📚 Organic Synthesis | 有机合成
Organic synthesis is the strategic construction of target molecules from readily available starting materials through a sequence of controlled chemical reactions. It lies at the heart of pharmaceutical development, materials science, and everyday chemical manufacturing. In A-Level Chemistry, we focus on planning multistep routes, choosing appropriate reagents and conditions, and understanding the scope of key functional group interconversions.
有机合成是利用一系列可控的化学反应，从易得的起始原料有策略地构建目标分子的过程。它是药物研发、材料科学和日常化学品制造的核心。在 A-Level 化学中，我们重点学习多步路线的规划、选择合适的试剂与条件，并理解关键官能团相互转化的适用范围。
1. Introduction to Organic Synthesis | 有机合成导论
The aim of an organic synthesis is to convert a given starting material into a desired target molecule, often with a higher level of structural complexity. A typical synthetic plan involves identifying the functional groups present, deciding which bonds need to be formed or broken, and matching these transformations to known reactions. Yield, atom economy, availability of reagents, number of steps, and ease of purification all influence the choice of route.
有机合成的目标是将给定的起始原料转化成所需的目标分子，通常结构更为复杂。一个典型的合成计划包括识别现有官能团、决定需要形成或断裂的化学键，并将这些转化与已知反应匹配。产率、原子经济性、试剂的可获得性、步骤数以及纯化的难易程度都会影响路线的选择。
Retrosynthetic analysis is a problem-solving technique where we work backwards from the target molecule to simpler precursors, disconnecting bonds that can be formed by reliable reactions. For example, an ester can be disconnected to a carboxylic acid and an alcohol; a secondary alcohol can be imagined from a ketone and a Grignard reagent.
逆合成分析是一种解决问题的技巧，我们从目标分子倒推至更简单的前体，切断那些可以通过可靠反应形成的键。例如，酯可以断开成羧酸和醇；二级醇可以设想由酮和格氏试剂制备。
2. Functional Group Interconversions | 官能团相互转化
The backbone of any organic synthesis is the ability to convert one functional group into another with high selectivity. Primary alcohols can be oxidised to aldehydes using acidified potassium dichromate(VI) with immediate distillation, and further to carboxylic acids under reflux with excess oxidant. Secondary alcohols yield ketones upon oxidation, while tertiary alcohols resist oxidation under these conditions.
任何有机合成的核心都是高选择性地将一种官能团转化为另一种。伯醇可以用酸化重铬酸钾(VI)氧化成醛，需立即蒸出以防止进一步氧化；在过量氧化剂和回流条件下则生成羧酸。仲醇氧化生成酮，而叔醇在此条件下不易被氧化。
Halogenoalkanes undergo nucleophilic substitution reactions that install new groups: with aqueous hydroxide ions they form alcohols; with cyanide ions in ethanol they yield nitriles (extending the carbon chain by one atom); with excess ammonia they produce primary amines. The nitrile group can later be hydrolysed to a carboxylic acid or reduced to an amine.
卤代烷可发生亲核取代反应以引入新基团：与氢氧根水溶液反应生成醇；与氰根离子的乙醇溶液反应得到腈（碳链延长一个碳原子）；与过量氨反应生成伯胺。腈基随后可水解成羧酸或还原成胺。
RCH₂Br + KCN → RCH₂CN + KBr
Alkenes can be converted to alcohols by acid-catalysed hydration or to halogenoalkanes by electrophilic addition of HX or X₂. These addition reactions provide entry points to many other functional groups.
烯烃可通过酸催化水合变成醇，或通过与 HX 或 X₂ 的亲电加成生成卤代烷。这些加成反应为众多其他官能团提供了切入点。
3. Extending the Carbon Chain | 延长碳链
Building larger carbon skeletons is essential when the target molecule contains more carbon atoms than the starting material. Grignard reagents, RMgX, are powerful nucleophiles that attack the partially positive carbon of carbonyl compounds. With methanal they give primary alcohols, with other aldehydes they give secondary alcohols, and with ketones they form tertiary alcohols. The reaction with carbon dioxide yields carboxylic acids after acidic work-up.
