📚 Introduction to Organic Chemistry | 有机化学入门
Organic chemistry is the branch of chemistry that studies the structure, properties, composition, reactions, and synthesis of carbon-based compounds. From the fuels that power our cars to the pharmaceuticals that heal our bodies, organic molecules form the molecular backbone of life. This introductory chapter builds the essential vocabulary and concepts needed to navigate the entire subject: how carbon bonds, how we draw and name molecules, how we classify them, and the types of isomerism and reactions that define their behaviour. Mastering these foundations will unlock every subsequent topic in the A-Level syllabus.
有机化学是研究碳基化合物的结构、性质、组成、反应与合成的化学分支。从驱动汽车的燃料到治愈身体的药物，有机分子构成了生命的分子骨架。本章介绍有机化学的基本词汇与概念，帮助你掌握碳原子如何成键、如何绘制和命名分子、如何对它们进行分类，以及定义它们行为的异构现象与反应类型。打好这些基础，就能开启A-Level课程中所有后续主题的大门。
1. What Is Organic Chemistry? | 什么是有机化学？
Initially, chemists believed that organic compounds could only be produced by living organisms under the influence of a 'vital force'. This theory was overturned in 1828 when Friedrich Wohler synthesised urea (an organic compound found in urine) from ammonium cyanate, an inorganic salt. Today, organic chemistry is defined simply as the chemistry of carbon compounds, with the exception of carbon oxides, carbonates, hydrogencarbonates, carbides and cyanides, which are traditionally studied as inorganic substances. Carbon forms millions of compounds, far more than any other element, owing to its unique ability to catenate and form stable bonds with many other atoms.
最初，化学家认为有机化合物只能在“生命力”的影响下由生物体产生。1828年，弗里德里希·维勒利用无机盐氰酸铵合成了尿素（一种在尿液中发现的有机化合物），推翻了这一理论。如今，有机化学被简单地定义为碳化合物的化学，但不包括碳的氧化物、碳酸盐、碳酸氢盐、碳化物和氰化物，这些传统上被归入无机物。碳能够形成数百万种化合物，数量远超其他任何元素，这得益于其独特的成链能力以及与其他多种原子形成稳定键的本领。
2. The Unique Bonding of Carbon | 碳独特的键合方式
Carbon has an electronic configuration of 1s²2s²2p², with four electrons in its outer shell. To achieve a stable octet, carbon forms four covalent bonds. Its ability to form strong single, double and triple bonds with itself (catenation) and with other elements such as hydrogen, oxygen, nitrogen and the halogens leads to an endless variety of chains, rings and branched structures. The four bonding pairs of electrons repel each other to positions of minimum energy, giving rise to tetrahedral geometry (bond angle 109.5°) around sp³ hybridised carbon, trigonal planar (120°) around sp² carbon and linear (180°) around sp carbon.
碳的电子排布为1s²2s²2p²，外层有四个电子。为达到稳定的八电子构型，碳原子会形成四个共价键。它能与自身形成牢固的单键、双键和三键（成链能力），也能与氢、氧、氮和卤素等元素成键，从而产生无穷无尽的直链、环状和支链结构。四对成键电子相互排斥，占据能量最低的位置，在sp³杂化碳周围形成四面体构型（键角109.5°），在sp²碳周围形成三角平面（120°），在sp碳周围形成直线形（180°）。
3. Representing Organic Molecules | 有机分子的表示方法
Chemists use several types of formulae to display organic structures with different levels of detail. The empirical formula shows the simplest whole‑number ratio of atoms of each element in the compound (e.g. CH₂O for ethanoic acid). The molecular formula shows the actual number of atoms of each element in one molecule (e.g. C₂H₄O₂ for ethanoic acid). A structural formula shows how the atoms are arranged, often written in a condensed form (e.g. CH₃COOH). A displayed formula draws every atom and every bond. Finally, the skeletal formula omits carbon and hydrogen atoms entirely, representing carbon‑carbon bonds as lines, with functional groups drawn explicitly; each vertex and line end represents a carbon atom.
