Complete mineralization of 4-chlorophenol in water can be achieved by photocatalytic degradation of oxygenated solutions containing suspended TiO2. The chemical pathways of this degradation are complex, and in this paper, that which begins with hydroquinone is examined. Hydroxylation to form 1,2,4-benzenetriol is the first step, though a very small amount of cleavage of the C1−C2 bond is observed. The first major group of acyclic compounds derives from oxidative cleavage of either the C1−C2 or C3−C4 bond of 1,2,4-benzenetriol. It is argued that this results from single electron oxidation and capture by superoxide. Many smaller compounds have also been identified, and routes to various ones of them are proposed. Nearly all of the compounds are verified by use of authentic samples.

Photolysis of dibenzothiophene sulfoxide results in the formation of dibenzothiophene and oxidized solvent. Though quantum yields are low, chemical yields of the sulfide are quite high. Yields of the oxidized solvents can also be high. Typical products are phenol from benzene, cyclohexanol, and cyclohexene from cyclohexane and 2-cyclohexenol and epoxycyclohexane from cyclohexene. A number of experiments designed to elucidate the mechanism of the hydroxylation were carried out, including measurements of quantum yields as a function of concentration, solvent, quenchers, and excitation wavelength. These data are inconsistent with a mechanism involving a sulfoxide dimer, which also does not properly account for the solvent oxidations. It is suggested that the active oxidizing agent may be atomic oxygen O( 3 P) or a closely related noncovalent complex, based on the nature of the oxidation chemistry, comparison to known rate constants for O( 3 P) reactivity, and the quantum yield data.

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The TiO2-mediated photocatalytic degradation of 4-chlorocatechol is studied as a branch of the degradation of 4-chlorophenol. In addition to some basic kinetic studies, the identities of many of the cyclic and acyclic intermediates, verified in most cases with authentic samples, are reported. From 4-chlorocatechol, the major product is hydroxylation to form 5-chloro-1,2,4-benzenetriol. A small amount of 4-chloropyrogallol is also produced. Substitution to give 1,2,4-benzenetriol is observed as is oxidative cleavage of the C1−C2 bond to give the diacid. The major products of all of the triols are those of oxidative cleavages, occurring mainly between ortho hydroxy-substituted carbons to give diacids but also between one hydroxy and one unsubstituted carbon to give acid-aldehydes. Many smaller intermediates in the degradations are identified, and pathways are proposed for the larger compounds.

The quenching rate constant of the triplet state by molecular oxygen and the efficiency of singlet oxygen generation have been measured for 12 amines in cyclohexane and benzene. For the best electron donors, the average rate constant for quenching by energy transfer is 6.0 × 10 9 M -1 s -1 . For the same compounds, the rate constant of quenching via enchancement of internal conversion is 1.2 × 10 10 M -1 s -1 . The energy transfer component of the total quenching rate constant is almost twice as fast as the maximum from the standard Porter model. The mechanisms of quenching via quintet, singlet, and triplet channels are discussed for amines and aromatic hydrocarbons, and intersystem crossing out of the quintet manifold is proposed.