当目标分子所含碳原子多于起始原料时，构建更大的碳骨架至关重要。格氏试剂 RMgX 是强亲核试剂，能进攻羰基化合物中带部分正电荷的碳原子。与甲醛反应生成伯醇，与其他醛反应生成仲醇，与酮反应生成叔醇。与二氧化碳反应后再经酸性后处理则可得到羧酸。
CH₃MgBr + CO₂ → CH₃COO⁻MgBr⁺ → CH₃COOH
The reaction of halogenoalkanes with potassium cyanide, as mentioned above, increases the chain length by one carbon and introduces a nitrile group. Nitriles can be reduced by LiAlH₄ to primary amines or hydrolysed under acidic conditions to carboxylic acids, both useful transformations in building complex structures.
如前所述，卤代烷与氰化钾反应可增加一个碳原子并引入腈基。腈可被 LiAlH₄ 还原为伯胺，或在酸性条件下水解成羧酸，两者都是在构建复杂结构时非常有用的转化。
For aromatic compounds, Friedel-Crafts alkylation and acylation allow direct attachment of alkyl or acyl groups to a benzene ring, providing efficient ways to build carbon-carbon bonds on aromatic scaffolds.
对于芳香族化合物，傅克烷基化和酰基化反应可以直接将烷基或酰基连接到苯环上，为在芳香骨架上构建碳-碳键提供了高效的方法。
4. Aliphatic Building Blocks: Alkanes and Alkenes | 脂肪族构建模块：烷烃和烯烃
Alkanes from petroleum fractions are often the starting point. Cracking (thermal or catalytic) breaks longer alkanes into shorter alkanes and alkenes, the latter being much more reactive and serving as key intermediates. Ethene and propene are particularly valuable; ethene can be hydrated to ethanol, converted to epoxyethane, or polymerised.
来自石油馏分的烷烃往往是合成的起点。裂化（热裂化或催化裂化）将较长的烷烃断裂为更短的烷烃和烯烃，烯烃的反应活性高得多，是关键的中间体。乙烯和丙烯尤为重要；乙烯可水合生成乙醇、转化为环氧乙烷或发生聚合。
Alkenes react with hydrogen halides to form halogenoalkanes (Markovnikov addition usually predominates), with halogens to give dihalides, and with steam over a phosphoric acid catalyst to produce alcohols. Symmetrical alkenes give a single product; unsymmetrical alkenes can produce structural isomers that must be considered when planning a synthesis.
烯烃与卤化氢反应生成卤代烷（通常遵循马氏规则），与卤素生成二卤代物，与水蒸气在磷酸催化下生成醇。对称烯烃得到单一产物；不对称烯烃可能产生结构异构体，在规划合成时必须加以考虑。
The controlled oxidation of alkenes can also be useful: cold, dilute KMnO₄ yields diols, whereas hot, concentrated KMnO₄ cleaves the double bond to give carbonyl compounds or carboxylic acids, depending on the substitution pattern.
烯烃的控制氧化也很实用：冷稀 KMnO₄ 溶液生成二醇，而热浓 KMnO₄ 则切断双键，根据取代情况生成羰基化合物或羧酸。
5. Alcohols as Key Intermediates | 醇作为关键中间体
Alcohols occupy a central position in synthesis because they can be transformed into a wide variety of other functional groups. Primary alcohols are oxidised to aldehydes and then to carboxylic acids; secondary alcohols give ketones. The choice of oxidising agent and conditions (distillation vs reflux) determines the product obtained.
醇在合成中占据中心位置，因为它们可以转化为多种其他官能团。伯醇被氧化成醛继而变为羧酸；仲醇氧化生成酮。氧化剂的选择以及反应条件（蒸馏与回流）决定了最终产物的种类。
Alcohols can be dehydrated to alkenes by heating with concentrated H₂SO₄ or by passing the vapour over hot Al₂O₃. This elimination reaction is particularly useful when an alkene is needed as a synthetic intermediate.
醇与浓硫酸共热或让醇蒸气通过热的氧化铝，可发生脱水消除生成烯烃。当需要烯烃作为合成中间体时，这一消除反应尤其有用。
Halogenation of alcohols with PCl₅ (for chloroalkanes), SOCl₂ (for chloroalkanes with gaseous by-products), or P/I₂ mixtures (for iodoalkanes) provides clean routes to halogenoalkanes. Esterification with carboxylic acids or acyl chlorides yields esters, linking alcohol chemistry to carbonyl derivatives.
