Structural isomerism (chain, position and functional group isomers), stereoisomerism including cis-trans (E-Z) isomerism in alkenes, the conditions needed for each, and why isomers can have different properties.

What this dot point is asking

CCEA wants you to explain structural isomerism and its types (chain, position and functional group isomers), explain stereoisomerism including cis-trans (E-Z) isomerism in alkenes, state the conditions needed for each, and explain why isomers can have different physical and chemical properties. It builds on naming and on the reactions of organic families, because the type of isomer affects how a molecule behaves.

Structural isomerism

There are three types of structural isomerism. Chain isomers differ in the arrangement of the carbon skeleton: for example, butane (a straight four-carbon chain) and 2-methylpropane (a branched chain) both have the formula C4H10. Position isomers have the same skeleton and functional group, but the functional group is in a different position: for example, propan-1-ol and propan-2-ol differ in whether the hydroxyl is on carbon 1 or carbon 2. Functional group isomers have the same molecular formula but different functional groups: for example, the molecular formula C2H6O can be ethanol (an alcohol) or methoxymethane (an ether). Recognising the type helps you draw all the isomers of a given formula systematically.

Stereoisomerism and cis-trans (E-Z) isomerism

The single bonds in a molecule can rotate freely, but the double bond in an alkene cannot, so the groups attached to it are locked on one side or the other. This is why but-2-ene exists as two isomers (methyl groups on the same side or opposite sides), but but-1-ene does not (one double-bond carbon has two identical hydrogens). The two stereoisomers have the same atoms joined in the same order but different shapes, which gives them slightly different physical properties.

Why isomers differ in properties

Because isomers have different structures or spatial arrangements, they can differ in their properties. Chain isomers differ in boiling point: a branched isomer has a lower boiling point than its straight-chain isomer because branching reduces the surface contact between molecules, weakening the intermolecular forces. Position and functional group isomers can differ in chemical reactivity because the functional group is in a different place or is a different group entirely. Stereoisomers differ in shape, which can change physical properties and, in biological molecules, can drastically change how a molecule fits an enzyme or receptor. This last point is hugely important in pharmacology, where one stereoisomer of a drug may be active and another inactive or harmful.

Examples in context

Example 1. Branching and fuel volatility. Petrol contains hydrocarbon chain isomers. Branched isomers are more volatile and burn more smoothly than straight-chain isomers of the same formula, which is why refiners adjust the proportion of branched molecules. This shows how chain isomerism affects useful physical properties.

Example 2. Stereoisomers in medicines. Many drugs are stereoisomers where only one form fits the target receptor or enzyme correctly, so only one isomer is therapeutically active and the other may be inactive or cause side effects. This is why drug manufacturers often produce a single stereoisomer, directly linking isomerism to the health-science focus of the qualification.

Try this

Q1. Define structural isomers. [1 mark]

• Cue. Compounds with the same molecular formula but a different structural arrangement of atoms.

Q2. State the two conditions needed for cis-trans (E-Z) isomerism. [2 marks]

• Cue. Restricted rotation about a carbon-to-carbon double bond, and each double-bond carbon carrying two different groups.

Q3. Explain why a branched chain isomer has a lower boiling point than its straight-chain isomer. [2 marks]

• Cue. Branching reduces surface contact between molecules, weakening intermolecular forces, so less energy is needed to separate them.