Isomerism

Isomers are compounds with the same molecular formula but different structures or arrangements of atoms, so they have different properties.

Structural isomers differ in how the atoms are linked:

• Chain isomerism: different carbon skeletons. C₅H₁₂ can be pentane (straight), isopentane (one branch) or neopentane (two branches on one carbon).
• Position isomerism: the same skeleton and group, but the group sits in a different position: propan-1-ol and propan-2-ol.
• Functional group isomerism: different functional groups. C₃H₆O can be propanal (an aldehyde) or propanone (a ketone).
• Metamerism: different alkyl groups on either side of the same functional group. Diethylamine (C₂H₅–NH–C₂H₅) and methylpropylamine (CH₃–NH–C₃H₇) are metamers.

Stereoisomers have the same linkage of atoms, but a different arrangement in space:

• Geometrical isomers differ in the positions of groups on either side of a C=C bond, which cannot rotate: cis (same side) and trans (opposite sides).
• Optical isomers are mirror images of each other that cannot be superimposed, like your left and right hands.

Do not confuse isomers with resonance structures. In resonance structures, only the electrons are arranged differently; the atoms stay in the same places, and they all describe one molecule. Isomers are different compounds.

A pair can show more than one kind of isomerism. Pentan-2-one and pentan-3-one are position isomers (the C=O is in a different place), and also metamers (different alkyl groups on each side of it).