A-hydroxyl radicals

Peroxodiphosphoric acid (PDPA) may also be used to convert sulphoxides to sulphones in good yields. An initial study of this reaction80 concluded that the mechanism was a free radical process, involving the reaction of a hydroxyl radical with the sulphoxide as shown in equation (26). This was later claimed to be incorrect the reaction actually occurs by the initial decomposition of PDPA to PMPA which then reacts as described above81. 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

As noted jseviously, the prqx)sed reaction sequaice of intoost initially produces a hydroxyl radical,... 

The antiulcer agent rebamipide ((2-(4-chlorobenzoy-lamino)-3-[2(lH)-quinolinon-4-yl]propionic acid) dose-dependently decreased hydroxyl radical signal generated by the Fenton reaction in an e.s.r. study. Rebamipide is active as a hydroxyl radical scavenger and inhibitor of superoxide production by neutrophils (Yoshikawa etal., 1993). 

Tomiyama T, Shoji A, Kataoka K, Suwa Y, Asano S, Kaneko H, Endo N. Inhibition of amyloid beta protein aggregation neurotoxicity by rifampicin. Its possible function as a hydroxyl radical scavenger. J Biol Chem 1996 271 6839-6844. 

The exciton reacts further instantaneously with a surface hydroxide ion and a Ti4+ ion of the lattice, forming a hydroxyl radical and a (formal) Ti3+ ion. 

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

Fig. 4.12 The formation of phosphoaldehyde via recombination of the radical derived from vinylphosphonic acid with a hydroxyl radical (de Graaf et al., 1997)...

The oxidation of "OH in acetonitrile at glassy carbon elec-trodes yields a hydroxyl radical ( OH) in the primary step (1)... 

From the above it is clear that DMPO can undergo the addition-oxidation mechanism with water as the nucleophile, provided a suitable oxidant is present. With a primary alcohol competing, the O-connected alkoxy spin adduct is formed in addition to HO-DMPO". On the other hand, with a hydroxyl radical source a competing alcohol will undergo hydrogen abstraction by HO" and form an a-hydroxyalkyl radical which forms a C-connected spin adduct. This criterion clearly can distinguish between the two mechanisms at least in model systems (for recent examples, see Reszka and Chignell, 1995 Janzen et al., 1995 Thomas et al., 1996). 

One of the best known ortho-effects is the loss of a hydroxyl radical resulting from the interaction between a nitro group and a hydrogen atom in an adjacent substituent through a cyclic TS62. 

Tomiyama, T., Shoji, A., Kataoka, K., Suwa, Y., Asano, S., Kaneko, H., and Endo, N., Inhibition of amyloid beta protein aggregation and neurotoxicity by rifampicin its possible function as a hydroxyl radical scavenger, ]. Biol. Chem., 271, 6839,1996. 

Figure 6.9 Abstraction of a phenolic hydroxyl proton by a hydroxyl radical generated by Fenton s reaction with hydrogen peroxide.

Reaction of a hydroxyl radical with a CFjCOOH end group in Nation. (From Curtin, D. E. et al. 2004. Journal of Power Sources 131 41-48.)... 

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

Finally, nitrogen dioxide is oxidized to nitric acid in a number of ways. For example, it may react with a hydroxyl radical, producing the acid ... 

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