醇与 PCl₅（制取氯代烷）、SOCl₂（制取氯代烷，副产物为气体）或红磷/碘混合物（制取碘代烷）发生卤化反应，可提供纯净的卤代烷。与羧酸或酰氯的酯化反应得到酯，从而将醇化学与羰基衍生物联系起来。
6. The Chemistry of Carbonyl Compounds | 羰基化合物的化学
Aldehydes and ketones are susceptible to nucleophilic attack at the carbonyl carbon. Addition of HCN (generated in situ from NaCN and acid) produces hydroxynitriles, which can be hydrolysed to hydroxycarboxylic acids or reduced to amino alcohols. This reaction is important for chain extension and introduction of additional functionality.
醛和酮容易在羰基碳上发生亲核进攻。与 HCN（由 NaCN 和酸现场生成）的加成生成羟基腈，后者可水解成羟基羧酸或还原成氨基醇。此反应对碳链延长和引入额外官能团非常重要。
CH₃COCH₃ + HCN → CH₃C(OH)(CN)CH₃
Reducing agents such as NaBH₄ in water or LiAlH₄ in dry ether reduce aldehydes to primary alcohols and ketones to secondary alcohols. These reductions are widely used when an alcohol target is more accessible from a carbonyl precursor.
还原剂如 NaBH₄（在水中）或 LiAlH₄（在干醚中）可将醛还原为伯醇、将酮还原为仲醇。当醇类目标更易从羰基前体获得时，这些还原反应被广泛采用。
Aldehydes can be selectively oxidised to carboxylic acids using acidified dichromate or Tollen’s reagent; the latter leaves ketones unaffected. Identification tests such as 2,4-DNPH (orange precipitate for both aldehydes and ketones) and the iodoform reaction (for methyl ketones and ethanol) are also useful for monitoring transformations.
醛可用酸化重铬酸盐或托伦斯试剂选择性地氧化成羧酸；后者对酮无效。鉴别试验如 2,4-二硝基苯肼（醛、酮均生成橙色沉淀）和碘仿反应（针对甲基酮和乙醇）也可用于监控转化过程。
7. Carboxylic Acids and Their Derivatives | 羧酸及其衍生物
Carboxylic acids are often the target of an oxidation sequence, but they are also starting points for many derivatives. Esterification with an alcohol in the presence of a strong acid catalyst (e.g. concentrated H₂SO₄) is an equilibrium process that requires careful control to maximise yield. Acyl chlorides, prepared by reacting carboxylic acids with SOCl₂ or PCl₅, are much more reactive and react vigorously with alcohols, phenols, ammonia, and amines to give esters, amides, and substituted amides.
羧酸常是氧化序列的目标，但它们也是众多衍生物的起点。在强酸催化剂（如浓硫酸）存在下与醇的酯化反应是一个平衡过程，需要精心控制以获得最高产率。由羧酸与 SOCl₂ 或 PCl₅ 反应制得的酰氯活性高得多，能与醇、酚、氨和胺剧烈反应，分别生成酯、酰胺和取代酰胺。
CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl
Amides can also be formed by heating ammonium salts of carboxylic acids, but the acyl chloride route is generally faster and cleaner. Acid anhydrides are another class of reactive derivatives that acylates nucleophiles under milder conditions than acyl chlorides, often preferred in industry for safety and control.
酰胺也可以通过加热羧酸的铵盐来制备，但酰氯路线通常更快、更干净。酸酐是另一类活性衍生物，可在比酰氯更温和的条件下酰化亲核试剂，因安全性和可控性而在工业上常被优先选用。
Hydrolysis of esters, amides, and nitriles under acidic or basic conditions returns the carboxylic acid; this reversibility is useful for protective group strategies or for installing the acid group at a late stage.
酯、酰胺和腈在酸性或碱性条件下水解均可回到羧酸；这种可逆性对于保护基策略或在后期引入羧基非常有用。
8. Aromatic Compounds in Synthesis | 合成中的芳香族化合物
Aromatic rings provide a rigid, stable framework that can be functionalised with high regioselectivity. Nitration of benzene with a mixture of concentrated HNO₃ and H₂SO₄ at 50 °C introduces a nitro group, which can be reduced (using Sn and concentrated HCl, followed by NaOH) to form phenylamine, a crucial intermediate for dyes and pharmaceuticals.
