Molecular orbital calculations have been carried out on 2-fluorotetrahydropyran and 2-chlorotetrahydropyran as models for examining the anomeric effect in glycosyl halides. The calculated population of chair conformers depends on the solvent and for the axial form of fluoro derivative decreases from 90.6 % in dioxan to 37.0 % in water. For chloro derivative the dependence is less pronounced; th...