Sur les esters phosphoriques

Abstract

Orthophosphoric acid can be clearly distinguished from carboxylic acids in it’s behaviour upon esterification as well as in the scission of its esters. The monoesters in particular are formed with difficulty by direct reaction with hydroxylic derivatives. Their hydrolysis is in general very difficult especially in alkaline medium. The speed of hydrolysis varies as a fonction of pH in a complicated inanner (curve with 1 maximum and 2 minima).
These particularities can be traced back to the fact that in orthophosphoric acid the central atom in the grouping, i.e. phosphorous, is co-ordinatively saturated and that the group PO₄ is exceptionally stable. The mechanisms of esterification and hydrolysis of the esters with formation of intermediary compounds by addition of a group to the central atom – such as is the case in the transformation of corresponding carboxylic acids – can therefore not occur in the particular case of phos-phoric acid. For the “direct” esterification of orthophosphoric acid a mechanism through intermediate formation of polyphosphoric groupings (which then undergo alcoholysis) is proposed. Hydrolysis will probably proceed through formation of an intermediate complex cation [(RO)₃PO]⁺ which then undergoes an interposition reaction or dissociation leading to the products of hydrolysis.
The rather complicated aspect of the change of the rate of hydrolysis as a function of pH, as well as the mounting difficulty of hydrolysis when passing from a triester (neutral), to diester (dialkylphosphoric acid) to monoester (monoalkylphosphoric acid) can now be explained. It becomes understandable why these phosphoric esters are generally hydrolysed more readily in acidic than in alkaline medium.
Concerning the interaction of the phosphoric ester function and other functional groups of the same molecule, it can be shown that an unsaturated carbon atom in β-position to the ester group produces an occasionally enormous labilisation of the phosphoric ester grouping in alkaline medium. The scission of the ester can either resuit in proper hydrolysis or in β-elimination of the phosphoryloxy group.