The kinetics and transient optical absorption spectra of the intermediates formed in the reaction of •OH, N3 •, and e- aq with 1-arylazo-2-naphthol and its ortho- and para-substituted chloro, methyl and methoxy derivatives at pH 7.0 have been studied by pulse radiolysis. The rate constants for the reaction of •OH and the nucleophilic hydrated electron (e- aq) with these compounds are (1.0−1.2) × 1010 M-1 s-1 and (0.8−2.5) × 1010 M-1 s-1. The reaction of N3 • with these azo dyes is more selective (ρ+ = −2.0) with the rate constants varying between 0.1 × 109 and 8 × 109 M-1 s-1 depending upon the substituent. The transient optical absorption spectra of the intermediates formed in the reaction of the dyes with •OH are different from the corresponding spectra of the intermediates observed in the reaction of the dyes with N3 •. The •OH radical reacts with the dyes by addition to form the •OH adducts, whereas N3 • reacts with the dyes by direct one-electron transfer to form the one-electron oxidized dye radical. The pK A of the dye radical of the para-methoxy derivative was determined to be 5.2. This radical decays bimolecularly, predominantly by disproportionation, whereas the •OH adducts of the dyes also decay bimolecularly but predominantly by dimerization. The initially formed radical anion in the reaction of e- aq with the unsubstituted dye protonates rapidly to give the hydrazyl radical, which decays bimolecularly by disproportionation. Of the radicals studied, the •OH-induced oxidation of the dyes is the most effective process leading to decolorization of the dyes at pH 7.0.

The reaction of OH radicals with a number of substituted halobenzenes (C, H, -, X, Y, where X = F, CI or Br and Y = CH3, CH,CI, CHCI,, CF, or OCH,) produces a transient absorption band whose A,,, is in the range 31 0-340 nm. This band is assigned to the OH adduct and decayed with second-order kinetics, the bimolecular rate constant being in the region of 0.6 x lo9 -5.0 x lo9 dm3 mol-' s-I. The OH radical reacts mainly by addition to the benzene ring. The rate constants for the reaction of OH radicals, as determined from formation kinetics studies, are between 1.7 and 9.3 x I O9 dm3 mol-' s-l, and are found to depend on the nature of the substituents. A linear correlation is observed with the Hammett parameter.

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Recoil chlorine ¡Chlorotoluenes /Hot atom chemistry /Ipso attack AbstractThe chemical reactions of recoil 38 C1 atoms have been studied in liquid ortho, meta and para chlorotoluenes and in a 1 :1:1 mixture. In the pure liquids, the total organic yield is less in meta isomer compared to ortho and para isomers and is mainly due to a lower 3 * Cl-for-Cl substitution yield. In systems scavenged with 2 mol % I,, these substitution yields are about same for each of the three isomers. In a 1 :1 :1 mixture of the o : m : ρ isomers, containing 2 mol % I 2 , the ratios of the (hot) 38 Cl-for-Cl substitution yields are close to statistical. The distribution for thermal exchange reactions is however, selective. The 3, Cl-for-H substitution yield is less than 1 % per Η atom-aromatic and methylic-and is scavenger independent. The 3, Cl-for-CH 3 substitution yield is also about 1 %, whereas in a 1 :1 :1 mixture the o : m ; ρ ratio is not far from statistical.

The reactions of •OH, O•- and SO4 •- with 2-, 3-, and 4-cresols were studied by pulse radiolysis, laser flash photolysis, and product analysis techniques. The rates of OH reaction with cresols are very high (k ≈ 1 × 1010 M-1 s-1), whereas O•- was found to be less reactive (k ≈ 2.4 × 109 M-1 s-1). The second-order rate constants for SO4 •- reaction with cresols are in the range (3−6) × 109 M-1 s-1. The transient absorption spectra measured in OH reaction exhibited peaks in the range 295−325 nm with a red shift for the meta isomer. The absorption spectra obtained for O•- reaction with 2-cresol has a peak at 360 nm, which is different from those measured with the m and p isomers (λ max = 310 and 380 nm). The absorption spectra of the transient species in SO4 •- reaction obtained by pulse radiolysis and flash photolysis techniques are similar, with absorption maxima centered around 290 and 390 nm in all three isomers. The intermediates formed in •OH, O•-, and SO4 •- reactions are assigned to OH adducts, substituted benzyl radicals, and radical cations, respectively. The rates for oxygen addition to OH adducts of 2-, 3-, and 4-chlorotoluenes and cresols are high, with k f values lying in the range (1−2.7) × 108 M-1 s-1. The relatively higher stability constants of peroxyl radicals formed with cresols (K = k f/k r = (2−5) × 104 M-1) than with chlorotoluenes suggest that the product-forming reaction competes effectively with the reverse reaction in cresols. Dihydroxytoluenes with OH groups ortho to each other were only formed in oxygenated solutions of cresols, and the mechanism involves the addition of oxygen to 1,3-type OH adducts at the carbon carrying the hydroxyl function.

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