Ozonolysis of phenols in aqueous solution

In the ozonolysis of phenol in aqueous solution at pH 3, 7 and 10 the following products were quantified: catechol, hydroquinone, 1,4-benzoquinone, cis,cis-muconic acid, H2O2., 2,4-dihydroxybiphenyl and 4,4-dihydroxybiphenyl. At pH 10, material balance (products vs. phenol consumption) is obtained. Singlet dioxygen, O-2((1)Delta(g)), and (OH)-O-. are formed as short-lived intermediates. The precursor of the latter, O-3(.-), and a phenoxyl radical is suggested to arise from electron transfer from phenol/phenolate to ozone. Addition of (OH)-O-. to phenol gives rise to dihydroxycyclohexadienyl radicals which add dioxygen and eliminate HO2. thereby forming catechol/hydroquinone. In competition and catalysed by H+ and OH-, the dihydroxycyclohexadienyl radical eliminates water yielding a phenoxyl radical. At pH 10, they readily oxidise catechol and hydroquinone. This reforms phenol (accounting for the low phenol consumption) and yields higher-oxidised products, eventually 1,4-benzoquinone. cis,cis-Muconic acid can be accounted for by the Criegee mechanism, while O-2((1)Delta(g)) is released on the way to (some of the) catechol and hydroquinone. Similar reactions proceed with hydroquinone (products: 1,4-benzoquinone, 2-hydroxy-1,4-benzoquinone and H2O2, with high yields of O-2((1)Delta(g)) and (OH)-O-.) and with catechol (products: 2-hydroxy-1,4-benzoquinone, cis,cis-muconic acid, H2O2, with high yields of O-2((1)Delta(g)) and (OH)-O-.). Material balance is not obtained for these two systems. Pentachlorophenolate, pentabromophenolate and 2,4,6-triiodophenolate ions give rise to halide ions, O-2((1)Delta(g)) (58%/48%/10%) and (OH)-O-. (27%/2%/0%). It is suggested that together with O-2((1)Delta(g)) the corresponding ortho- and para-quinones plus a halide ion are formed. Further halide ion is released upon the hydrolysis of these and other products. For pentachlorophenolate the material balance with respect to the short-lived intermediates is 85%. With the bromo- and iodophenolates the O-2((1)Delta(g)) yields are substantially lowered, most likely due to release of triplet (ground state) dioxygen induced by the heavy atom effect.