Knowledge Bank

The potential barrier to the planar configuration is greatly reduced in the molecule 1,1-difluorocyclobutance relative to cyclobutane and 3,3-difluoro-oxetane relative to trimethylene oxide. By comparing changes in angle deformation (including ring strain) and torsional potential contributions to the ring puckering potential due to fluorine substitution, we find the torsional change is as large as the three-fold torsional barrier in the non-ring molecule propane. A relatively large repulsive torsional term has been added to the ring puckering potential function to account for this large reduction in the barrier. The repulsive term arises in a four-membered ring molecule when two eclipsed members of a ethane-type barrier are brought closer together to form the ring. The electronegative fluorine nuclei withdraw electron density from the eclipsed-ring bonds, thereby reducing the magnitude of the repulsive term. This effect in other fourmembered ring molecules will be discussed.