化学家使用多种类型的化学式来展示有机化合物的结构，详细程度各不相同。实验式表示化合物中各元素原子的最简整数比（例如乙酸的实验式为CH₂O）。分子式表示一个分子中各元素原子的实际数目（例如乙酸的分子式为C₂H₄O₂）。结构式表示原子的排列方式，通常以简写形式书写（例如CH₃COOH）。展示式画出每一个原子和每一个键。最后，骨架式则完全省略碳原子和氢原子，以线段表示碳碳键，官能团则明确画出；每个顶点和线端代表一个碳原子。
4. Homologous Series and General Formulae | 同系物与通式
A homologous series is a family of organic compounds that share the same functional group and therefore similar chemical properties. Successive members differ by a –CH₂– unit, and their physical properties (such as boiling point) change gradually with increasing chain length. Each series can be described by a general formula. For example, alkanes follow CₙH₂ₙ₊₂, alkenes with one double bond follow CₙH₂ₙ, and alcohols with one –OH group follow CₙH₂ₙ₊₁OH. Recognising which homologous series a molecule belongs to is the first step in predicting its reactions.
同系物是一类具有相同官能团、因而化学性质相似的有机化合物。相邻成员相差一个–CH₂–单元，其物理性质（如沸点）随碳链增长而逐渐变化。每一系列都可以用一个通式来描述。例如，烷烃的通式为CₙH₂ₙ₊₂，含有一个双键的烯烃为CₙH₂ₙ，含有一个–OH基的醇为CₙH₂ₙ₊₁OH。判断一个分子属于哪个同系物，是预测其反应的第一步。
5. Functional Groups – The Heart of Reactivity | 官能团——反应性的核心
ined by their functional groups. For instance, all alkenes contain the C=C double bond, which enables electrophilic addition, while all carboxylic acids contain the – The following table lists the functional groups you must recognise at the start of your A‑Level course.
官能团是一个原子或一组原子，它赋予分子特有的化学性质。不同的同系物由其官能团定义。例如，所有烯烃都含有C=C双键，这使它们能够发生亲电加成；而所有羧酸都含有–COOH基团，这是它们具有酸性的原因。下表列出了A-Level课程开始时必须识别的官能团。
| Homologous Series | 中文名称 | Functional Group | 官能团结构 |
|---|---|---|---|
| Alkane | 烷烃 | None (C–C, C–H only) | — |
| Alkene | 烯烃 | C=C | >C=C< |
| Halogenoalkane | 卤代烷 | –X (F, Cl, Br, I) | –Cl, –Br etc. |
| Alcohol | 醇 | –OH | –O–H |
| Aldehyde | 醛 | –CHO | O=CH– |
| Ketone | 酮 | C=O between two carbons | –C–(C=O)–C– |
| Carboxylic acid | 羧酸 | –COOH | O=C–O–H |
| Ester | 酯 | –COO– | O=C–O– |
| Amine | 胺 | –NH₂ (primary) | –N–H₂ |
| Nitrile | 腈 | –C≡N | –C≡N |
6. IUPAC Nomenclature Basics | IUPAC命名法基础
Systematic naming follows rules set by the International Union of Pure and Applied Chemistry (IUPAC). The name of a molecule consists of a stem that indicates the longest continuous carbon chain (meth‑, eth‑, prop‑, but‑, pent‑, hex‑, etc.), a suffix that identifies the principal functional group (e.g. ‑ane, ‑ene, ‑ol, ‑al, ‑one, ‑oic acid), and prefixes that show the identity and position of side chains and lower‑priority groups. The chain is numbered so that the principal functional group gets the lowest possible locant. Substituents are listed alphabetically, with multiplying prefixes (di‑, tri‑, tetra‑) ignored when deciding order. Hyphens separate numbers from letters, and commas separate numbers.
系统命名遵循国际纯粹与应用化学联合会(IUPAC)制定的规则。一个分子的名称由三部分组成：词干表示最长的连续碳链（甲‑、乙‑、丙‑、丁‑、戊‑、己‑等），后缀标明主要的官能团（例如‑烷、‑烯、‑醇、‑醛、‑酮、‑酸），前缀表示侧链和次要基团的身份与位次。碳链的编号应使主官能团获得尽可能小的位次。取代基按字母顺序列出，决定顺序时忽略倍数前缀（二、三、四）。连字符分隔数字与字母，逗号分隔数字。
As an example, consider the molecule CH₃CH(CH₃)CH₂OH. The longest chain has four carbon atoms (butan‑), the principal functional group is –OH (suffix ‑ol), and there is a methyl substituent on carbon 2. The correct name is 2‑methylbutan‑1‑ol. Notice that the hydroxyl group is at position 1, and the methyl group is at position 2.
