Photoreactions of aromatic compounds. Part XV: Photosolvolysis of some 5‐substituted 3‐nitrophenyl phosphates and of 3‐nitrophenyl sulfate

Abstract

AbstractQuantum yields have been determined for the photosolvolysis of 5‐chloro‐, 5‐bromo‐ and 5‐methyl‐3‐nitrophenyl phosphate in alkaline solutions of different pH and ionic strength. Quantum yields are found to be the same for light of 313 mμ and 265 mμ. The 5‐bromo‐ and 5‐methyl‐compounds show a behaviour comparable to that of 3‐nitrophenyl phosphate itself, be it with lower quantum yields under the same reaction conditions; the data suggest a shortening of the lifetime of the reacting excited species. 5‐Chloro‐3‐nitrophenyl phosphate reacts with water in the pH range studied with a relatively high quantum yield (0.15), probably due to the favourable electronic effect of the chlorine substituent. There is no indication of a photoreaction with OH−, not even in 0.1 molar NaOH and ionic strength ∼ 1; for this compound the reaction with the solvent (H2O) may be so fast that reactions with solutes (OH−) cannot compete. Another compound for which only a reaction with water and not with OH was observed, is m‐nitrophenyl sulfate. Here again, the quantum yield of photohydrolysis is high (0.5) and practically constant over the range pH2‐13. This makes it unlikely that protonation or deprotonation is one of the essential steps of the reaction. Cl−‐ and especially Br−‐ions only when present in high concentration (∼ 1 mole) quench the photosolvolysis of m‐nitroanisole (by OH−), and probably also the reaction of m‐nitrophenyl sulfate with water. This suggests that the reacting excited species is in the first singlet rather than in a triplet state.