The nine homologous series
| Series | Functional group | Name ending / prefix | Example |
|---|
| Alkanes | only C–C single bonds | -ane | propane |
| Alkenes | C=C double bond | -ene | but-1-ene |
| Alkynes | C≡C triple bond | -yne | propyne |
| Haloalkanes | –X (F, Cl, Br, I) | fluoro-, chloro-, bromo-, iodo- | 2-chloropropane |
| Alcohols | –OH (hydroxyl) | -ol | ethanol |
| Aldehydes | –CHO (formyl, at the end of the chain) | -al | propanal |
| Ketones | C=O inside the chain (carbonyl) | -one | propanone |
| Carboxylic acids | –COOH (carboxyl) | -oic acid | ethanoic acid |
| Esters | –COO– between two chains | alkyl alkanoate | methyl ethanoate |
Chain prefixes: meth (1), eth (2), prop (3), but (4), pent (5), hex (6), hept (7), oct (8).
Naming in five steps
- Find the longest continuous carbon chain that contains the functional group. This gives the stem.
- Number the chain from the end that gives the functional group the lowest number. (For alkanes and haloalkanes, give the substituents the lowest numbers.)
- Name the branches (methyl, ethyl) and halogens with their position numbers.
- List substituents in alphabetical order; use di-, tri-, tetra- for repeats (these prefixes are ignored when alphabetising).
- Punctuation: commas between numbers, hyphens between numbers and letters, no spaces (except in acids and esters).
✏️ Worked examples
1CH₃CH(CH₃)CH(CH₃)CH₃ — longest chain 4 C, methyl groups on C2 and C3: 2,3-dimethylbutane.
2CH₃CH=CHCH(CH₃)CH₃ — 5 C chain with the double bond; number from the left so C=C starts at C2; methyl on C4: 4-methylpent-2-ene.
3CH₃CHBrCH(CH₃)CH₃ — 4 C chain; Br on C2, methyl on C3; bromo before methyl alphabetically: 2-bromo-3-methylbutane.
4CH₃CH(CH₃)CH₂OH — 3 C chain containing the C bonded to OH; number from the OH end: 2-methylpropan-1-ol.
Naming esters
An ester forms when an alcohol reacts with a carboxylic acid (esterification, catalysed by concentrated H₂SO₄). The name has two words: the alkyl group from the alcohol first, then the alkanoate from the acid.
✏️ Worked example
1Propan-1-ol + ethanoic acid → propyl ethanoate + water.
2Reading a formula: in CH₃COOCH₂CH₃, the C=O carbon belongs to the acid part (2 C → ethanoate) and the chain on the single-bonded O comes from the alcohol (2 C → ethyl): ethyl ethanoate.
Structural isomers
Structural isomers are organic molecules with the same molecular formula but different structural formulae.
- Chain isomers: same functional group, different carbon chain (butane and 2-methylpropane, C₄H₁₀).
- Positional isomers: same chain and functional group, group in a different position (propan-1-ol and propan-2-ol).
- Functional isomers: different functional groups — aldehydes and ketones (propanal and propanone, C₃H₆O); carboxylic acids and esters (propanoic acid and methyl ethanoate, C₃H₆O₂).
Common mark losses: choosing a chain that misses the functional group; numbering from the wrong end; forgetting hydrogen atoms on a structural formula; writing “2-methyl-butane” or “2,3 dimethylbutane”.