Chlorine, dissociation effect

Electron withdrawing effects of the cx-chlorine give chloroacetic acid a higher dissociation constant than that of acetic acid. 

Cooling water pH affects oxidizing antimicrobial efficacy. The pH determines the relative proportions of hypochlorous acid and hypochlorite ion or, in systems treated with bromine donors, hypobromous acid and hypobromite ion. The acid forms of the halogens are usually more effective antimicrobials than the dissociated forms. Under some conditions, hypochlorous acid is 80 times more effective in controlling bacteria than the hypochlorite ion. Hypochlorous acid predominates below a pH of 7.6. Hypobromous acid predominates below pH 8.7, making bromine donors more effective than chlorine donors in alkaline cooling waters, especially where contact time is limited. 

The standard entropy change for the atom-molecule reactions is in the range 5-20 mole and the halogen molecule dissociation has an eiiU opy change of about 105 e.u. The halogen molecule dissociation energy decreases from chlorine to iodine, but the atom-molecule reactions become more endothermic from chlorine to iodine, and this latter effect probably influences the relative contributions to the mechanism from chain reaction and biinolecular reaction. 

It is always important to keep in mind the relative nature of substituent effects. Thus, the effect of the chlorine atoms in the case of trichloroacetic acid is primarily to stabilize the dissociated anion. The acid is more highly dissociated than in the unsubstituted case because there is a more favorable energy difference between the parent acid and the anion. It is the energy differences, not the absolute energies, that determine the equilibrium constant for ionization. As we will discuss more fully in Chapter 4, there are other mechanisms by which substituents affect the energy of reactants and products. The detailed understanding of substituent effects will require that we separate polar effects fiom these other factors. 

The effect of pH alone on chlorine efficiency is shown in Figure 3. Chlorine exists predominantly as HOCl at low PH levels. Between pH of 6.0 and 8.5, a dramatic change from undissociated to completely dissociated hypochlorous acid occurs. Above pH 7.5, hypochlorite ions prevail while above 9.5, chlorine exists almost entirely as OCl. Increased pH also diminishes the disinfecting efficiency of monochloramine. 

The bactericidal effect of hypochlorite is due to the presence of available chlorine. This is hypochlorous acid (HOC1) and hypochlorite ions (OCl ), where HOC1 is the most effective disinfectant (Betts and Everis, 2005). The dissociation of HOC1 is dependent on pH and at 20 °C and between pH 6.0 and 8.0 the percentages of HOC1 is approximately 97% and 23%, respectively (Beuchat, 1998). This suggests that the pH should be as low as possible, but... 

The effect of one chlorine atom in different positions can be seen from the dissociation constants for the monochlorobutyric acids. In the series of acids, a chlorine atom can be attached to the carbon atom adjacent to the COOff group or on one of the other carbon atoms. The dissociation constant for butyric acid is 1.5 X 10 1 When a chlorine atom is attached in the three available positions, the dissociation constants are as follows ... 

Most acid dissociation constants pKa exceed environmental pH values, the exceptions being the highly chlorinated phenols. As a result, these substances tend to have higher apparent solubilities in water because of dissociation. The structure-property relationships apply to the un-ionized or protonated species thus, experimental data should preferably be corrected to eliminate the effect of ionization, thus eliminating pH effects. 

Kinetic studies of the stoichiometric carbonylation of [Ir(CO)2l3Me] were conducted to model the rate-determining step of the catalytic cycle [73,85]. The reaction can form both fac,cis and mer,trans isomers of [Ir(CO)2l3 (COMe)] (Scheme 13), the product ratio varying with the solvent and temperature used. An X-ray crystal structure was obtained for the fac,cis isomer. Carbonylation of [Ir(CO)2l3Me] is rather slow and requires temperatures > 80 °C in chlorinated solvents (e.g. PhCl). However, the presence of protic solvents (e.g. methanol) has a dramatic accelerating effect. This is interpreted in terms of the protic solvent aiding iodide dissociation by solvation. 

Dissociative adsorption occurring on four adjacent silver atoms (AgadJ) is responsible for nonselective oxidation to yield CO2. An optimum chlorine coverage of about 25% of surface silver atoms effectively blocks dissociative adsorption of oxygen by physical blocking ensuring increased selectivity. 

Alkanes. The chlorination of ethane known to produce more 1,1-dichloroethane than 1,2-dichloroethane is explained by the so-called vicinal effect.115 One study revealed285 that this observation may be explained by the precursor 1,2-dichloroethane radical (the 11 2-chloroethyl radical) thermally dissociating into ethylene and a chlorine atom [Eq. (10.54)]. Indeed, this radical is the major source of ethylene under the conditions studied. At temperatures above 300°C, the dissociation dominates over the chlorination reaction [Eq. (10.55)], resulting in a high rate of ethylene formation with little 1,2-dichloroethane ... 

At 200° C. sulphuryl chloride converts sulphur into monochloride, this result probably being preceded by dissociation of the sulphuryl chloride into chlorine and sulphur dioxide. In the presence of aluminium chloride this reaction can be effected even at the ordinary temperature, and aluminium chloride is known to favour the dissociation of sulphuryl chloride ... 

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