芳香环提供了刚性的稳定骨架，并能以高区域选择性进行官能化。苯在 50 °C 下与浓硝酸和浓硫酸混合物发生硝化反应引入硝基，后者可被还原（用锡和浓盐酸，随后加 NaOH）生成苯胺——染料和药物的重要中间体。
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
Phenylamine can be diazotised with NaNO₂ and HCl below 10 °C to form a diazonium salt, which couples with phenols or aromatic amines to produce brightly coloured azo compounds. This sequence is a classic example of using aromatic substitution to construct complex molecules from simple precursors.
苯胺可在 10 °C 以下与 NaNO₂ 和 HCl 发生重氮化反应生成重氮盐，后者与酚或芳胺偶联，生成色泽鲜艳的偶氮化合物。这一序列是利用芳香取代反应由简单前体构建复杂分子的经典范例。
Halogenation of benzene with Cl₂ or Br₂ requires a halogen carrier such as AlCl₃ or FeBr₃. Friedel-Crafts alkylation (using RX and AlCl₃) and acylation (using RCOCl and AlCl₃) attach carbon fragments directly to the ring. The acyl group introduced can later be reduced (Clemmensen or Wolff-Kishner) to an alkyl chain, giving a two-step route to alkylbenzenes without rearrangement problems.
苯与 Cl₂ 或 Br₂ 的卤化反应需要卤素载体如 AlCl₃ 或 FeBr₃。傅克烷基化（使用 RX 和 AlCl₃）和酰基化（使用 RCOCl 和 AlCl₃）将碳碎片直接连接到环上。引入的酰基随后可被还原（克莱门森还原或沃尔夫-凯惜纳还原）为烷基链，提供一种无重排问题的两步法制备烷基苯。
9. Retrosynthetic Analysis and Planning | 逆合成分析与规划
Retrosynthesis begins with the target molecule and, by thinking backwards through imagined bond disconnections, identifies simpler starting materials. Each disconnection must correspond to a known forward reaction. A synthon is an idealised fragment generated in the disconnection, which is then matched to a real reagent. For instance, disconnecting a secondary alcohol yields a ketone synthon and an alkyl anion synthon, the latter being supplied by a Grignard reagent.
逆合成分析从目标分子开始，通过反向思维想象切断化学键，确定更简单的起始原料。每次切断必须对应于一个已知的正向反应。合成子是切断中产生的理想化碎片，随后匹配到真实的试剂上。例如，切断一个仲醇得到一个酮合成子和一个烷基负离子合成子，后者由格氏试剂提供。
When planning a route, chemists evaluate the number of steps, the expected yield of each step (overall yield = product of individual step yields), the atom economy, and the hazards of reagents. A shorter route with high-yielding steps is generally preferred, but sometimes a longer route with milder conditions is chosen to avoid side reactions or difficult separations.
在规划路线时，化学家会评估步骤数、每步的预期产率（总产率 = 各步产率之积）、原子经济性和试剂的危险性。通常优先选择步骤少且各步产率高的路线，但有时为了避开副反应或难以分离的混合物，也会选择步骤较长但条件较温和的路线。
10. Practical Techniques in Organic Synthesis | 有机合成中的实验技术
Carrying out reactions successfully requires mastery of laboratory techniques. Heating under reflux allows reactions to proceed at elevated temperatures without loss of volatile components. Simple distillation separates a liquid product from non-volatile impurities, while fractional distillation can separate mixtures of liquids with close boiling points.
成功实施反应要求掌握实验技术。回流加热可使反应在较高温度下进行而不损失挥发性组分。简单蒸馏将液体产物与不挥发性杂质分离，而分馏则可分离沸点相近的液体混合物。
Purification methods include solvent extraction (washing with aqueous solutions to remove acids, bases, or salts), drying with anhydrous salts like MgSO₄, and recrystallisation for solids from a suitable solvent. The purity of a solid can be assessed by its melting point: a sharp melting range close to the literature value indicates high purity.
纯化方法包括溶剂萃取（用水溶液洗涤以去除酸、碱或盐）、用无水盐如 MgSO₄ 干燥，以及用适当溶剂对固体进行重结晶。固体的纯度可通过其熔点评估：熔点尖锐且接近文献值则表明纯度高。
Safety considerations are integral: many organic solvents and reagents are flammable, toxic, or corrosive. Syntheses should be designed to minimise the use of hazardous substances and to generate as little waste as possible, in line with the principles of green chemistry. Where possible, catalytic and atom-economic reactions are favoured.
安全考量必不可少：许多
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