Isomers are compounds with the same molecular formulae but that are structurally different in some way. It is important to be able to recognise isomers because they can have different chemical, physical properties and biological properties.

Constitutional isomers differ in the order in which the atoms are connected so they can contain different functional groups and / or bonding patterns (e.g. branching)

• example: 1-propanol, 2-propanol and ethyl methyl ether (C₃H₈O)

 

Stereoisomers have the same functional groups and connectivities, they differ only in the arrangement of atoms and bonds in space.

Conformational isomers (or conformers or rotational isomers or rotamers) are stereoisomers produced by rotation about σ bonds, and are often rapidly interconverting at room temperature

• example 1: butane : anti (left) and syn (center). The C2-C3 σ bond rotation is animated (right). Try rotating the model to look along the C-C to see the two forms.
 

 

• example 2: cyclohexane : chair (left) and boat (right).These two forms can be interconverted by twisting the ring structure.