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.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20216 marks(a) Draw and name the four structural isomers of . (b) For each, classify the type of structural isomerism shown relative to 1-bromobutane. (c) State which isomer would be a tertiary haloalkane.Show worked answer →
A 6 mark question rewards the four isomers, their classification, and the tertiary identification.
(a) The four isomers are:
- 1-bromobutane, .
- 2-bromobutane, .
- 1-bromo-2-methylpropane, .
- 2-bromo-2-methylpropane, .
(b) 2-bromobutane is a position isomer of 1-bromobutane (same chain, Br on a different carbon). 1-bromo-2-methylpropane is a chain isomer (branched skeleton). 2-bromo-2-methylpropane differs in both chain and position.
(c) 2-bromo-2-methylpropane is the tertiary haloalkane, because the carbon bearing the bromine is bonded to three other carbon atoms.
Markers reward the four correct structures and names, the chain/position classification, and 2-bromo-2-methylpropane as tertiary.
WACE 20235 marksBut-2-ene exists as cis and trans isomers, but but-1-ene does not. (a) Explain why cis-trans isomerism is possible for but-2-ene but not but-1-ene. (b) Predict and explain which isomer of but-2-ene has the higher boiling point.Show worked answer →
A 5 mark answer needs the structural requirement and the polarity-based property prediction.
(a) Cis-trans isomerism requires that rotation about the double bond is restricted (the pi bond locks the geometry) and that each carbon of the C=C carries two different groups. In but-2-ene () each double-bond carbon has a methyl and a hydrogen, two different groups, so cis and trans forms exist. In but-1-ene () one double-bond carbon () carries two identical hydrogens, so swapping them gives the same molecule and no isomerism is possible.
(b) Cis-but-2-ene has the higher boiling point. In the cis isomer the two methyl groups are on the same side, so the molecule is slightly polar (the small bond dipoles do not cancel), giving weak dipole-dipole attraction in addition to dispersion forces. In the trans isomer the methyl groups are on opposite sides, the dipoles cancel and the molecule is non-polar, so it has only dispersion forces and a lower boiling point.
Markers reward the restricted-rotation and two-different-groups requirement, the identical-H reason for but-1-ene, and cis having the higher boiling point due to its small net polarity.