Past exam of the chemistry B course of the University of Cambridge / 2025 / Paper 1 / B4 / b / iv

The anomeric effect is the preference, in suitable cyclic acetals, for an electronegative substituent at an anomeric carbon to occupy an axial position even when simple steric reasoning favours equatorial. An important orbital contribution is donation from an endocyclic oxygen lone pair into the antibonding orbital of the adjacent exocyclic C–O bond, $n_{\mathrm{O}}\to {\sigma }_{\mathrm{C}-\mathrm{O}}^{\ast }$. The chair's axial arrangement supplies favourable antiperiplanar overlap. Electrostatic and solvation contributions can also affect the observed conformational preference.