A homologous series is a family of carbon compounds with the same functional group. Find the functional group first, and the family follows.
This lesson is part of SPM Chemistry carbon compounds. Naming comes next in naming and drawing organic structures.
Which families do I need?
Five families are central in the syllabus.
| Series | General formula | Functional group | Example |
|---|---|---|---|
| Alkane | CₙH₂ₙ₊₂ | C–C single bonds only | C₂H₆ |
| Alkene | CₙH₂ₙ | C=C double bond | C₃H₆ |
| Alcohol | CₙH₂ₙ₊₁OH | –OH | C₂H₅OH |
| Carboxylic acid | CₙH₂ₙ₊₁COOH | –COOH | CH₃COOH |
| Ester | RCOOR′ | –COO– | CH₃COOC₂H₅ |
Each member of a series differs from the next by CH₂, so the general formula lets you test a molecular formula quickly.
Worked example: four unknown formulas
Classify C₄H₁₀, C₃H₆, C₃H₇OH and C₂H₅COOH.
- C₄H₁₀: fits CₙH₂ₙ₊₂ with n = 4, so it is an alkane (butane).
- C₃H₆: fits CₙH₂ₙ with n = 3. With a C=C bond, it is an alkene (propene).
- C₃H₇OH: the –OH is present and the carbon part is C₃H₇, which fits CₙH₂ₙ₊₁. It is an alcohol (propanol).
- C₂H₅COOH: the –COOH group is present, with a C₂H₅ chain. It is a carboxylic acid (propanoic acid, three carbons in total).
The mistake that loses marks
A student counts C₂H₅COOH as a two-carbon acid because the formula shows C₂. The carbon inside –COOH also counts, so the acid has three carbons and is propanoic acid.
The fix is to count every carbon in the structure, including the one in the functional group. Then CH₃COOH has two carbons and is ethanoic acid.
A trap: the same formula, two families
C₃H₆O₂ can be propanoic acid, CH₃CH₂COOH, or methyl ethanoate, CH₃COOCH₃. The molecular formula alone cannot decide, and the drawing or the reaction does.
Acids turn blue litmus red and release carbon dioxide from carbonates. Esters do not, and they have a sweet, fruity smell.
Check yourself
State the series of (a) C₆H₁₄, (b) CH₃CH₂CH₂OH, (c) C₅H₁₀ with one C=C bond.
Answer
(a) Alkane, because 6 carbons and 14 hydrogens fit CₙH₂ₙ₊₂ (2 × 6 + 2 = 14).
(b) Alcohol, because it carries –OH. It is propan-1-ol.
(c) Alkene, because 5 carbons and 10 hydrogens fit CₙH₂ₙ and the C=C bond is given.
What to study next
Use the families in naming and drawing organic structures, then see the same formulas appear as isomers.
The mole and stoichiometry steps tool helps once you start calculating from formulas. For a teacher to work through your classifications, see online one-to-one Chemistry tuition.