In organic chemistry, the Cieplak effect refers to a proposal (from Andrzej Stanislaw Cieplak in 1980) to try to explain why nucleophiles (organic chemicals that donate an electron pair to form a chemical bond) preferentially add to one face of a carbonyl group over another. These types of reactions have been called 'Stereo-selective', and are regularly observed.

Technical details, see Wikipedia

A significant proportion of theorists dispute Cieplak's theory, and various other alternatives have been proposed. Also see Wikipedia