Reactions of OH Radicals

The radiation chemical investigations on die nature of OH radical reaction with organic halides has been die subject of ciaient interest [109,110] particularly with aromatic halides containir dectron donating substitiients [111-136]. The measurement of the rate constants, the oxidation of OH-adduct various 

Kinetic parameters of the transient qtecies formed on reaction of OH radicals 

4 Reaction with alltyl halides in neutral solutions 

The same is true fiv l,n-bromochloroaikanes. (4) C2H5I on reaction OH radicals showed absorption bands in 310 and 3S0 nm region whereas in CF3CH2I, the absoption is observed only below 280 nm (Fig lb). 

5 Reaction vHth allgri halides in acidic solutions 

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The pKa value of the equilibrium was found to be equal to 10.2. Meissner and coworkers36 studied also the reaction of OH radicals with DMSO, however, the product of this reaction has no optical absorption in the range 270-800 nm and they measured only the rate of this reaction by a competition method and obtained k = 4.2 x 109m-1s-1. 

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers37 38. They used the system T1m-H202 to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfmic radicals in this reaction either directly or by spin trapping experiments39-44. 

In the case of diaryl sulfoxides the formation of both the aryl radical and the hydroxycyclohexadienyl radical was observed optically. Veltwisch and coworkers45 studied also the reaction of OH radicals from radiolysis of aqueous solutions of mixed (alkyl phenyl) sulfoxides (PhSOR). They found the formation of both alkylsulfinic and phenylsulfinic acids. 

Sumiyoshi and coworkers48 suggested that the reaction of OH radical with the sulfoxides S—C bond can be studied by looking for the sulfinic acid which is expected from the reaction of OH radical and a sulfoxide37,45 (reaction 24). 

Methanol is a very minor product and the observation that its polarization is more intense in N20-saturated solution than in He-saturated solution suggests that it is formed by reaction of OH" radical, probably by a degradation of the radical formed by the addition of OH to DMSO other than the main one given in reaction 21. 

Koulkes-Pujo and coworkers5 5 studied the formation of methane in the reaction of OH radicals and H atoms with aqueous DMSO in acidic media. In the radiolysis of deaerated acidic aqueous solution of DMSO they found that G(CH4) increases monotonously with CH4 concentration up to 0.8 m DMSO. Similar results were obtained for C2H6 but the yields of C2H6 are much lower than that of CH4. 

In the case of PCSO the addition of N20 leads to increased formation of cysteic acid, alanine and dipropyl sulfide and to a decrease in the yield of dipropyl disulfide. The addition of KBr decreases the yield of all the four products. These findings indicate that cysteic acid and alanine are formed by the reaction of OH radicals in parallel reactions as given in Figure 7. 

FIGURE 7. The reactions of OH radical with alkyl-L-cysteine sulfoxide. 

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

Atkinson R, J Arey, B Zielinska, SM Aschmann (1987b) Kinetics and products of the gas-phase reactions of OH radicals and NjOj with naphthalene and biphenyl. Environ Sci Technol 21 1014-1022. 

Due to the high rate of reaction observed by Meissner and coworkers it is unlikely that the reaction of OH with DMSO is a direct abstraction of a hydrogen atom. Gilbert and colleagues proposed a sequence of four reactions (equations 20-23) to explain the formation of both CH3 and CH3S02 radicals in the reaction of OH radicals with aqueous DMSO. The reaction mechanism started with addition of OH radical to the sulfur atom [they revised the rate constant of Meissner and coworkers to 7 X 10 M s according to a revision in the hexacyanoferrate(II) standard]. The S atom in sulfoxides is known to be at the center of a pyramidal structure with the free electron pair pointing toward one of the corners which provides an easy access for the electrophilic OH radical. 

Interestingly, one-electron oxidants partly mimic the effects of OH radicals in their oxidizing reactions with the thymine moiety of nucleosides and DNA. In fact, the main reaction of OH radicals with 1 is addition at C-5 that yields reducing radicals in about 60% yield [34, 38]. The yield of OH radical addition at C-6 is 35% for thymidine (1) whereas the yield of hydrogen abstraction on the methyl group that leads to the formation of 5-methyl-(2 -de-oxyuridylyl) radical (9) is a minor process (5%). Thus, the two major differences in terms of product analysis between the oxidation of dThd by one-electron oxidants and that by the OH radical are the distribution of thymidine 5-hydroxy-6-hydroperoxide diastereomers and the overall percentage of methyl oxidation products. 

