Conformational isomerism
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Conformational isomerism is a form of isomerism and describes the phenomenon of molecules with the same structural formula having different chemical conformations or conformers due to atoms rotating about a bond.
There are three principle effects that make some conformers more stable than others:
- Bond interaction with the back lobes of orbitals on adjacent atoms is possible only when atoms on adjacent atoms are staggered. This is virtually the only reason that ethane's preferred conformation is the staggered one.
- Steric repulsion will also make some conformers more favourable than others.
- Finally, bond moments of polar bonds will influence which conformers are most stable.
Different conformers can interconvert by rotation around single bonds, without breaking chemical bonds. 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.
The population of different conformers follows a Boltzmann distribution:
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.
Two important forms of conformational isomerism exist:
- linear alkane conformations with staggered, eclipsed and gauche conformers, and
- cyclohexane conformations with chair and boat conformers.
Another example of conformational isomerism is protein folding, where some shapes are stable and functional, but others are not. Conformational isomerism is also found in atropisomers.
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[edit] 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.
[edit] 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.
[edit] 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.
