Homolytic cleavage of water

Unlike conventional chemical reactions, the altered reactivity of chemical reactions undergoing ultrasonic irradiation is principally due to acoustic cavitation which essentially involves the free radical formation. The ultrasound produces highly reactive free radical species like H and OH radicals from the homolytic cleavage of water. Further they may react with any of other free radicals present or with neutral molecules like 02 and O3 to produce peroxy species, superoxide, hydrogen peroxide or hydrogen. When the aqueous solution is saturated with 02, extra... 

The hydrido complex may also arise from sources other than molecular hydrogen. The homolytic cleavage of water by pentacyanocobaltate (II) (9, 16) (Reaction 2) permits various substrates to be reduced stoichiometrically in the absence of molecular hydrogen (20). 

While the availability of hydrogen atoms and/or hydride ions for photoreductions in organic solvents is apparent, the usual presence of reduced products from the photolysis of pesticides in aqueous media remains anomalous. Energy for the homolytic cleavage of water (about 118 kcal/mole) probably would not be derived from the sunlight spectrum, and the unlikely backward ionic dissociation of water to form hydride and hydroxonium (HO" ) ions never has been demonstrated. 

RuCl2(H20) ]+ This species was made from RUCI3 in HCl from pH 0.4-2.0. Kinetic studies suggest that in the epoxidation by [Ru(7l2(H20)4]X02/water-dioxane of cyclo-octene and -hexene homolytic cleavage of the 0-0 bond plays an essential part [771, 772], and that this is so for similar oxidation of alkanes (e.g. of cyclohexane to cyclohexanol) [771],... 

Negative reaction constants p1 for the oxidation of sulfides by [10-1-3]—(r-butylperoxy)iodanes are consistent with a mechanism involving rate-limiting formation of a sulfonium species by nucleophilic attack of sulfide on the iodine(III) atom followed by attack of water to give sulfoxide.151 However, in dichloromethane, inhibition by galvinoxyl implicates a free radical mechanism perhaps by homolytic cleavage of the weak iodine(III)-peroxy bond. 

The Co—carbon bond in AdoCbl is stable in water, but is inherently labile, with a bond dissociation energy of around 30—35 kcal mol This instability is exploited by the AdoCbl-dependent isomerases to effect radical-based rearrangements which, as pointed out above, are initiated by homolytic cleavage of the Co-carbon bond. In the absence of substrate, the homolysis products are not observed, yet in their presence the homolytic cleavage... 

In contrast, irradiation of 22, Eq. (8), at 254 nm in acetonitrile/water at pH 7 gives bibenzyl and dibenzosuberenone, clearly indicating homolytic cleavage of the benzylic bond [52]. 

The formation of the deuterated hydrocarbon, 54, on irradiation of the methyl ether, 53, in acetonitrile/water, Eq. (25), suggests homolytic cleavage of the benzylic-ether bond followed by disproportionation of the in-cage radical pair [87]. Finally, orf/io-nitrobenzylic chromophores have been used to photorelease carbohydrates from the corresponding ethers [76]. 

The ESI product ion spectrum of florfenicol (Scheme 4,1) does not show a molecular ion peak at m/z 358 (Fig. 10.4). The significant peak corresponding to the loss of water is formed by heterolytic fragmentation as shown in Scheme 4 (II, m/z 340). The II decomposes to III (m/z 320) by a neutral loss of HF, and this fragmentation is promoted by the formation of a substituted tropylium ion (Scheme 4). The product ion spectrum of florfenicol is characterized by the unusual feature of a most abundant peak occurring at odd mass, namely at m/z 241. The IV, a radical ion, is formed by homolytic cleavage of the sulfur—carbon bond in III, and loss of the methanesulfinic radical (Scheme 4). 

The results of several photolysis reactions of sulphur-containing rings can be rationalized by postulating this process. One example is the photolysis of lipoic acid (182) which yielded 185 (in water) or 186 (in methanol). The proposed mechanism is a homolytic scission of the S—S bond to the diradical 183 and migration of the tertiary hydrogen atom as a radical, to form the thionthiol 184 which reacts with the solvent. A similar mechanism which involves a primary homolytic cleavage of a C—S bond was assumed to occur in the photolysis of mercaptols . 

---