IUPAC nomenclature gives each organic compound a systematic name based on its structure. For IB Chemistry SL and HL, identify the parent carbon chain, number it correctly, name its substituents and functional groups, and assemble these components in the required order.
The Naming Process
- Identify the principal functional group. This determines the suffix and usually receives the lowest possible number.
- Choose the parent chain. Select the longest continuous carbon chain containing the principal functional group and any relevant multiple bond.
- Number the parent chain. Start from the end that gives the principal functional group the lowest locant. Multiple bonds and substituents are then considered according to naming priority.
- Identify and number substituents. Examples include methyl-, ethyl-, fluoro-, chloro-, and bromo-.
- Assemble the name. Write numbered substituents as prefixes, followed by the parent chain and functional-group suffix.
| Structural feature | Name component | Example |
|---|---|---|
| Four-carbon parent chain | but- | butane |
| Carbon-carbon double bond | -ene | but-2-ene |
| Alcohol group | -ol | butan-2-ol |
| Halogen substituent | fluoro-, chloro-, bromo-, iodo- | 2-bromopropane |
| Alkyl substituent | methyl-, ethyl- | 3-methylpentane |
For example, CH₃CH(CH₃)CH(OH)CH₃ has a four-carbon parent chain. Numbering from the end nearest the hydroxyl group places it on carbon 2 and the methyl substituent on carbon 3, giving 3-methylbutan-2-ol.
Repeated substituents use prefixes such as di-, tri-, and tetra-. Different substituents are listed alphabetically; multiplicative prefixes are not considered when alphabetizing. Commas separate numbers, while hyphens separate numbers from words.
A common misconception is that the longest chain should always receive the lowest substituent numbers. The principal functional group normally has priority, so its position controls the numbering first.
Exam Technique
This is assessed under Structure 3.2.5. When asked to name a compound, first mark the principal functional group and parent chain, then check locants, alphabetical order, hyphens, commas, and suffix spelling before writing the final name.
