# Explain why pent-2-ene can exist as cis and trans isomers but pent-1-ene cannot.

> WJEC A-Level Chemistry (Wales) — 4.1 Stereoisomerism · Explain · 5 marks

> Stereoisomers are molecules that have the same molecular formula and the same structural formula but differ in the arrangement of atoms in space. Cis-trans isomerism is one type of stereoisomerism found in some alkenes.

## Mark scheme (5 marks)

1. Cis-trans isomerism requires restricted rotation about the carbon–carbon double bond
2. For cis-trans isomerism to occur, each carbon of the double bond must have two different groups attached to it
3. In pent-2-ene, one carbon of the double bond has a hydrogen and a methyl group attached, and the other carbon has a hydrogen and an ethyl group attached — both carbons have two different groups
4. In pent-1-ene, one carbon of the double bond (C1) has two hydrogen atoms attached — both groups are the same
5. Because one carbon of the double bond in pent-1-ene has two identical groups, cis and trans arrangements cannot be distinguished / no cis-trans isomerism is possible

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