Topic 4 of 34
Isomers can differ in connectivity or in arrangement around a fixed framework
First compare which atoms are joined, then consider restricted rotation.
A-Level 9476 (2026-2027)
Isomers can differ in connectivity or in arrangement around a fixed framework
First compare which atoms are joined, then consider restricted rotation.
Constitutional (structural) isomers have the same molecular formula but different connectivity. They may differ in carbon skeleton, functional-group position or functional-group class. Butane and 2-methylpropane differ in skeleton; propan-1-ol and propan-2-ol differ in OH position; ethanol and methoxymethane have different functional groups despite both being C2H6O.
Cis-trans isomerism in an alkene arises because rotating about C=C would disrupt sideways p-orbital overlap and break the π bond. Each double-bond carbon must have two different substituents. If either carbon has two identical groups, there is no such pair. Where the cis/trans names are unambiguous, cis places the matching groups on the same side and trans on opposite sides. E/Z nomenclature is not required.
The two arrangements of but-2-ene
The cis drawing has both CH3 groups above the C=C bond. The trans drawing has one CH3 above and the other below. In each structure, each double-bond carbon also carries H.
Worked example
Enumerate without counting rotated drawings twice
List the acyclic alkene isomers with formula C4H8, including cis-trans forms.
- Choose the straight four-carbon skeleton: but-1-ene and but-2-ene are the distinct double-bond positions.
- Choose the branched skeleton: 2-methylpropene.
- But-2-ene gives cis and trans forms. But-1-ene has CH2 at one alkene end; 2-methylpropene also has a CH2 end, so neither gives cis/trans.
- A drawing numbered from the opposite end is not a new connectivity.
But-1-ene, cis-but-2-ene, trans-but-2-ene and 2-methylpropene: four stereochemically distinct acyclic alkenes. If the question asks for every compound with this formula, cyclic possibilities must also be considered.
Worked example
Finish the search when the question also permits rings
How does the answer change if C4H8 can be an alkene or a cycloalkane?
- An acyclic saturated four-carbon hydrocarbon would be C4H10. The loss of two H can represent one C=C or one ring, so investigate both classes.
- The four alkene arrangements have already been found. For a ring, choose four ring carbons with no branch: cyclobutane.
- Next choose three ring carbons and place the remaining carbon in one methyl branch: methylcyclopropane. The three possible branch positions on an otherwise unsubstituted triangle are equivalent by rotation.
- There is no two-carbon ring with ordinary single C-C bonds. Neither of these singly connected ring structures adds a cis-trans pair or a chiral centre: in methylcyclopropane the two routes around the ring are equivalent.
- Compare molecular formulae and connectivity, then count stereoisomers within each connectivity. This keeps a rotated ring drawing from becoming a false extra answer.
Five constitutional isomers, or six distinct isomers when cis- and trans-but-2-ene are counted separately: the four alkene arrangements above, cyclobutane and methylcyclopropane.
The two cyclic C4H8 skeletons
A square represents cyclobutane, with four CH2 corners. A triangle with one line extending from its top corner represents methylcyclopropane: that corner is CH, the other two corners are CH2, and the line end is CH3.