English

Noun

1. any of a set of stereoisomers characterised by a conformation that corresponds to a distinct potential energy minimum
2. A particular folded state or conformation of a protein.
3. A person who conforms.

In chemistry, conformational isomerism is a form of stereoisomerism involving the phenomenon of molecules with the same structural formula existing as different conformational isomers or conformers due to atoms rotating about a bond. Different conformers can interconvert by rotation around single bonds, without breaking chemical bonds. The existence of more than one conformation, usually with different energies, is due to hindered rotation sp3 hybridised carbon carbon σ bonds. The comparitive stabilities of different conformers of a molecule are usually explained through differences in Steric repulsion. 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. Rotamers are a set of conformers and the rotation barrier is the activation energy required to jump from one conformer to another conformer.

Two important forms of conformational isomerism exist:

1. Linear alkane conformations with staggered, eclipsed and gauche conformers, and
2. Cyclohexane conformations with chair and boat conformers.

Another example of conformational isomerism is the folding of molecules, where some shapes are stable and functional, but others are not. Conformational isomerism is also found in atropisomers.

The population of different conformers follows a Boltzmann distribution:

\frac = \frac \exp \left (\frac \right)

The subscripts i and j represent the highest and lowest energy. g is the number of conformations found at that particular energy, the degeneracy. N is the population of molecules in a particular conformation.

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 such as cyclohexanes. 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 because double or triple bonds have one or two pi bonds that prevent rotation about the longitudinal axis. 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.

References

See also

• Isomer
• Steric effects
• Molecular configuration

External Links

• Polymer structure: configuration and conformation