Photoisomerization and photodegradation of metanilic acid

Abstract

The photolysis of metanilic acid (1) as the sodium metanilate, through quartz under helium at h = 254 nm leading to the isomeric anilinesulphonic acids (2 and 3) and anilinei(4) has been studied in water. The effects of p H and solvent on products, the acid-base catalysis of the photoreaction (pHproduct profile) and the effects of substrate concentration, radical scavenging, triplet sensitization and quenching studies are reported.A mechanism involving a single triplet and a series of o-complexes is proposed.Thermal isomerization of orthanilic acid (2) to sulphanilic acid (3) in concentrated sulphuric acid at high temperatures was first reported almost one hundred years ago' and has been the subject of considerable study in recent time^.^.^ Metanilic acid(1) does not isomerize under similar conditions. There are a number of reports on the photochemistry of sulphonic acids and sulphonates & l o and two major reaction pathways have been recognized for arenesulphonic acids or their sodium ~a l t s . ~* ~, ~The primary photochemical process in aqueous solution appears to be C-S bond cleavage via the II,II* excited state, leading to desulphonation and the formation of the corresponding aromatic hydrocarbon [eqn. (l)]. A competing process of desulphonylation leading to the formation of the corresponding phenol, or its quinonoid oxidation products, if oxygen is present, may also occur [eqn. (2)].ArSO, -% ArOSO; -ArO-+ SO2 (2)Desulphonation proceeds via the excited triplet state and desulphon ylation involves an initial rearrangement of the sulphonate to the isomeric sulphite possibily via an oxathiiran intermediate and a singlet excited state. We were not aware of any studies on the photochemistry of aminobenzenesulphonic acids and we decided therefore to examine the photochemistry of metanilic acid as a starting point. * 1 cal = 4.184 J. Quenching Studies.-Stern-Volmer (S-V) plots for the appearance of 2-4 on irradiation of metanilic acid in water Paper 0/03438J