Definition of Umpolung

Any process by which the normal alternating donor and acceptor reactivity pattern of a chain, which is due to the presence of O or N heteroatoms, is interchanged.

Reactivity umpolung is most often achieved by temporary exchange of heteroatoms (N, O) by others, such as P, S, and Se.

The original meaning of the term has been extended to the reversal of any commonly accepted reactivity pattern. For example, reaction of R-CCX (X = halide) as a synthon for "R-CC+ (i.e. electrophilic acetylene) is an umpolung of the normal more common acetylide, R-CC- (i.e. nucleophilic) reactivity. SEEBACH (1979).