以分子CH₃CH(CH₃)CH₂OH为例。最长碳链有四个碳原子（丁‑），主官能团是–OH（后缀‑醇），在碳2上有一个甲基取代基。正确名称为2‑甲基‑1‑丁醇。注意羟基在1位，甲基在2位。
7. Classification of Carbon and Hydrogen Atoms | 碳原子和氢原子的分类
Carbon atoms in an organic molecule are classified as primary (1°), secondary (2°), tertiary (3°) or quaternary (4°) according to the number of other carbon atoms they are bonded to. A primary carbon is attached to one other carbon; a secondary to two; a tertiary to three; a quaternary to four. Hydrogen atoms are described by the carbon they are attached to: a hydrogen on a primary carbon is a primary hydrogen. This classification is essential when discussing the stability of carbocations and the mechanisms of substitution and elimination reactions.
有机分子中的碳原子根据其所连接的其他碳原子个数，可分为伯(1°)、仲(2°)、叔(3°)和季(4°)碳。伯碳连接一个其他碳原子，仲碳连接两个，叔碳连接三个，季碳连接四个。氢原子则由其所连接的碳原子来描述：连接在伯碳上的氢称为伯氢。在讨论碳正离子的稳定性以及取代反应和消除反应的机理时，这一分类至关重要。
8. Structural Isomerism | 结构异构
Structural isomers have the same molecular formula but different structural formulae, meaning their atoms are connected in a different order. There are three distinct types: chain isomerism, in which the carbon skeleton is arranged differently (e.g. butane and methylpropane, both C₄H₁₀); position isomerism, where the functional group occupies different positions on the same backbone (e.g. propan‑1‑ol and propan‑2‑ol, both C₃H₈O); and functional group isomerism, where the atoms are rearranged to give different functional groups (e.g. ethanol and methoxymethane, both C₂H₆O, containing –OH and –O– respectively).
结构异构体具有相同的分子式，但结构式不同，即原子连接顺序不同。它分为三种类型：碳链异构，指碳骨架排列不同（例如丁烷和甲基丙烷，分子式均为C₄H₁₀）；位置异构，指相同的官能团连接在同一主链的不同位置（例如1‑丙醇和2‑丙醇，分子式均为C₃H₈O）；以及官能团异构，指原子重新排列后形成不同的官能团（例如乙醇和甲氧基甲烷，分子式均为C₂H₆O，分别含有–OH和–O–基团）。
9. Stereoisomerism – Geometric (Cis‑Trans) Isomers | 立体异构——几何（顺反）异构
the same molecular formula and the same sequence of bonded atoms, but differ in the three‑l arrangement of their atoms. The first type of stereoisomerism you meet is geometric (cis‑ ic compounds where there is restricted rotation about a bond. For an alkene to exhibit cis‑ carbon of the C=C bond must carry two different groups. The cis isomer has the two highest‑ his subtle structural difference leads to distinct physical and often chemical properties.
立体异构体具有相同的分子式和相同的原子连接顺序，但原子在三维空间的排列方式不同。你最先遇到的立体异构类型是几何（顺反）异构，它发生于因双键或环而限制旋转的烯烃和环状化合物中。一个烯烃若要表现出顺反异构，其C=C双键的每个碳原子必须连接两个不同的基团。顺式异构体中，两个优先基团位于双键的同侧；反式异构体中，它们位于异侧。这种细微的结构差异会导致不同的物理性质，并常常带来不同的化学性质。
10. Introduction to Reaction Types | 反应类型简介
Most organic reactions fall into a few fundamental categories. Addition occurs when two molecules combine to form a single product; it is typical of unsaturated compounds like alkenes (e.g. addition of H₂ to ethene). Substitution involves one atom or group replacing another; halogenoalkanes undergo nucleophilic substitution. Elimination is the reverse of addition, removing atoms from adjacent carbons to create a double bond. Oxidation of organic molecules often involves gain of oxygen or loss of hydrogen (e.g. primary alcohol to aldehyde to carboxylic acid), while reduction involves gain of hydrogen or loss of oxygen. Recognising the reaction type helps predict products and choose appropriate reagents.
大多数有机反应可归入几个基本类别。加成反应指两个分子结合生成单一产物，这是不饱和化合物如烯烃的典型反应（例如乙烯与H₂的加成）。取代反应指一个原子或基团被另一个替换；卤代烷烃可发生亲核取代。消除反应是加成的逆过程，从相邻碳原子上移除原子以形成双键。有机分子的氧化常伴随着氧原子的增加或氢原子的减少（例如伯醇氧化成醛再成羧酸），而还原则伴随着氢原子的增加或氧原子的减少。识别反应类型有助于预测产物和选择合适的试剂。
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