Isomerism
Same molecular formula, different molecule. The trick is knowing which type you're looking at — and the exact condition each one needs.
Structural isomerism
Same molecular formula, different structural formula (the atoms are joined up differently). Three kinds:
Chain isomerism
Different carbon skeleton (branched vs straight).
e.g. butane and 2-methylpropane (both C4H10)
Position isomerism
Same functional group in a different place on the chain.
e.g. propan-1-ol and propan-2-ol (both C3H8O)
Functional group isomerism
Same atoms arranged into a different functional group.
e.g. propanal and propanone (both C3H6O)
E/Z (geometric) isomerism
A type of stereoisomerism — same structural formula, different arrangement in space. It arises when there is:
- a C=C double bond (no rotation about it), and
- two different groups on each carbon of the double bond.
Assign priority to the groups on each carbon by atomic number (CIP rules). If the two higher-priority groups are on the same side, it's Z (zusammen, together); on opposite sides, it's E (entgegen, opposite). The older cis/trans naming is the special case where each carbon carries an H.
Optical isomerism
Stereoisomerism around a chiral centre: a carbon atom bonded to four different groups. The two isomers (enantiomers) are non-superimposable mirror images — like your left and right hands.
- Enantiomers are chemically identical but rotate the plane of plane-polarised light by equal amounts in opposite directions.
- A racemic mixture (racemate) is a 50:50 mix of both enantiomers — the rotations cancel, so there is no overall effect on polarised light.
- A racemate forms when a planar group is attacked from both sides equally — e.g. nucleophilic addition to a C=O (making a hydroxynitrile) or an SN1 substitution via a planar carbocation.
Spot the functional groups first
Functional-group isomerism is easier once you can name every group on sight.
Functional groups referenceFree revision tool by Oscar Song Tutoring
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