Phosphorus analogs of amines are referred to as phosphines. The chemistry of phosphines and the related phosphite esters is primarily characterized by their strong nucleophilic and reducing properties. Trivalent phosphorus exhibits significant nucleophilicity, leading to the rapid formation of phosphonium salts upon reaction with reactive alkyl halides. For instance, while the resonance delocalization of the nitrogen electron pair in triphenylamine, (C6H5)3N, renders it relatively inert in SN2 reactions, the corresponding phosphorus compound, triphenylphosphine, readily undergoes a rapid and exothermic SN2 reaction to yield a phosphonium salt, as depicted in the first equation below, however, the resulting phosphonium salts (highlighted in the box) are often unstable and, upon heating, undergo a second SN2 reaction to yield dialkyl phosphonate esters, as illustrated in equation 2 below.