Why can molecules with the same molecular formula have different structures and properties?
Explain and identify structural isomerism and cis-trans (geometric) isomerism, and relate isomerism to differences in physical and chemical properties
A focused answer to the WACE Year 12 Chemistry dot point on structural and cis-trans isomerism, how to identify each type, and how isomerism affects properties, with a worked example and common mistakes.
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What this dot point is asking
Isomers are different compounds that share the same molecular formula but differ in the arrangement of their atoms. Because the arrangement differs, isomers usually have different physical properties, and sometimes different chemical properties, even though their formula is identical. The WACE course requires structural isomerism and cis-trans (geometric) isomerism.
Structural (constitutional) isomerism
Structural isomers have the same molecular formula but a different connectivity, the atoms are bonded in a different order. There are three types you should recognise.
- Chain isomerism
- The carbon skeleton differs (straight chain versus branched). For example butane () and 2-methylpropane both have the formula . The branched isomer has weaker dispersion forces, so it boils at a lower temperature.
- Position isomerism
- The functional group is on a different carbon of the same chain. For example propan-1-ol and propan-2-ol both are , differing only in where the OH sits.
- Functional-group isomerism
- The atoms are arranged into a different functional group entirely. For example can be ethanol (an alcohol) or methoxymethane (an ether), which have very different properties.
Cis-trans (geometric) isomerism
Cis-trans isomerism arises because a C=C double bond cannot rotate freely (the pi bond locks the geometry). For it to occur, each carbon of the double bond must carry two different groups.
- In the cis isomer the two higher-priority groups are on the same side of the double bond.
- In the trans isomer they are on opposite sides.
For example but-2-ene () exists as cis-but-2-ene (both methyl groups the same side) and trans-but-2-ene (methyl groups opposite). But-1-ene shows no cis-trans isomerism because one double-bond carbon () carries two identical hydrogens.
The two geometric isomers differ in shape, so they differ in properties: the cis isomer is usually more polar and packs less neatly, giving a different boiling and melting point from the trans isomer.
Why isomerism matters for properties
Because isomers differ in shape, branching and polarity, they differ in melting and boiling point, solubility and density. More branching lowers the boiling point (weaker dispersion forces, smaller surface contact). A more polar isomer is more water-soluble. These structure-property links are routinely examined.
When answering isomerism questions in the WACE examination, state the molecular formula is the same for all isomers, name the type of isomerism, and link the structural difference to the property difference you are asked about.