Effect of basicity on the decomposition of the conjugate base of 4-nitrophenyl N-aryl-P-phenylphosphonamidates

Abstract

The hydrolyses at 25 °C in 50% ethanol–water and 1 M ionic strength of the title compounds in alkaline solutions obey the equation k_obs.=k_o/(1 +K_w/K_a[OH^–]) where k_o, is the rate constant for decomposition of the conjugate base and K_a is the ionisation constant. The parameters obey good Hammett relationships ρ(k_o)=–1.96 and ρ(K_w/K_a)=–2.2 for five substituents varying from 3-chloro to 4-methoxy. The substituent effect monitors the charge development on the nitrogen during the E1 reaction and, taken with previous charge data for the leaving oxygen, indicates a transition state with considerable metaphosphorimidate [PhP(O)NAr] character. The substituent effect on nitrogen cannot however differentiate between E1 cB and B_Ac2 mechanisms of hydrolysis.