A-Level Chemistry: Systematic Nomenclature of Organic Compounds | A-Level 化学：有机化合物的系统命名法

📚 A-Level Chemistry: Systematic Nomenclature of Organic Compounds | A-Level 化学：有机化合物的系统命名法

Systematic nomenclature is the language through which chemists communicate the structure of organic molecules with precision. In the CIE A-Level syllabus, the IUPAC system is not just a list of rules — it is a logical framework that links structure to name, and name back to structure.

系统命名法是化学家精确沟通有机分子结构的语言。在 CIE A-Level 考纲中，IUPAC 体系不只是规则列表，更是一套将结构、名称与反向推导紧密相连的逻辑框架。

1. Why IUPAC Nomenclature Matters | 为何 IUPAC 命名法如此重要

Common names such as “acetone” or “ethylene glycol” are convenient in everyday conversation, but they fail to reveal the molecular structure unambiguously. IUPAC nomenclature ensures that every organic compound has exactly one systematic name that can be translated directly into a structural formula.

像“丙酮”或“乙二醇”这样的俗名在日常对话中很方便，但无法唯一地揭示分子结构。IUPAC 命名法确保每个有机化合物拥有一个能直接转化为结构式的唯一系统名称。

Consider the molecular formula C₄H₁₀O: it corresponds to two different alcohols and one ether. Only systematic names — butan-1-ol, butan-2-ol and ethoxyethane — can distinguish them clearly.

以分子式 C₄H₁₀O 为例：它对应两种不同的醇和一种醚。只有系统名称——丁-1-醇、丁-2-醇和乙氧基乙烷——才能将它们清晰区分。

2. The Foundation: Parent Chain and Prefixes | 根基：主链与前缀

The first step in naming any acyclic compound is to identify the longest continuous carbon chain. This chain gives the parent name: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- for 1 to 10 carbon atoms.

命名任何开链化合物的第一步是找出最长的连续碳链。该链提供母体名称：甲、乙、丙、丁、戊、己、庚、辛、壬、癸分别对应 1 至 10 个碳原子。

The suffix indicates the principal functional group, while prefixes are used for substituents. For example, a five-carbon chain with a methyl substituent on carbon 2 is named 2-methylpentane.

后缀表示主要官能团，前缀用于取代基。例如，在 2 号碳上带有甲基取代基的五碳链命名为 2-甲基戊烷。

CH₃–CH(CH₃)–CH₂–CH₂–CH₃ = 2-methylpentane | 2-甲基戊烷

3. Numbering the Chain: The Lowest Locant Rule | 链编号：最低位次规则

After identifying the parent chain, we assign locants (numbers) to the carbon atoms. The chain must be numbered so that the principal functional group receives the lowest possible number.

确定主链后，需为碳原子编号。编号方向的选择必须使主要官能团获得尽可能小的位次数字。

If there are multiple substituents, the first point of difference determines the direction. Compare 1,2-dimethylpentane with 2,3-dimethylpentane — the former is correct because 1 < 2 at the first differing position.

当存在多个取代基时，由第一个不同点决定编号方向。比较 1,2-二甲基戊烷与 2,3-二甲基戊烷——前者正确，因为在第一个不同位次上 1 < 2。

Always give the principal group the lowest locant.

始终将主官能团放在最低位次。
Then minimise locants for substituents in alphabetical order of comparison.

然后按取代基字母顺序比较，使位次总和最小。

4. Alkanes and Branched Chains | 烷烃与支链

Alkanes are the simplest family. The suffix is -ane. For branched alkanes, each substituent is named as an alkyl group: -yl replaces -ane (methyl, ethyl, propyl, etc.).

烷烃是最简单的系列，后缀为“-烷”。对于支链烷烃，每个取代基作为烷基命名：将“-ane”改为“-yl”（甲基、乙基、丙基等）。

When multiple identical substituents appear, use the multiplicative prefixes di-, tri-, tetra- and list every locant explicitly. The prefixes di-, tri- etc. are ignored when alphabetising, but iso-, sec- and tert- are not.

当多个相同取代基出现时，使用倍数前缀二、三、四，并逐一列出所有位次。字母排序时忽略二、三等倍数前缀，但 iso-、sec-、tert- 不忽略。

CH₃–CH₂–CH(CH₃)–CH(CH₃)–CH₃ = 2,3-dimethylpentane | 2,3-二甲基戊烷

5. Alkenes and Alkynes | 烯烃与炔烃

For unsaturated hydrocarbons, the parent chain must contain the double or triple bond, and the suffix becomes -ene or -yne. The chain is numbered to give the multiple bond the lowest locant.

