Kinetics of Hydrolysis of p-methoxyphenyl Phosphorotriamidate in the pH Region

Abstract

Hydrolysis of p-methoxyphenyl phosphorotriamidate has been
carried out in the pH range 1.24 to 10.0 in 30% acetic acid at
65°C. A higher value of rate coefficients at pH 1.24 is based on the
presence of conjugate acid form while the neutral form is active in
the entire pH range examined. Three maxima are obtained at pH
4.17, pH 6.43 and pH 10, the first two corresponding to the maxima
and minmma observed for the corresponding mono- and di-esters,
at which pH both the esters. hydrolyse via their monoanion forms.
Both the values are probably the mixed rates and are possibly not
analysed by the methods used for this purpose. Arrhenius
parameters favour bimolecular route of hydrolysis for the neutral
form; solvent effect study carried out at pH 1.61 and 4.17 indicates
that the rates are elevated (1.61 pH) nearly two and a half times,
which is attributed to the lowering of dielectric constant of acetic
acid medium which favours the reactions involving protonation. At
pH 4.17, when the solvent in increased by nearly one and a half
times, the rates are reduced but when the acetic acid is as high as
90% the rates are found to be doubled. Thus most probably here
the solvent helps to stabilise the basic leaving group by some sort
of salt formation. Comparative kinetic data of some triesters via
their neturai forms has been used to decide the P—N bond cleavage,
depending upon the similarity in behaviour in all cases.