📚 S3.2 Functional Groups: Classification of Compounds | 官能团：有机化合物的分类
Organic chemistry is built on a relatively small number of functional groups – specific atoms or groups of atoms within a molecule that are responsible for its characteristic chemical reactions. Understanding these groups is the key to predicting behaviour, naming compounds systematically, and linking structure to reactivity.
有机化学建立在少数几个官能团之上——它们是分子中特定的原子或原子团，决定了分子特有的化学反应。理解这些官能团是预测化学行为、系统命名化合物以及将结构与反应性联系起来的关键。
1. What is a Functional Group? | 什么是官能团？
A functional group is an atom or a group of atoms in an organic molecule that governs its chemical properties. Regardless of the size of the carbon skeleton, molecules containing the same functional group will undergo similar reactions. For instance, all alcohols contain the hydroxyl (-OH) group and can be oxidised, while all carboxylic acids contain the carboxyl (-COOH) group and display acidic behaviour.
官能团是有机分子中决定其化学性质的一个原子或一组原子。无论碳骨架大小如何，含有相同官能团的分子都会发生相似的化学反应。例如，所有醇都含有羟基（-OH）并可以被氧化，而所有羧酸都含有羧基（-COOH）并表现出酸性。
2. Homologous Series | 同系物系列
A homologous series is a family of organic compounds that share the same functional group, with each successive member differing by a -CH₂- unit. Members of a homologous series show a gradual trend in physical properties, such as increasing boiling point with molar mass, and they can be represented by a general molecular formula. For alkanes this is CₙH₂ₙ₊₂, and for alkenes it is CₙH₂ₙ.
同系物是一类具有相同官能团的有机化合物家族，每个相邻成员相差一个 -CH₂- 单元。同系物在物理性质上表现出渐变趋势，例如沸点随摩尔质量增加而升高，并且可以用一个通式来表示。烷烃的通式为 CₙH₂ₙ₊₂，烯烃的通式为 CₙH₂ₙ。
3. Alkanes: The Simplest Hydrocarbons | 烷烃：最简单的烃类
Alkanes are saturated hydrocarbons containing only C-C and C-H single bonds. They have no functional group, which makes them relatively unreactive compared to other classes. Their chemistry is dominated by combustion and free-radical substitution reactions. Examples include methane (CH₄), ethane (C₂H₆) and propane (C₃H₈). The suffix used in IUPAC naming is "-ane".
烷烃是只含碳碳单键和碳氢单键的饱和烃。它们没有官能团，因此与其他类别的化合物相比活性较低。烷烃的化学性质以燃烧和自由基取代反应为主。例子包括甲烷（CH₄）、乙烷（C₂H₆）和丙烷（C₃H₈）。IUPAC 命名时使用的后缀是“-ane”。
4. Alkenes and Alkynes: Unsaturated Hydrocarbons | 烯烃和炔烃：不饱和烃
oup, making them unsaturated. They undergo addition reactions, such as hydrogenation and halogenation, and polymerisation. The general formula is CₙH₂ₙ, and the suffix is "-ene". Alkynes contain a triple bond (C≡C), have the general formula CₙH₂ₙ₋₂, and use the suffix "-yne". The pi bonds in these groups are electron-rich and attack electrophiles.
烯烃含有碳碳双键（C=C）作为官能团，使其不饱和。它们发生加成反应，如加氢和卤代，以及聚合反应。通式为 CₙH₂ₙ，后缀是“-ene”。炔烃含有三键（C≡C），通式为 CₙH₂ₙ₋₂，后缀为“-yne”。这些基团中的 π 键富电子，会进攻亲电试剂。
5. Halogenoalkanes: Substitution with Halogens | 卤代烷：卤素取代
The functional group of a halogenoalkane is a halogen atom (F, Cl, Br, I) attached to an alkyl chain, denoted as -X. Their reactivity centres on nucleophilic substitution, where the halogen is replaced by a nucleophile such as OH⁻ or CN⁻. The rate depends on whether the carbon is primary, secondary or tertiary. Examples include chloromethane (CH₃Cl) and bromoethane (C₂H₅Br).
卤代烷的官能团是连接在烷基链上的卤素原子（F, Cl, Br, I），用 -X 表示。它们的反应活性集中在亲核取代上，卤素被 OH⁻ 或 CN⁻ 之类的亲核试剂取代。反应速率取决于碳是伯、仲还是叔碳。例子包括氯甲烷（CH₃Cl）和溴乙烷（C₂H₅Br）。
6. Alcohols and Ethers: Oxygen-Containing Groups | 醇和醚：含氧官能团
Alcohols contain the hydroxyl group (-OH) bonded to a saturated carbon. They are classified as primary, secondary or tertiary depending on that carbon. Alcohols can be oxidised, form esters, and undergo dehydration. The suffix is "-ol", as in ethanol (CH₃CH₂OH). Ethers have an oxygen atom bridging two alkyl or aryl groups (R-O-R'). They are relatively inert and often used as solvents; diethyl ether (CH₃CH₂OCH₂CH₃) is a classic example.
醇含有与饱和碳相连的羟基（-OH）。根据该碳原子的类型，醇可分为伯、仲或叔醇。醇可被氧化、生成酯并发生脱水。后缀是“-ol”，例如乙醇（CH₃CH₂OH）。醚有一个氧原子桥接两个烷基或芳基（R-O-R'）。它们相对惰性，常被用作溶剂；乙醚（CH₃CH₂OCH₂CH₃）是一个经典例子。
7. Carbonyl Compounds: Aldehydes and Ketones | 羰基化合物：醛和酮
s, the carbonyl is at the end of a carbon chain and is attached to at least one hydrogen atom; the functional group is written as -CHO. Aldehydes are easily oxidised to carboxylic acids. The suffix is "-al", e.g. ethanal (CH₃CHO). In ketones, the carbonyl is bonded to two alkyl groups (R-CO-R'), and they resist mild oxidation. The suffix is "-one", as in propanone (CH₃COCH₃).
