Preparation of Pyrophosphate Esters

One of the oldest methods of forming pyrophosphate (diphosphate) esters is to heat silver pyrophosphate with an alkyl iodide. 

A method in use commercially is to heat a phosphate triester with phosphorus pentoxide or thionyl chloride (5.308). The tetraethyl analogue of (5.308), that is, TEPP (5.309) is an effective insecticide (Chapter 12.12). It can be distilled at low pressure, but if impure, or at normal pressure, it disproportionates into triethyl phosphate and a non-volatile ethyl metaphosphate polymer (5.310). 

FIGURE 5.35 Crystal structures of phosphate esters, (a) (C Hj CHjOljPfOlOH, (b) (CgHjO) , (c) (p-ClCgH50)2P(0)0H. (Adapted from D.E.C. Corbridge, The structural chemistry of phosphorous compounds. Topics Phos. Chem., 3, 57, 1966 S. Ohashi, Chromatography of the phosphorus oxyacids. Pure Appl. Chem., 44, 415, 1975 US 3,634,262.) 

Pyrophosphate esters can be obtained by heating dialkyl halophosphates with triesters (5.311) or dialkyl sodinm salts (5.312) (Table 5.26). 

Condensation occurs on heating the monoaryl silver salt (5.313), and direct exchange can be effected with some pyrophosphate esters (5.314). 

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