Any process by which the normal alternating donor and acceptor reactivity pattern of a chain, which is due to the presence of $\ce{O}$ or $\ce{N}$ heteroatoms, is interchanged. Reactivity umpolung is most often achieved by temporary exchange of heteroatoms ($\ce{N}$, $\ce{O}$) by others, such as $\ce{P}$, $\ce{S}$ and $\ce{Se}$. The original meaning of the term has been extended to the reversal of any commonly accepted reactivity pattern. For example, reaction of $\ce{R–C#CX}$ ($\ce{X}$ = halide) as a synthon for $\ce{'R–C#C^{+}}$ (i.e. electrophilic acetylene) is an umpolung of the normal more common acetylide, $\ce{R–C#C^{−}}$ (i.e. nucleophilic) reactivity.