Kinetic order hypochlorous acid

Kinetic studies using acidified hypochlorous acid are rather more complicated than these with hypobromous acid. Much higher concentrations of mineral acid are necessary so that the activities of the reacting entities do not correspond closely to their molecular concentrations, and the kinetic order of reaction varies according to the acid concentration and the reactivity of the aromatic. 

With 77 % aqueous acetic acid, the rates were found to be more affected by added perchloric acid than by sodium perchlorate (but only at higher concentrations than those used by Stanley and Shorter207, which accounts for the failure of these workers to observe acid catalysis, but their observation of kinetic orders in hypochlorous acid of less than one remains unaccounted for). The difference in the effect of the added electrolyte increased with concentration, and the rates of the acid-catalysed reaction reached a maximum in ca. 50 % aqueous acetic acid, passed through a minimum at ca. 90 % aqueous acetic acid and rose very rapidly thereafter. The faster chlorination in 50% acid than in water was, therefore, considered consistent with chlorination by AcOHCl+, which is subject to an increasing solvent effect in the direction of less aqueous media (hence the minimum in 90 % acid), and a third factor operates, viz. that in pure acetic acid the bulk source of chlorine ischlorineacetate rather than HOC1 and causes the rapid rise in rate towards the anhydrous medium. The relative rates of the acid-catalysed (acidity > 0.49 M) chlorination of some aromatics in 76 % aqueous acetic acid at 25 °C were found to be toluene, 69 benzene, 1 chlorobenzene, 0.097 benzoic acid, 0.004. Some of these kinetic observations were confirmed in a study of the chlorination of diphenylmethane in the presence of 0.030 M perchloric acid, second-order rate coefficients were obtained at 25 °C as follows209 0.161 (98 vol. % aqueous acetic acid) ca. 0.078 (75 vol. % acid), and, in the latter solvent in the presence of 0.50 M perchloric acid, diphenylmethane was approximately 30 times more reactive than benzene. 

Oxidation of oxalic acid with dimethyl-V,V-dichlorohydantoin and dichloroisocya-nuric acid is of first order with respect to the oxidant. The order with respect to the reductant is fractional. The reactions are catalysed by Mn(II). Suitable mechanisms are proposed.129 A mechanism involving synchronous oxidative decarboxylation has been suggested for the oxidation of a-amino acids with l,3-dichloro-5,5-dimethylhydantoin.130 Kinetic parameters have been determined and a mechanism has been proposed for the oxidation of thiadiazole and oxadiazole with trichloroiso-cyanuric acid.131 Oxidation of two phenoxazine dyes, Nile Blue and Meldola Blue, with acidic chlorite and hypochlorous acid is of first order with respect to each of the reductant and chlorite anion. The rate constants and activation parameters for the oxidation have been determined.132... 

Shilov et al studied the rate of oxidation of formate ions in phosphate and carbonate buffers, and showed that the reaction with molecular chlorine is negligible in solutions of pH > 6. At 20 °C the rate of reaction with hypochlorous acid is constant in the range pH 5.5-7, then it decreases with increase of pH, and becomes negligible at pH 13. The kinetics are second-order with respect to hypochlorous acid, and first with respect to formate ions. In alkaline solution hydroxide ion catalysis is apparent viz. 

The oxidations of lactic acid, d-gluconic, and pyruvic acids have been studied by Shilov and Yasnikov. They found that the rate of reaction in each case is very low in acid solution, and that the rate increases with increase of pH to a maximum at pH 7. In solutions of pH 7 the kinetics are first-order with respect to both active chlorine and the substrate. Shilov and Yasnikov deduced that hypochlorous acid is the principal active species, and that the mechanism involves the formation of an ester of hypochlorous acid as the first step, followed by elimination of hydrochloric acid and the formation of a carbonyl group. 

Kinetic studies of the chlorination of benzaldehyde with CBT in water-acetic acid mixtures [86IJC(A)478] show the reaction is the first order in CBT and zero order in substrate. Reaction rate increases with a decrease in pH and the addition of chloride ion (the first order by [Cl]). It is proposed that the chlorinating agent under these conditions is hypochlor-ous acid, which is formed from CBT as a result of protonation and subsequent hydrolysis. Its reaction with benzaldehyde is fast (Scheme 58). 

---