Atom-water reaction pathways

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

The observation of lsO-exchange between the carbonyl oxygen atom of an ester and solvent is evidence for the formation of a tetrahedral addition compound, but not proof that this actually lies on the reaction pathway. But the behaviour of the exchange and hydrolysis reactions is so similar that there can be little doubt that this is, in fact, the case. The evidence is most complete for alkaline hydrolysis (see p. 163). One further piece of evidence obtained under acidic conditions is the observation47 that there is no exchange between the solvent and the carbonyl-lsO-labelled ester when methyl 2,4,6-trimethyl-benzoate is hydrolyzed in 3.09,5.78 and 11.5 M sulphuric acid. Other evidence makes it clear that this ester is hydrolyzed by the Aac1 mechanism, and that no reversible addition of water is expected. 

The use of /V-chloramines, in principle, allows the facile generation of aminyl radicals upon UV photolysis in neutral media. A radical chain can be envisioned for the formation of 2-chIoromethylpyrrolidines (Scheme 7). In practice, however, there is a slow step in this sequence, step A and/or B, such that other reaction pathways, disproportionation or H-abstraction from the solvent, compete. Surzur has studied the reaction in Scheme 7 in the alcoholic solvents MeOH and /-PrOH, which serve as hydrogen atom sources, and achieved acceptable ratios of cyclic products 25 and 26 to acyclic amine 27 (70TL3107). Other /V-chloroalkenylamines gave similar results (71TL903 80TL287). /8-chloro-substituted amine products such as 25 were the sole products when the reactions were carried out in acetic acid-water mixtures these reactions involve aminium cation radicals and are discussed further in Section III,B. 

Chlorohydrins are compounds characterized by alpha halo-alpha alkoxy groups bound to a common carbon atom. These compounds undergo rapid hydrolysis at this shared carbon atom. Bis(2-chloroisopropyl)ether, a chlorohydrin, has two such carbon atoms, and both react very rapidly with water. In fact, the reactions are so fast that acid and alkaline contributions have not been determined. It is likely, however, that base accelerates the reaction kinetics. The proposed reaction pathway for this compound is based on the reported pathway for bis(chloroethyl)ether (Figure 13.4). The reported rate constant for bis(chlorome-thyl)ether, of 0.23 sec-1 was based on an observed half-life of a few minutes. Similarly, for bis(2-chloroisopropyl)ether, both of the chloro substituents are reactive, and a half-life of a few minutes can be assigned to this compound, as well. 

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