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 SN2 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 SN2 reaction (equation 2 below).
Oxidation States of Phosphorus Compounds
The difference in oxidation states between nitrogen and phosphorus is less pronounced than between oxygen and sulfur. Organophosphorus compounds having phosphorus oxidation states ranging from –3 to +5, as shown in the following table, are well known (some simple inorganic compounds are displayed in green). As in the case of sulfur, the P=O double bonds drawn in some of the formulas do not consist of the customary sigma & pi-orbitals found in carbon double bonds. Phosphorus is a third row element, and has five empty 2d-orbitals that may be used for p-d bonding in a fashion similar to p-p (π) bonding. In this way phosphorus may expand an argon-like valence shell octet by two electrons (e.g. phosphine oxides).
Phosphorus Compounds as Reducing Agents
Trivalent phosphorus is easily oxidized. In contrast with ammonia and amines, phosphine and its mono and dialkyl derivatives are pyrophoric, bursting into flame on contact with the oxygen in air. The affinity of trivalent phosphorus for oxygen (and sulfur) has been put to use in many reaction systems, three of which are shown here. The triphenylphosphine oxide produced in reactions 1 & 3 is a very stable polar compound, and in most cases it is easily removed from the other products. Reaction 2 is a general formulation of the useful Corey-Winter procedure for converting vicinal glycols to alkenes.
Triphenylphosphine is also oxidized by halogens, and with bromine yields dibromotriphenylphosphorane, a crystalline salt-like compound, useful for converting alcohols to alkyl bromides. As in a number of earlier examples, the formation of triphenylphosphine oxide in the irreversible SN2 step provides a thermodynamic driving force for the reaction.