Harris GW, Kleindienst TE, Pitts JN Jr. 1981. Rate constants for the reaction of OH radicals with CfpCN, and CFfe = CH-CN in the temperature range 298-424 K. Chemical Physics Letters 80 479-483. 

Atkinson, R. (1987) A structure-activity relationship for the estimation of the rate constants for the gas phase reactions of OH radicals with organic compounds. Int. J. Chem. Kinetics 19, 790-828. 

Ohta, T. (1983) Rate constants for the reactions of OH radicals with alkyl substituted olefins. Int. J. Chem. Kinet. 16, 879-886. 

Hansen, D.A., Atkinson, R., Pitts, J.N. Jr. (1975) Rate constants for the reaction of OH radicals with a series of aromatic hydrocarbons. J. Phys. Chem. 79, 1763-1766. 

Michael and Hart10 found that the reaction of OH radicals (formed by pulse radiolysis of aqueous solutions saturated with N20) with 1,3- and 1,4-cyclohexadienes leads to formation of an intermediate absorbing at 310 nm. In the case of 1,4-cyclohexadiene, another band at A, < 240 nm was also found. In this system there are both H atoms and OH radicals, however the yield of the OH radicals is 10 times higher than that of the H- atoms. Michael and Hart10 assumed that the band at 310 nm is due to CeWi ... 

Von Sonntag and coworkers14 repeated Michael and Hart s study of the reaction of OH radical with 1,3- and 1,4-cyclohexadienes and extended it. They found that in the case of 1,4-cyclohexadiene, 50% of the OH radicals abstract an hydrogen atom, while only about 25% of the OH radicals abstract an hydrogen atom from 1,3-cyclohexadiene. The remaining OH radicals probably add to the double bond. The addition to the double bond was confirmed by final products analysis in the case of the 1,4-isomer. When N20-saturated aqueous solution of 1,4-cyclohexadiene (10-2 M) together with lower (10-4 M) concentration of the thiol (1,4-dithiothreitol) was y-radiolysed, it was found that 4-hydroxycyclohexene was produced with a yield of 0.29 prnol J 1, i.e. a yield of 50% of the OH radicals (equation 9). 

Atkinson R, Aschman SM, Carter WPL. 1982. Kinetics of the reactions of OH radicals with -alkanes at 299K. Int J Chem Kinetics 14 781-788. 

Photolytic. The estimated half-life of sec-butyl alcohol for the reaction of OH radicals in air ranges from 129 d to 23 yr (Anbar and Neta, 1967). 

The titanium-mediated photocatalytic oxidation of a pyridine solution was conducted by Low et al. (1991). They proposed that the reaction of OH radicals with pyridine was initiated by the addition of a OH radical forming the 3-hydro-3-hydroxypyridine radical followed by rapid addition of oxygen forming 2,3-dihydro-2-peroxy-3-hydroxypyridine radical. This was followed by the opening of the ring to give At-(formylimino)-2-butenal which decomposes to a dialdehyde and formamide. The dialdehyde is oxidized by OH radicals yielding carbon dioxide and water. Formamide is unstable in water and decomposes to ammonia and formic acid. Final products also included ammonium, carbonate, and nitrate ions. 

CASRN 56-40-6 molecular formula C2H5NO2 FW 75.07 Chemical/Physical Products identified from the oxidation of glycine and OH radicals (generated from H2O2/UV) in oxygenated water were oxalic acid, formic acid, and ammonium ions. In oxygen-free water, oxalic and formic acids were not produced, i.e., glycine oxidized directly to ammonium ions. The rate constant for the reaction of OH radicals with the zwitterion ion is 1.7 X 10 /M-sec and with the anionic form is 1.9 x 10 /M-sec (Vel Leitner et al., 2002). 

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