A homologous series is a family of compounds with the same functional group and general formula, where each member differs from the next by a CH₂ unit. Isomers are compounds with the same molecular formula but different structural formulae. One idea is about a family; the other is about two molecules of identical formula.
This lesson follows identifying a functional group and belongs to organic families and naming.
What makes a set of molecules a homologous series?
The alkanes are the standard example. Each has only single bonds and fits the general formula CₙH₂ₙ₊₂.
| n | Name | Molecular formula |
|---|---|---|
| 1 | methane | CH₄ |
| 2 | ethane | C₂H₆ |
| 3 | propane | C₃H₈ |
| 4 | butane | C₄H₁₀ |
Moving down the table, each step adds one carbon and two hydrogens, which is one CH₂. The members have similar chemical properties because they share a functional group, and their physical properties, such as boiling point, change gradually with chain length.
What makes two molecules isomers?
Isomers have the same number of each kind of atom but joined differently. The molecular formula is identical; the structural formula is not. The two isomers of C₄H₁₀ are:
- Butane: CH₃CH₂CH₂CH₃, a straight chain.
- Methylpropane: CH₃CH(CH₃)CH₃, a three-carbon chain with a CH₃ branch on the middle carbon.
How do you decide which word applies?
- Write the molecular formula of each molecule.
- If the formulae are identical and the structures differ, they are isomers.
- If the formulae differ by one or more CH₂ units and the functional group is the same, they belong to the same homologous series.
- If neither fits, they are in different families or unrelated.
Worked example
Decide how each pair is related.
(a) Propane and butane. (b) Propan-1-ol, CH₃CH₂CH₂OH, and propan-2-ol, CH₃CH(OH)CH₃.
(a) Propane is C₃H₈ and butane is C₄H₁₀. The formulae differ by CH₂, and both are alkanes with only single bonds. They are members of the same homologous series, not isomers.
(b) Count atoms in propan-1-ol: carbons 3, hydrogens 3 + 2 + 2 + 1 = 8, oxygen 1, so C₃H₈O. In propan-2-ol: carbons 3, hydrogens 3 + 1 + 3 + 1 = 8, oxygen 1, so also C₃H₈O. The formulae match but the -OH sits on a different carbon. They are isomers.
Notice that propan-1-ol and propan-2-ol are also both alcohols. Isomers can belong to the same family, and whether they do depends on how the structures differ.
The mistake to watch for
A frequent error is to call two molecules isomers because they “look alike” or are in the same family.
Mistaken answer: Ethane and propane are isomers because they are both alkanes.
The student matched the family and ignored the formulae, C₂H₆ and C₃H₈, which are different.
A second version is to think a bent drawing of butane is a new isomer. Bending a chain on paper does not change which atoms are joined to which. The correction in both cases is to compare the molecular formula first, then check how the atoms are connected.
Check yourself
1. Ethene (C₂H₄) and propene (C₃H₆): isomers or members of one series? Give a reason.
Show answer
Same homologous series. Both are alkenes with a C=C bond, and the formulae differ by one CH₂. Their molecular formulae are different, so they are not isomers.
2. Write the molecular formula of CH₃CH₂CH₂CH₂CH₃ and of CH₃CH(CH₃)CH₂CH₃. Are they isomers?
Show answer
The first has 5 carbons and 3 + 2 + 2 + 2 + 3 = 12 hydrogens, so C₅H₁₂. The second has 5 carbons and 3 + 1 + 3 + 2 + 3 = 12 hydrogens, so also C₅H₁₂. The formulae are identical and the structures differ (a straight chain and a branched chain), so they are isomers.
3. A student says ethanol (C₂H₆O) and ethanoic acid (C₂H₄O₂) are isomers. Is that correct?
Show answer
No. Their molecular formulae are different, C₂H₆O and C₂H₄O₂, so they cannot be isomers. They are also in different families, alcohol and carboxylic acid.
Where this leads next
The next lesson, drawing a small molecule with correct bonding, gives you the tools to draw both isomers of C₄H₁₀ without breaking bonding rules. The organic families practice set then mixes both ideas.
Students who understand the definitions but lose marks on the wording of explanations often gain from online one-to-one Chemistry tuition, where a teacher can read your sentence and tighten it with you.