Conformational isomerism

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 (of butane)

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

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

File:Conformers2.jpg

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 substituents 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.

Conformer dependent reactions

The E2 elimination mechanism relies on the base or acid attacked substituent being in an antiperiplanar configuration along a bond with respect to the leaving group. This prerequisite for reaction is important in understanding organic elimination reaction pathways, especially those involving halogenated cyclic alkanes. Two adjacent substituents on a cyclic alkane can only undergo an E2 elimination if they are both axial to the ring and hence antiperiplanar. A combination of axial and equatorial substituents cannot react through an E2 mechanism, though ring flips (with associated reconformation) may allow reactions to occur if they are not precluded by an energy barrier or steric lock through isopropyl or larger substituents.

Conditions

Conformational isomerism only occurs around single bonds as a result of the requirement of breaking one or more pi bonds to rotate substituents about a sigma bond axis in double and triple bonded atoms. Conformers sufficiently constrained to exhibit measurable isomerism are unique from various flavours of stereoisomers in the fact that changes in stereochemistry are independent from any mechanism and instead rely only on molecular energy.

See also

• diastereomer
• enantiomer
• isomer
• chirality
• Steric effects