醛和酮都含有羰基（C=O）。在醛中，羰基位于碳链末端，至少与一个氢原子相连；官能团写作 -CHO。醛容易被氧化成羧酸。后缀是“-al”，例如乙醛（CH₃CHO）。在酮中，羰基与两个烷基相连（R-CO-R'），它们不易被温和氧化。后缀是“-one”，例如丙酮（CH₃COCH₃）。
8. Carboxylic Acids and Esters | 羧酸和酯
Carboxylic acids have the carboxyl group (-COOH), which is essentially a carbonyl bonded to a hydroxyl group. They are weak acids, forming carboxylate salts with bases. The suffix is "-oic acid", e.g. ethanoic acid (CH₃COOH). Esters are derived from carboxylic acids by replacing the acidic hydrogen with an alkyl group, giving the functional group -COO-. They are known for their fruity smells and are formed by condensation reactions between an acid and an alcohol. Naming uses the alkyl part first, then the acid part as "-oate": methyl ethanoate (CH₃COOCH₃).
羧酸含有羧基（-COOH），它本质上是一个羰基与一个羟基相连。它们是一元弱酸，能与碱生成羧酸盐。后缀是“-oic acid”，例如乙酸（CH₃COOH）。酯是由羧酸衍生而来，用烷基取代酸性氢，得到官能团 -COO-。它们以水果香味著称，并由酸和醇的缩合反应生成。命名时先写烷基部分，再将酸部分写作“-oate”：乙酸甲酯（CH₃COOCH₃）。
9. Amines, Amides, and Nitriles: Nitrogen-Containing Groups | 胺、酰胺和腈：含氮官能团
Amines contain the amino group (-NH₂) attached to an alkyl or aryl group. They act as bases and nucleophiles, and are classified as primary, secondary or tertiary depending on the number of carbon groups attached to nitrogen. The suffix is "-amine", e.g. ethylamine (CH₃CH₂NH₂). Amides feature the group -CONH₂, formed when a carboxylic acid reacts with ammonia or an amine. Nitriles contain the cyano group (-C≡N), with the suffix "-nitrile", such as ethanenitrile (CH₃CN). Nitriles can be hydrolysed to carboxylic acids or reduced to amines.
胺含有连接在烷基或芳基上的氨基（-NH₂）。它们具有碱性和亲核性，根据与氮相连的碳基团数目可分为伯胺、仲胺或叔胺。后缀是“-amine”，例如乙胺（CH₃CH₂NH₂）。酰胺含有 -CONH₂ 基团，由羧酸与氨或胺反应生成。腈含有氰基（-C≡N），后缀为“-nitrile”，例如乙腈（CH₃CN）。腈可水解释放出羧酸，也可被还原为胺。
10. Functional Group Isomerism | 官能团异构
Compounds with the same molecular formula but different functional groups belong to different homologous series and are called functional group isomers. For example, C₃H₆O₂ can be propanoic acid (a carboxylic acid), methyl ethanoate (an ester) or hydroxyacetone (a ketone with an alcohol group). Similarly, C₃H₉N can be propylamine (an amine) or ethylmethylamine, but also N-methylethanamine, illustrating that carbon skeleton isomerism coexists with functional group differences. Recognising functional group isomers is a core skill in organic identification.
具有相同分子式但官能团不同的化合物属于不同的同系物，称为官能团异构体。例如，C₃H₆O₂ 可以是丙酸（羧酸）、乙酸甲酯（酯）或羟基丙酮（含酮和醇基团）。同样，C₃H₉N 可以是丙胺（胺）或乙基甲胺，同时也存在碳骨架异构。识别官能团异构体是有机物鉴定的核心技能。
11. Classifying Compounds by Skeleton: Aliphatic, Alicyclic, and Aromatic | 按骨架分类：脂肪族、脂环族和芳香族
Beyond functional groups, organic molecules are also classified by their carbon framework. Aliphatic compounds consist of open chains or branched chains. Alicyclic compounds contain rings that are not aromatic, such as cyclohexane or cyclohexene. Aromatic compounds contain one or more benzene rings (C₆H₆) and follow Hückel's rule. The arenes, with the functional group being the phenyl group (C₆H₅-), show distinct electrophilic substitution reactions rather than addition typical of alkenes.
除了官能团，有机分子还根据碳骨架分类。脂肪族化合物由开链或支链组成。脂环族化合物含有非芳香性的环，如环己烷或环己烯。芳香族化合物含有一个或多个苯环（C₆H₆）并遵循休克尔规则。芳烃的官能团可视为苯基（C₆H₅-），它们发生典型的亲电取代反应，而不是烯烃的加成反应。
12. IUPAC Naming and the Importance of Functional Groups | IUPAC 命名和官能团的重要性
ides which acts as the suffix. For example, -COOH outranks -OH and -C=O. By learning the structure and priority of each functional group, chemists can deduce the correct systematic name and draw the molecular structure unambiguously, forming the foundation of worldwide chemical communication.
国际纯粹与应用化学联合会（IUPAC）命名系统基于含有主官能团的最长连续碳链给出母体名称，后缀或前缀由该官能团决定。当存在多个官能团时，通过优先次序决定哪个作为后缀。例如 -COOH 优先于 -OH 和 -C=O。通过学习每种官能团的结构和优先级，化学家可以推断出正确的系统名称，并明确无误地画出分子结构，这构成了全球化学交流的基础。
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