Naming organic compounds and isomerism
Organic chemistry has millions of compounds, so names follow strict rules that tell you the structure exactly. Isomers share a molecular formula but have different structures, which is why naming matters so much.
Part 1 of 3: Learn it
In short
- Find the longest carbon chain, number it to give the lowest locants, and list substituents alphabetically.
- Structural isomers have one molecular formula but a different structural formula.
- E/Z isomers have restricted rotation about C=C with two different groups on each carbon.
Where this is in your specification
Spec points: AQA 7405 3.3.1, OCR A H432 4.1.1, Edexcel 9CH0 Topic 6
| Board | Topic: Organic chemistry: functional groups |
|---|---|
| AQA 7405 | 3.3.1 to 3.3.6 |
| Edexcel 9CH0 | Topics 6 and 7 |
| OCR H432 | 4.1 and 4.2 |
| Higher C813 76 | 2(a) Systematic carbon chemistry; 2(b) Alcohols; 2(c) Carboxylic acids; 2(d) Esters, fats and oils |
Homologous series and functional groups
| Series | Functional group | Name ending or prefix | Example |
|---|---|---|---|
| Alkanes | C-C single bonds only | -ane | propane |
| Alkenes | C=C | -ene | but-2-ene |
| Halogenoalkanes | C-Hal | chloro-, bromo- | 2-bromopropane |
| Alcohols | -OH | -ol | butan-2-ol |
| Aldehydes | -CHO | -al | propanal |
| Ketones | C=O inside the chain | -one | propanone |
| Carboxylic acids | -COOH | -oic acid | ethanoic acid |
Members of a homologous series share a general formula, differ by CH₂ and have similar chemical properties.
Naming rules
- Find the longest continuous carbon chain; this gives the stem (meth, eth, prop, but, pent, hex).
- Number the chain from the end that gives the functional group, then the substituents, the lowest numbers.
- Name the side chains (methyl, ethyl) and halogens, with di- or tri- for repeats, in alphabetical order.
- Separate numbers from each other with commas and from letters with hyphens: 2,2-dimethylbutane.
Isomerism
- Chain isomers: different carbon skeletons, such as butane and 2-methylpropane (both C₄H₁₀).
- Position isomers: the same functional group in a different place, such as propan-1-ol and propan-2-ol.
- Functional group isomers: the formula is shared but the functional group differs, such as propanal and propanone (both C₃H₆O).
- E/Z isomers: around a C=C bond, which cannot rotate. Using priority rules, Z has the two higher-priority groups on the same side, and E on opposite sides.
Name CH₃CH₂CH(OH)CH₃.
Show the answer
Butan-2-ol.
Part 2 of 3: See it worked
Worked examples
Example 1
Name CH₃CH(CH₃)CH₂CH₃.
- Longest chain: 4 carbons, so butane
- A methyl group on carbon 2 (numbering from the nearer end)
Answer: 2-methylbutane.
Example 2
Explain why but-2-ene has E and Z isomers but but-1-ene does not.
- In but-2-ene, each carbon of the C=C has two different groups: H and CH₃
- Rotation about the double bond is restricted, so the CH₃ groups can be on the same side or opposite sides
- In but-1-ene, one carbon of the C=C carries two H atoms, so swapping them makes no difference
Answer: Only but-2-ene has two different groups on both carbons of the double bond.
Common mistakes
- Choosing a chain that is not the longest because it is drawn in a straight line.
- Numbering from the end that gives higher locants.
- Listing substituents in order of position instead of alphabetical order.
- Saying E/Z isomers have different structural formulas. They have the same structure, arranged differently in space.
What type of isomers are propanal and propanone?
Show the answer
Functional group isomers.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Name CH₃CH₂CH(OH)CH₃.
Butan-2-ol.
What type of isomers are propanal and propanone?
Functional group isomers.
What is the general formula of the alkenes?
CₙH₂ₙ.
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