对于不饱和烃，主链必须包含双键或三键，后缀变为“-烯”或“-炔”。链编号应使多重键获得最低位次。

More than one double bond is indicated as -diene, -triene, etc. Every double bond must receive a separate locant. An example is hexa-1,3-diene: the two double bonds are at positions 1 and 3.

一个以上的双键用“-二烯”“-三烯”等表示。每个双键都必须有单独的位次。例如己-1,3-二烯：两个双键位于 1 位和 3 位。

When both a double and a triple bond appear, the suffix is ordered as -en-yne, and the double bond takes priority for lower locants — so pent-1-en-4-yne is correct, not pent-4-en-1-yne.

当双键和三键同时出现时，后缀写为“-烯-炔”，编号时双键优先获得低位次——因此戊-1-烯-4-炔正确，而非戊-4-烯-1-炔。

6. Haloalkanes | 卤代烷烃

Halogen substituents are always treated as prefixes: fluoro-, chloro-, bromo-, iodo-. They have no priority in selecting the parent chain or the suffix; they simply modify the alkane name.

卤素取代基始终作为前缀处理：氟、氯、溴、碘。它们在选择主链或后缀时没有优先权，只是修饰烷烃名称。

Number the chain to give the lowest locants to the principal functional group first, then to multiple substituents. In 1-bromo-2-chloropropane, bromo and chloro are both substituents; the chain is numbered to give the lowest sum.

编号时先给主要官能团最低位次，再考虑多个取代基。在 1-溴-2-氯丙烷中，溴和氯均为取代基；链编号使位次总和最小。

CH₃–CHCl–CH₂Br = 1-bromo-2-chloropropane | 1-溴-2-氯丙烷

7. Alcohols and Phenols | 醇与酚

The suffix -ol identifies the presence of a hydroxyl group (–OH). The parent chain must include the carbon bearing the –OH, and this carbon receives the lowest possible locant.

后缀“-醇”标识羟基（–OH）的存在。主链必须包含连接 –OH 的碳原子，该碳获得尽可能低的位次。

If the –OH group is on a benzene ring, the compound is named as a phenol derivative. For example, C₆H₅OH is simply phenol; 2-methylphenol has a methyl group at position 2 relative to the –OH.

如果 –OH 连在苯环上，该化合物以酚的衍生物命名。例如 C₆H₅OH 就是苯酚；2-甲基苯酚在相对 –OH 的 2 位上有甲基。

Polyhydric alcohols use the suffixes -diol, -triol. The simple compound HOCH₂CH₂OH is ethane-1,2-diol. Note that the final -e of ethane is retained when a vowel follows in the suffix.

多元醇使用“-二醇”“-三醇”后缀。简单的 HOCH₂CH₂OH 是乙烷-1,2-二醇。注意当后缀以元音开头时，要在“乙烷”后保留“e”吗？实际上标准 IUPAC 中保留“-e”。

8. Aldehydes and Ketones | 醛与酮

Aldehydes contain the –CHO group. As the highest priority group in this pair, they take the suffix -al. Long-chain aldehydes are numbered so that the carbonyl carbon is always carbon 1.

醛含有 –CHO 基团。作为这一对中优先度最高的基团，它们使用后缀“-醛”。长链醛编号时羰基碳始终为 1 号碳。

Ketones contain C=O within the chain, given the suffix -one. The carbonyl carbon receives the lowest locant. Propanone is the systematic name for the common solvent often called acetone.

酮的 C=O 位于链内，后缀为“-酮”。羰基碳获得最低位次。“丙-2-酮”是常用溶剂“丙酮”的系统名称。

When both –CHO and –OH are present, the –CHO takes naming priority as the suffix, and –OH becomes the prefix hydroxy-. Thus HOCH₂CH₂CHO is 3-hydroxypropanal.

当 –CHO 与 –OH 同时存在时，–CHO 作为后缀优先，“–OH”变为前缀“羟基”。因此 HOCH₂CH₂CHO 是 3-羟基丙醛。

9. Carboxylic Acids and Esters | 羧酸与酯

Carboxylic acids contain –COOH and take the suffix -oic acid. The carboxyl carbon is always carbon 1. Since it necessarily lies at the chain end, no locant is normally needed for the –COOH group itself.

