Preparation of Orthophosphate Esters

These esters may in principle be prepared by the direct esterification of orthophosphoric acid with alcohol, but in practice this method is much too slow. Phosphoric acid and ethyl alcohol, for example, show little reaction after 7 h boiling. This direct method can, however, in some cases be made to work when condensing agents such as carbodi-imides are employed. 

A simple but little-used method is by the oxidation of the corresponding phosphite (6.328,6.333). 

Alkyl phosphates are produced by hydrolysis of the appropriate halophosphate (5.338, 5.339) shown below. 

Trialkyl phosphates are obtained by oxidative hydrolysis of the corresponding phospho-rothioates (9.390). 

The oldest and cheapest method of forming esters is by reacting phosphorus pentoxide with an alcohol to form a mixture of primary and secondary esters (5.291). The two esters so formed are usually separated by virtue of the difference in solubility of their alkaline earth salts, but the method may produce some phosphoric acid and triesters as imparities. 

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