Conformational isomerism

You don't need to be Editor-In-Chief to add or edit content to WikiDoc. You can begin to add to or edit text on this WikiDoc page by clicking on the edit button at the top of this page. Next enter or edit the information that you would like to appear here. Once you are done editing, scroll down and click the Save page button at the bottom of the page.

Please Take Over This Page and Apply to be Editor-In-Chief for this topic: There can be one or more than one Editor-In-Chief. You may also apply to be an Associate Editor-In-Chief of one of the subtopics below. Please mail us [1] to indicate your interest in serving either as an Editor-In-Chief of the entire topic or as an Associate Editor-In-Chief for a subtopic. Please be sure to attach your CV and or biographical sketch.

Overview

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.

There are three principle effects that make some conformers more stable than others:

1. 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.
2. Steric repulsion will also make some conformers more favourable than others.
3. 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:

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.

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.

See also

• isomer
• Steric effects

ar:تزامر تشكيلي cs:Konformace de:Konformationit:Conformazione he:קונפורמציה (כימיה) nl:Conformatiesv:Konformation