羧酸含有 –COOH，后缀为“-酸”。羧基碳永远是 1 号碳。由于它必然位于链端，通常无需为 –COOH 本身标注位次。

Esters are named as alkyl alkanoates: The alkyl group from the alcohol comes first, followed by the alkanoate derived from the carboxylic acid. CH₃COOCH₂CH₃ is ethyl ethanoate, not ethylethanoate — do not combine them.

酯命名为“烷基某某酸酯”：来自醇的烷基在前，来自羧酸的烷酸根随后。CH₃COOCH₂CH₃ 是乙酸乙酯（ethyl ethanoate），注意两个词之间保留空格。

Functional group priority in CIE A-Level: carboxylic acid > ester > aldehyde > ketone > alcohol > alkene > alkyne > alkane.

CIE A-Level 中的官能团优先顺序：羧酸 > 酯 > 醛 > 酮 > 醇 > 烯烃 > 炔烃 > 烷烃。

10. Amines and Amides | 胺与酰胺

Amines are named by locating the longest chain bearing the –NH₂ group and adding the suffix -amine. For example, CH₃CH₂CH₂NH₂ is propan-1-amine. Secondary and tertiary amines may be named as N-substituted primary amines.

胺的命名是找到连接 –NH₂ 的最长链，加后缀“-胺”。例如 CH₃CH₂CH₂NH₂ 是丙-1-胺。仲胺和叔胺可以命名为 N-取代的伯胺。

Amides contain –CONH₂ and use the suffix -amide, with the carbonyl carbon as carbon 1. Substituents on nitrogen are indicated by the prefix N-. Thus CH₃CONHCH₃ is N-methylethanamide.

酰胺含有 –CONH₂，后缀为“-酰胺”，羰基碳为 1 号碳。氮上的取代基用前缀 N- 表示。因此 CH₃CONHCH₃ 是 N-甲基乙酰胺。

Ethanoamide is the simplest amide derived from acetic acid; it is also commonly written as acetamide in older nomenclature.

乙酰胺是乙酸衍生的最简单酰胺；旧式命名中也写作 acetamide（乙酰亚胺），但 IUPAC 推荐 ethanamide。

11. Nitriles and Other Functional Groups | 腈与其它官能团

Nitriles contain the –C≡N group. The suffix is -nitrile, and the carbon of the –CN group is included in the parent chain number count but not as a plain carbon atom. CH₃CH₂CN is propanenitrile.

腈含有 –C≡N 基团，后缀为“-腈”，–CN 中的碳计入主链编号。CH₃CH₂CN 是丙腈。

When a nitrile coexists with a higher-priority group, –CN becomes the prefix cyano-. For example, 2-cyanopropanoic acid contains both –COOH (suffix) and –CN (prefix).

当腈基与更高优先度基团共存时，–CN 变为前缀“氰基”。例如 2-氰基丙酸同时含有 –COOH（后缀）和 –CN（前缀）。

12. Common Errors in Naming | 命名中的常见错误

Many A-Level candidates lose marks not from a lack of doctrine but from systematic micro-errors. The most frequent ones include: selecting the wrong parent chain when two chains have equal length but different substituents; forgetting to place commas between locants and hyphens between numbers and names; and ignoring alphabetical order among prefixes.

许多 A-Level 考生失分并非因为不懂原理，而是源于系统性的小错误。最常见的有：两条等长链时选错母体链；忘记在数字之间写逗号、数字与名称之间写短横线；以及忽略前缀之间的字母顺序。

Always count the longest chain carefully — a folded chain drawn in a zig-zag may look shorter than it is.

务必仔细数最长链——之字形绘制的链可能看起来比实际短。
Write locants with hyphens: 2-methylpentane, not 2 methylpentane or 2-methyl-pentane.

位次与名称之间用短横线：2-甲基戊烷，不要写成 2 甲基戊烷或 2-甲基-戊烷。
Alphabetise substituents before using di-/tri-: ethyl before dimethyl.

先按字母顺序排列取代基再使用二/三：乙基排在二甲基之前。
For unsaturated chains, the ene/yne locant must be as low as possible, even if it forces a substituent to a higher position.

对不饱和链，烯/炔位次必须最低，即使迫使某个取代基处于较高位次。

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A-Level Chemistry: Systematic Nomenclature of Organic Compounds | A-Level 化学：有机化合物的系统命名法