Nucleophilicity of Phosphorus Compounds

Phosphorous analogs of amines are called phosphines. The chemistry of phosphines and the related phosphite esters is dominated by their strong nucleophilicity and reducing character. The nucleophilicity of trivalent phosphorus results in rapid formation of phosphonium salts when such compounds are treated with reactive alkyl halides. For example, although resonance delocalization of the nitrogen electron pair in triphenylamine, (C₆H₅)₃N, renders it relatively unreactive in S_N2 reactions, the corresponding phosphorus compound, triphenylphosphine, undergoes a rapid and exothermic reaction to give a phosphonium salt, as shown below in the first equation. Phosphite esters react in the same manner, but the resulting phosphonium salts (shaded box) are often unstable, and on heating yield dialkyl phosphonate esters by way of a second S_N2 reaction (equation 2 below).