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, O2(1 delta g), and .OH are formed as short-lived intermediates. The precursor of the latter, O3.-, and a phenoxyl radical is suggested to arise from electron transfer from phenol/phenolate to ozone. Addition of .OH 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 oxidize 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 O2(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 O2(1 delta g) and .OH) and with catechol (products: 2-hydroxy-1,4-benzoquinone, cis,cis-muconic acid, H2O2 with high yields of O2(1 delta g) and .OH). Material balance is not obtained for these two systems. Pentachlorophenolate, pentabromophenolate and 2,4,6-triiodophenolate ions give rise to halide ions, O2(1 delta g) (58%/48%/10%) and .OH (27%/2%/0%). It is suggested that together with O2(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 O2(1 delta g) yields are substantially lowered, most likely due to release of triplet (ground state) dioxygen induced by the heavy atom effect.

In ozone reactions in aqueous solutions, • OH and O 2 •are often generated as short-lived intermediates and hydroperoxides are formed as labile or stable final products. Tertiary butanol reacts with ozone only very slowly but readily with • OH. In the presence of dioxygen, formaldehyde is a prominent final product, 30 ( 4%, whose ready determination can be used as an assay for • OH. Although dimethyl sulfoxide reacts much more readily with ozone, its fast reaction with • OH which gives rise to methanesulfinic acid can also be applied for the determination of • OH, at least in fast ozone reactions. The formation of O 2 •can be assayed with tetranitromethane (TNM), which yields nitroform anion (NF -) at close to diffusion-controlled rates. TNM is stable in neutral and acid solution but hydrolyzes in basic solution (k ) 2.7 M -1 s -1 ), giving rise to NFplus nitrate ion (62%) and CO 2 plus 4 nitrite ions (38%). TNM reacts with O 3 (k ) 10 M -1 s -1 ), yielding 4 mol of nitrate (plus CO 2 ) and 4 mol of O 3 are consumed in this reaction. NFreacts with O 3 (k ) 1.4 × 10 4 M -1 s -1 ) by O-transfer. The resulting products, (NO 2 ) 3 COand (NO 2 ) 2 CdO, rapidly hydrolyze (k > 10 s -1 ), and most of the nitrite released is further oxidized by ozone to nitrate. In the case of slow ozone reactions, these reactions have to be taken into account; i.e. the NO 3yield has to be measured as well. For the determination of hydroperoxides, Fe 2+ -based assays are fraught with considerable potential errors. Reliable data may be obtained with molybdate-activated iodide. The kinetics of this reaction can also be used for the characterization of hydroperoxides. Reactive hydroperoxides undergo rapid O-transfer to sulfides, e.g., k(HC(O)OOH + (HOCH 2 CH 2 ) 2 S] ) 220 M -1 s -1 , and the corresponding reaction with methionine may be used for their quantification (detection of methionine sulfoxide by HPLC). Distinction of organic hydroperoxides and H 2 O 2 by elimination of the latter by reaction with catalase can often be used with advantage but fails with formic peracid, which reacts quite readily with catalase (k ) 1.3 × 10 -3 dm 3 mg -1 s -1 ). Some examples of • OH and O 2 •formation in ozone reactions are given.

In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and approximately 410 nm) which is the precursor of these products increases nonlinearly (k=(2-->3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2-->10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500+/-1000 M(-1)). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (.)OH adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2) as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H(2)PO(4) (-) giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (.)OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (.)OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* react with alcohols by H abstraction (rates in units of M(-1) s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.

In ozone reactions, singlet dioxygen [O 2 ( 1 ∆ g )] is formed when ozone reacts by O-atom transfer. O 2 ( 1 ∆ g ) yields were determined for more than 50 compounds using as reference the reaction of hydrogen peroxide with hypochlorite. Close to 100% yields were found in the reaction of O 3 with sulfides, disulfides, methanesulfinic acid, and nitrite. In accordance with this, the only products are: methionine sulfoxide, methanesulfonic acid, and nitrate for the reaction of O 3 with methionine, methanesulfinic acid, and nitrite, respectively. In the case of aliphatic tertiary amines (trimethylamine, triethylamine, and DABCO), the O 2 ( 1 ∆ g ) yields range between 70 and 90%, the aminoxide being the other major product. With EDTA and nitrilotriacetic acid (NTA), the O 2 ( 1 ∆ g ) yield is around 20%. The interpretation of the data with DABCO required the determination of the quenching constant of O 2 ( 1 ∆ g ) by this amine, k q = 1.8 × 10 9 dm 3 mol Ϫ1 s Ϫ1 in H 2 O and D 2 O, two orders of magnitude lower than previously reported. Aromatic tertiary amines give even lower O 2 ( 1 ∆ g ) yields [N,N-dimethylaniline (7%), N,N,N Ј,N Ј-tetramethylphenylenediamine (9%)]. Substantial amounts of O 2 ( 1 ∆ g ) are also formed with the DNA model compounds, 2Ј-deoxyguanosine (40%) and 2Ј-deoxyadenosine (15%, in the presence of tert-butyl alcohol as ؒ OH scavenger). The pyrimidine nucleobases only yield O 2 ( 1 ∆ g ) when deprotonated at N(1). O 2 ( 1 ∆ g ) formation is also observed with hydrogen sulfide (15%), azide (17%), bromide (56%), iodide (12%), and cyanide ions (20%). The O 2 ( 1 ∆ g ) yield from the reaction of O 3 with phenols and phenolates is typically around 20%, but may rise closer to 50% in the case of pentachloro-and pentabromophenolate. Low O 2 ( 1 ∆ g ) yields are found with unsaturated acids such as dihydroxyfumarate (6%), muconate (2%), and acetylenedicarboxylate (15%). With saturated compounds, also, no O 2 ( 1 ∆ g ) (e.g. with propan-2-ol, acetaldehyde, acetaldehyde dimethylacetal and glyoxal) or very little O 2 ( 1 ∆ g ) (formic acid; 6%, at high formate concentrations) was detected. As shown with some examples, knowledge of the O 2 ( 1 ∆ g ) yield (in combination with that of other products) is a prerequisite for the elucidation of the mechanisms of O 3 reactions in aqueous solutions.