is mainly used to prepare simple halogenoalkanes when no better route exists.
反应条件至关重要：紫外光提供能量使 Cl-Cl 键发生均裂，产生氯自由基。在合成中，该路线很少只生成单一产物，因此通常仅在无更优路线时用于制备简单的卤代烷烃。
3. Halogenoalkanes as Key Intermediates | 卤代烷烃：关键中间体
Halogenoalkanes are the most versatile intermediates in aliphatic synthesis because the carbon-halogen bond is polarised, making the carbon atom susceptible to nucleophilic attack. Four reactions are particularly important.
卤代烷烃是脂肪族合成中最通用的中间体，因为碳-卤键具有极性，使碳原子容易受到亲核试剂进攻。以下四个反应尤为重要。
Nucleophilic substitution with aqueous NaOH produces an alcohol:
与氢氧化钠水溶液的亲核取代生成醇：
CH₃CH₂Br + NaOH (aq) → CH₃CH₂OH + NaBr (reflux)
Nucleophilic substitution with KCN extends the carbon chain by one carbon atom, forming a nitrile:
与氰化钾的亲核取代使碳链增加一个碳原子，生成腈：
CH₃CH₂Br + KCN → CH₃CH₂CN + KBr (ethanol, reflux)
Nucleophilic substitution with excess ammonia gives a primary amine:
与过量氨的亲核取代得到伯胺：
CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br (ethanol, reflux)
Elimination with ethanolic NaOH produces an alkene:
与氢氧化钠乙醇溶液的消除反应生成烯烃：
CH₃CH₂Br + NaOH (ethanol) → CH₂=CH₂ + NaBr + H₂O (reflux)
4. Alcohols and Oxidation | 醇与氧化反应
Alcohols are classified as primary, secondary or tertiary, and their oxidation behaviour differs accordingly. The oxidising agent used in AQA is acidified potassium dichromate, K₂Cr₂O₇ / H₂SO₄, which changes colour from orange to green.
醇分为伯醇、仲醇和叔醇，它们的氧化行为各不相同。AQA 使用的氧化剂是酸化的重铬酸钾 K₂Cr₂O₇ / H₂SO₄，反应中溶液由橙色变为绿色。
A primary alcohol can be oxidised to an aldehyde if the product is distilled off immediately, or to a carboxylic acid if heated under reflux:
伯醇若立即蒸出产物可被氧化为醛；若加热回流则可被氧化为羧酸：
CH₃
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