Conformational isomerism is the phenomenon of molecules with the same structural formula but different conformations (conformers) of atoms about a rotating bond. Steric repulsion will make some conformers more favourable than others - i.e. they will have a lower energy level. A simplified example is that of a butane molecule viewed in the Newman Projection shown - i.e. as if viewed down the central C-C bond with relative rotations of C² and C³ illustrated.

Potential energies

A measure of potential energy is stored in conformers with greater steric hindrance than the 'anti' conformer ground state.

• Gauche a.k.a. Staggered conformer - 16 kJ/mol
• Eclipsed H and CH₃ - 3.8 kJ/mol
• Eclipsed CH₃ and CH₃ - 19 kJ/mol

The eclipsed methyl groups exert a greater steric strain because of their greater electron density compared to lone hydrogen atoms.

Consequences

If the eclipsed conformations of an isomer have high enough potentials, they may prevent rotation of radicals to different staggered conformations at sufficiently low energy levels. This will result in a racemic mixture of conformations that may or may not have different reactivities in situations such as enzymatic reactions in which molecular shape is usually a key factor of operation.

See also

• diastereomer
• enantiomer
• isomer
• chirality