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9476 / 2027
Organic Chemistry

Lesson 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)

  • I can describe constitutional isomerism.Syllabus

    Syllabus 9476, 11.2(a). Describe constitutional isomerism.

  • I can explain cis-trans alkene isomerism.Syllabus

    Syllabus 9476, 11.2(b). Explain cis-trans alkene isomerism.

  • I can deduce possible isomers from a molecular formula.Syllabus

    Syllabus 9476, 11.2(i). Deduce possible isomers from a molecular formula.

  • I can identify stereochemical features from structures.Syllabus

    Syllabus 9476, 11.2(j). Identify stereochemical features from structures.

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 their atoms are joined differently. They can differ in the carbon chain, the position of a functional group, or the type of functional group. Butane and 2-methylpropane have different chains. Propan-1-ol and propan-2-ol have the OH in different places. Ethanol and methoxymethane are both C2H6O but have different functional groups.

Cis-trans isomerism happens in alkenes because the C=C bond cannot rotate. Rotating it would break the sideways overlap of p orbitals, which is the π bond. Each carbon of the double bond must carry two different groups. If either carbon has two identical groups, there are no cis and trans forms. In cis, the matching groups are on the same side. In trans, they are on opposite sides. You do not need E/Z names.

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.

Turning the entire drawing over does not change an isomer. Exchanging groups at only one end of the rigid double bond does.

Worked exampleEnumerate without counting rotated drawings twice

List the acyclic alkene isomers with formula C4H8, including cis-trans forms.

Try it first. Then show the worked stepsHide the worked steps
  1. Choose the straight four-carbon skeleton: but-1-ene and but-2-ene are the distinct double-bond positions.
  2. Choose the branched skeleton: 2-methylpropene.
  3. 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.
  4. A drawing numbered from the opposite end is not a new connectivity.
Answer

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 exampleFinish the search when the question also permits rings

How does the answer change if C4H8 can be an alkene or a cycloalkane?

Try it first. Then show the worked stepsHide the worked steps
  1. 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.
  2. The four alkene arrangements have already been found. For a ring, choose four ring carbons with no branch: cyclobutane.
  3. 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.
  4. 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.
  5. Compare molecular formulae and connectivity, then count stereoisomers within each connectivity. This keeps a rotated ring drawing from becoming a false extra answer.
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.

A ring bond uses one valency at each of its endpoints. Complete every carbon to four bonds to recover C4H8 for each skeleton.

Check your understanding

Propan-1-ol and propan-2-ol have the same molecular formula. What kind of isomers are they?

  1. Constitutional isomers, because the OH is joined to a different carbon
  2. Optical isomers, because each has a carbon with four different groups
  3. Cis-trans isomers, because the OH can point to either side of the chain
Show the answer

Constitutional isomers, because the OH is joined to a different carbon

Constitutional isomers have their atoms joined differently. Here the position of the functional group differs. There is no C=C, so cis-trans isomerism is not possible.