Chlorination effectiveness, factors affecting

The action spectrum for DDT and its structural analogs is known to be quite broad and cannot be attributed to simple enzyme inhibition, as in the case of the carbamates or the organophosphates. The presence of chloroaryl moieties, as well as steric effects at receptor sites, both appear to be factors affecting insecticidal activity. In addition to DDT itself, its metabolites DDE and DDA and DDD have some activity. Efforts to overcome insect resistance and to produce more biodegradable analogs led to the introduction of substituents other than chlorine for... 

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. 

It has also been demonstrated that the germicidal effectiveness of free and combined chlorine is markedly diminished with decreasing water temperature. In any situation in which the effects of lowered temperature and high pH value are combined, reduced efficiency of free chlorine and chloramines is marked. These factors directly affect the exposure time needed to achieve satisfactory disinfection. Under the most ideal conditions, the contact time needed with free available chlorine may only be on the order of a few minutes combined available chlorine under the same conditions might require hours. 

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. 

Only metabolites leached from the cell were affected. Elford and van den Ende reported that ozone at 20 ppm had a lethal effect on some bacteria deposited from aerosol mists on various surfaces. Relative humidity is an important factor, particularly when ozone concentration is low. They found little death at a humidity below 45%, at concentrations of 1 ppm, as opposed to a 90% kill in 30 min at 0.025 ppm with a humidity of around 70%. A 5-min exposure of Bacillus cereus to ozone at 0.12 mg/liter was the minimal lethal dose, whereas 0.10 mg/liter was effective for B. megaterium and E. coli. Spores of the Bacillus sp. were killed by ozone at 2.29 mg/liter. These responses were of the all-or-none type with ozone between 0.4 and 0.5 mg/liter of water. Time of exposure, from 1 to 32 min, was not important. Chlorine was effective at 0.27-0.30 mg/liter, with time an important consideration. These two gases did not affect E. coli in the same way. 

Although it has become obvious that a fluorine atom in position 6 is the best substituent for antimicrobial activity, the reasons for this effect remain unclear. A comparative study of numerous quinolones seems to indicate that this F-substitution in position 6 affords both a better affinity with gyrase (2-17 times) and the best cell penetration (1-70 times).The presence of this fluorine lowers the basicity of the nitrogen-containing substituent in position 7, and this fact could play a role. However, there are multiple factors to take into account and interpretations must be carefully considered. Thus, replacing the fluorine atom by a chlorine in pefloxacin does not affect the activity on the enzyme, but a decrease of the bacterial activity is observed. This shows the importance of the pharmacokinetic factors. ... 

As in the case of other CSPs, the chiral resolution on these CSPs is also affected by a change in the structures of the racemic compounds. The different selectiv-ities of amino acids on these CSPs may be considerd as the best example. The effect of structures of the racemates on the chiral resolution may be understood from the work carried out by Shieh et al. [71]. The authors studied the chiral resolution of amino acids as their Schiff s bases. These racemates differ slightly in their structure and the substituent, such as alkyl groups, hence showed different values of enantioselectivities. The values of retention and separation factors decreased by introducing bulky groups in the racemates. Aboul-Enein and Ali [70] observed the lower values of retention factors of miconazole in comparison to econazole and sulconazole. The authors explained this sort of behavior on the basis of the steric effect exerted by the extra chlorine atom in miconazole molecule. 

NMR analyses of PCDEs, which can be used to verify structures, have been performed, e.g.,by Nevalainen et al. [61,69,70]. They have recorded H and BC NMR spectra and studied relationship between conformation and NMR parameters. The steric interactions of two adjacent chlorine atoms and the intramolecular ring effect are the most important factors that affect chemical shifts of PCDEs [69],... 

Copper(I) complexes catalyse a variety of organic reactions which are of synthetic and industrial importance.305 In such processes that involve halide abstraction from aryl or alkyl halides, the abstraction step by a Cu(I) catalyst is believed to be the rate-determining step. In order to circumvent the property of facile disproportionation of Cu, various methods of stabilising Cu(I) and influencing reaction rates were considered.306 A kinetics study of ligand (L) effects on the reactivity of Cu(I)L complexes towards C13CC02 was undertaken. The results indicated that the rate of the chlorine abstraction reaction was affected by several factors. These were the redox potential of the Cu(II/I)L couple, the hybridisation on Cu(I) in the Cu(I)L complex, steric hindrance, and electron density on the central Cu(I) cation at the binding site of the chlorine atom to be abstracted. The volume of activation,... 

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