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The anomeric effect refers to the unexpected preference for an electronegative substituent in the 2-position of a tetrahydropyran to be axial.
In the axial model, the molecular orbitals show the oxygen donating its lone pair into the sigma anti-bonding orbital of the C-Cl bond. This produces a longer C-Cl bond and a shorter C-O bond in the axial anomer. The clearly defined orbitals show that the anomeric effect is important in this molecule.
Comparison of the anomeric effect in substituted tetrahydropyrans
Click here to look at a molecule with a weak anomeric effect.
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