3.1.3Organic Chemistry
Nomenclature and Classification — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 16 of 42
Part 1 of 4Naming Rules
1
Find the longest continuous carbon chain that contains the functional group, and use its length to choose the stem: meth, eth, prop, but, pent, hex.
2
Add the suffix for the main functional group, such as –ane, –ene or –ol, and use a number to show its position where needed.
3
Number the chain from the end that gives the functional group the lowest possible number.
4
Name side chains or halogens as prefixes (methyl, chloro, bromo) with the carbon number they are attached to.
5
List prefixes alphabetically, use di–, tri– for repeated groups, and separate numbers with commas and numbers from words with hyphens.
Part 2 of 4Stems and Examples
Stems
- The prefixes meth–, eth–, prop–, but–, pent– and hex– represent chains of one to six carbon atoms.
Branched alkane
- 2-methylbutane has a four-carbon main chain with a methyl group on carbon number two.
Alkene
- But-1-ene has a double bond starting at carbon one, while but-2-ene has it starting at carbon two.
Alcohol
- Propan-2-ol has the –OH group on the second carbon of a three-carbon chain.
Halogenoalkane
- 2-bromo-2-methylpropane has a bromine and a methyl group both attached to carbon number two.
Part 3 of 4Classification
Classification
Primary
- The carbon bonded to the functional group is attached to one other carbon atom, as in propan-1-ol.
Secondary
- The carbon bonded to the functional group is attached to two other carbon atoms, as in propan-2-ol.
Tertiary
- The carbon bonded to the functional group is attached to three other carbon atoms, as in 2-methylpropan-2-ol.
Use
- Alcohols and halogenoalkanes are classified this way because their reactions depend on the class.
Carbocations
- The same terms describe carbocations according to the number of alkyl groups on the positively charged carbon.
| Class | Carbons attached to the C bearing the group | Example |
|---|---|---|
| Primary | 1 | CH₃CH₂CH₂OH |
| Secondary | 2 | CH₃CH(OH)CH₃ |
| Tertiary | 3 | (CH₃)₃COH |
Part 4 of 4Exam Checklist & Tips
3.1.3 – Key Points for the Exam
- IUPAC The international system for naming chemical compounds.
- Stem The part of the name that shows the number of carbon atoms in the main chain.
- Prefix Part of a name before the stem that shows a substituent and its position.
- Primary A class in which the functional-group carbon is attached to one other carbon.
- Tertiary A class in which the functional-group carbon is attached to three other carbons.
Common Mistakes
- Choosing the longest chain that does not contain the functional group.
- Numbering from the wrong end, so the functional group does not get the lowest possible number.
- Counting the number of carbons on the whole molecule when classifying, instead of the carbons bonded to the functional-group carbon.
- Forgetting to list prefixes alphabetically, for example writing methyl before chloro.
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