Hydroxylation of Chlorotoluenes and Cresols: A Pulse Radiolysis, Laser Flash Photolysis, and Product Analysis Study

Abstract

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.

Oxidation of cinnamic acid derivatives: A pulse radiolysis and theoretical study

“…In our laboratory, we have been interested in the study of reactions of oxidizing and reducing radicals ( Å OH, N Å 3 and e À aq ) with substituted benzenes of the type C 6 H 5Àn X n Y, where X is a halogen and Y = NH 2 , Cl, Br, CH 3 , CH 2 Cl or OCH 3 in order to examine the structure-reactivity relationship [9][10][11][12][13][14][15][16][17][18][19][20][21]. In these systems, the Å OH radical attacks the benzene ring except in isomers of chloroaniline and substituted toluenes (especially in para substituted toluenes e.g.…”

Photoconversion of 2-methylphenol, 4-methylphenol, and 2-amino-4-methylphenol in water

“…The most likely mechanism for breaking of the OH bond in methyl-substituted phenols upon excitation up to 50,000 cm -1 (200 nm) is predissociation. The fluorescence quantum yield for 2-A-4-MP in water is lower due to the high efficiency of S 1 -T 3 conversion compared with 4-MP (0.004 and 0.170 respectively).The proposed photolysis products of the selected molecules have been rather well studied by pulsed photolysis and are described in [4][5][6][7][8][9]. It has been established that the OH radicals formed during photolysis of aqueous solutions of methylphenols attack both the aromatic ring and the CH 3 group.…”

“…The proposed photolysis products of the selected molecules have been rather well studied by pulsed photolysis and are described in [4][5][6][7][8][9]. It has been established that the OH radicals formed during photolysis of aqueous solutions of methylphenols attack both the aromatic ring and the CH 3 group.…”

“…Higher photoconversion efficiency for methylphenols in acidic medium (created when H 2 SO 4 is added) also is promoted by the fact that SO 4 • − radicals formed during excitation of the solution will react with the methylphenol molecule [5].…”

“…In neutral and alkaline media, photoconversions of aqueous solutions of methylphenols are possible with participation of anionic forms [4,5], which react with peroxyl radicals • OH arising upon photolysis of water. In alkaline medium, the peroxyl radical exists in the O • − form (OH → H + + O • − , pK 11.9).…”