Structural Isomerism

Two compounds are structural isomers when they have an identical molecular formula but differ in which atoms are bonded to which, giving different structural formulae.

Same formula, different connectivity

Both structures contain three carbon atoms and six hydrogen atoms, but the bonding pattern is different. They are therefore structural isomers.

Structural isomerism can arise in three ways:

1. chain isomerism — a different carbon skeleton
2. position isomerism — the same group in a different position
3. functional group isomerism — different functional groups.

Exam Tip: In a definition, explicitly state that structural isomers have the same molecular formula. A statement about atoms being arranged differently in space describes stereoisomerism instead.

Deducing possible structural isomers

When a molecular formula is given, work systematically so that distinct structures are found without counting the same structure twice.

1. Keep the number of atoms of every element fixed so that each proposed structure matches the molecular formula.
2. Consider whether different carbon skeletons are possible.
3. For each skeleton, place the same functional group or substituent at each genuinely different position.
4. Consider whether the atoms can instead form a different functional group.
5. Check for duplicates: reversing or rotating a drawing does not create a new structural isomer if the connectivity is unchanged.

Worked example: Deduce the structural isomers with molecular formula $\ce{C3H6Br2}$.

The three-carbon skeleton cannot be branched, so the distinct structures come from changing the positions of the two bromine atoms.

1,1-dibromopropane

1,2-dibromopropane

1,3-dibromopropane

2,2-dibromopropane

These are four distinct connectivities. Drawing the same three-carbon chain from the opposite end does not produce an additional isomer.