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3.2.6—Structural isomers

Syllabus
First assessment 2025
Objective
3.2.6
Level
SL

Structural Isomers

Structural isomers have the same molecular formula but different atom connectivities. Types include straight-chain/branched, position, and functional-group isomers.

isomerism branches independently to stereoisomerism and structural isomerism; stereoisomerism alone branches to conformational and configurational isomerism; structural isomerism is not shown as a subtype of stereoisomerism; no categories from separate textbook figures are merged.

Classify primary, secondary, and tertiary alcohols, halogenoalkanes, and amines by the carbon or nitrogen environment attached to the functional group.

Family Primary / secondary / tertiary test
Alcohol count carbon groups attached to the carbon bearing -OH: 1 / 2 / 3
Halogenoalkane count carbon groups attached to the carbon bearing X: 1 / 2 / 3
Amine count carbon groups attached directly to N: 1 / 2 / 3

C₄H₁₀O can represent different carbon skeletons, different –OH positions, or an ether instead of an alcohol. Draw each connectivity once, then compare molecular formulae. Rotating or redrawing one connectivity does not create a new structural isomer.

For alcohols and halogenoalkanes, classify the carbon carrying the functional group; for amines, classify the nitrogen by how many carbon groups are bonded to it. Do not use the position number alone: butan-2-ol is secondary because its OH-bearing carbon is attached to two other carbons.

Classifying Structural Isomers

[ 1 ]

Draw a structural isomer of molecule X.

Organic Analysis Summary

Retrieve the route: translate formulae, identify functional groups and series, name and classify isomers, then use mass, IR, and NMR evidence together to determine structure.

Check connectivity, functional-group evidence, formula/mass constraint, shifts and integration, splitting neighbours, and agreement across every technique.