Anomeric effect
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In organic chemistry, the anomeric effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the axial orientation instead of the less hindered equatorial orientation that would be expected from steric considerations. This effect was originally observed in pyranoses. For example, the axial, or alpha configuration of glucopyranose is more stable than the equatorial, or beta configuration. The anomeric effect got its name from the term used to designate the C-1 carbon of a pyranose, the anomeric carbon.
Two different explanations for the anomeric effect have been proposed. The simpler explanation, based on simple physical considerations, is that the equatorial configuration has the dipoles involving both heteroatoms partially aligned, and therefore repelling each other. By contrast the axial configuration has these dipoles roughly opposing, thus representing a more stable and lower energy state. An alternative and widely accepted explanation, is that there is a stabilizing interaction (hyperconjugation) between the axial unshared electron pair on the endocyclic heteroatom and the σ* orbital for the axial exocyclic C-X bond. Such an interaction is not possible for the equatorial isomer.
