Isomers have the same molecular formula but a different arrangement. Two big families: structural (different connectivity) and stereo (same connectivity, different 3-D arrangement).
Types of isomerism
Type
What differs
Example (both C₄H₁₀ or as noted)
Chain / branching
Carbon skeleton
butane vs 2-methylpropane
Positional
Position of the group on the chain
butan-1-ol vs butan-2-ol
Functional group
Different functional group, same formula
ethanol (C₂H₆O) vs methoxymethane (ether)
Geometric (E/Z)
3-D arrangement across a C=C
cis/trans but-2-ene
Optical
Non-superimposable mirror images
butan-2-ol (chiral C)
E/Z (geometric) isomerism happens when a C=C can’t rotate and each carbon of the double bond carries two different groups.
Assigning E / Z
On each carbon of the C=C, find the higher-priority group (higher atomic number).
Z (zusammen = together): higher-priority groups on the same side.
E (entgegen = opposite): higher-priority groups on opposite sides.
No E/Z if either carbon has two identical groups (e.g. CH₂=).
E / Z isomers of but-2-eneThe C=C can’t rotate, so these are genuinely different molecules with different physical properties.Optical isomerism occurs at a chiral carbon, one bonded to four different groups. The two mirror-image forms (enantiomers) are non-superimposable, like left and right hands.
Spotting a chiral centre
Look for a carbon with four different atoms/groups attached.
Enantiomers rotate plane-polarised light in opposite directions (optically active).
They are otherwise identical in most physical/chemical properties.
① A carbon in a C=CH₂ has two H’s: no E/Z. ② A carbon needs four different groups for chirality. If two are the same, it’s not chiral. ③ Don’t confuse functional-group isomers (e.g. aldehyde vs ketone, C₃H₆O) with positional isomers.