Kinetics aromatic chlorination

S. Y Wang, and J. P. Chen et al., Selective Debenzylation in the Presence of Aromatic Chlorine on Pd/C Catalysts Effects of Catalyst Types and Reaction Kinetics , paper presented at 20th Organic Reactions Catalysis Society Meeting, March 21-25, 2004, Hilton Head Island, SC, USA. 

Selective Debenzylation in the Presence of Aromatic Chlorine on Pd/C Catalysts Effects of Catalyst Types and Reaction Kinetics... 

Harrison RM, Perry R, Wellings RA. 1976a. Chemical kinetics of chlorination of some polynuclear aromatic hydrocarbons under conditions of water treatment processes. Environ Sci Technol 10 1156-1160. 

In our first study, we examined the chlorination of anisole in water by HOCl, with and without a-cyclodextrin. Without the cyclodextrin, the product is 60% para- and 40% ortho-chloroanisole, but with 9 mM a-cyclodextrin (cyclohexaamylose) the ratio was 96% para- and only 4% ortho-chloroanisole. Furthermore, the anisole was only 72% bound in the cyclodextrin. From detailed kinetic studies we showed that the para position was 5.3 times as reactive in the complex as in free solution while the ortho position was completely blocked. The reaction was also first order in [HOCl], while in simple water solution it was second order. This showed that binding did not simply block the ortho positions, it also catalyzed the chlorination of the para position, by reversibly forming a cyclodextrin hypochlorite and delivering the chlorine to the accessible para position (Fig. 1.4). This was the first example in which simple cyclodextrin acted as a selective turnover catalyst for a reaction in water solution because of hydrophobic binding of the substrate into the cavity. We also examined other aromatic chlorinations with cyclodextrins. 

Molecular chlorine is believed to be the active electrophile in uncatalyzed chlorination of aromatic compounds. Simple second-order kinetics are observed in acetic acid. The reaction is much slower in nonpolar solvents such as dichloromethane and carbon tetrachloride. Chlorination in nonpolar solvents is catalyzed by added acid. The catalysis by acids is probably the result of assistance by proton transfer during the cleavage of the Cl-Cl bond. ... 

It follows from the above that, in the reactions of fairly unreactive aromatics, the formation of Cl+ (either from HOC1 or H2OCl+) will be relatively fast compared with the subsequent reaction of this ion with the aromatic so that the kinetics will be governed mainly by the third term in equation (94). Hence de la Mare et al.204 found the rate of chlorination of benzene and toluene by acidified hypochlorous acid to depend on the concentration and nature of the aromatic and to increase with hydrogen ion concentration though (as in the case of positive... 

Kinetics studies of acid-catalysed chlorination by hypochlorous acid in aqueous acetic acid have been carried out, and the mechanism of the reactions depends upon the strength of the acetic acid an<( the reactivity of the aromatic. Different groups of workers have also obtained different kinetic results. Stanley and Shorter207 studied the chlorination of anisic acid by hypochlorous acid in 70 % aqueous acetic acid at 20 °C, and found the reaction rate to be apparently independent of the hydrogen ion concentration because added perchloric acid and sodium perchlorate of similar molar concentration (below 0.05 M, however) both produced similar and small rate increases. The kinetics were complicated, initial rates being proportional to aromatic concentration up to 0.01 M, but less so thereafter, and described by... 

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. 

Kinetic studies have been carried out using the 1 1-complex iodobenzene dichloride as a source of molecular chlorine. In acetic acid solutions, the dissociation of this complex is slower than the rate of halogenation of reactive aromatics such as mesitylene or pentamethylbenzene, consequently the rate of chlorination of these is independent of the aromatic concentration. Thus at 25.2 °C first-order chlorination rate coefficients were obtained, being approximately 0.2 x 10-3 whilst the first-order dissociation rate coefficient was 0.16 xlO-3 from measurements at 25.2 and 45.6 °C the corresponding activation energies... 

They argued that pre-equilibria to form Cl+ or S02C1+ may be ruled out, since these equilibria would be reversed by an increase in the chloride ion concentration of the system whereas rates remained constant to at least 70 % conversion during which time a considerable increase in the chloride ion concentration (the byproduct of reaction) would have occurred. Likewise, a pre-equilibrium to form Cl2 may be ruled out since no change in rate resulted from addition of S02 (which would reverse the equilibrium if it is reversible). If this equilibrium is not reversible, then since chlorine reacts very rapidly with anisole under the reaction condition, kinetics zeroth-order in aromatic and first-order in sulphur chloride should result contrary to observation. The electrophile must, therefore, be Cli+. .. S02CI4- and the polar and non-homolytic character of the transition state is indicated by the data in Table 68 a cyclic structure (VII) for the transition state was considered as fairly probable. 

Since chlorine is always in more than a hundred-fold excess compared to bromine the reaction is occurring by pseudo monomolecular kinetics. The reaction occurs via nucleophilic aromatic substitution by an addition-elimination mechanism, the so-called SjsfAr mechanism (ref. 24). 

Klecka GM, WJ Maier (1988) Kinetics of microbial growth on mixtures of pentachlorophenol and chlorinated aromatic compounds. Biotechnol Bioeng 31 328-335. 

The partial dechlorination of chlorinated anilines has been examined in anaerobic slurries, which has already been noted (Kuhn et al. 1990), while D. hafniense (frappieri) strain PCP-1 is able to dechlorinate a wide range of polychlorinated aromatic substrates including phenols, catechols, anilines, pentachloronitrobenzene, and pentachloropyridine (Dennie et al. 1998). Both pentachloro-aniline and 2,3,5,6-tetrachloroaniline were partially dechlorinated to dichloroanilines, although the orientation of the substituents was unresoved. The kinetics of dechlorination of pentachloroaniline have been described (Tas et al. 2006). 

The Claisen-Schmidt condensation of 2 -hydroxyacetophenone and different chlorinated benzaldehydes over MgO has been investigated through kinetic and FTIR spectroscopic studies. The results indicate that the position of the chlorine atom on the aromatic ring of the benzaldehyde substantially affects the rate of this reaction. In particular, the rate increases in the following order p-chlorobenzaldehyde < m-chlorobenzaldehyde < o-chlorobenzaldehyde. The difference between the meta and para-substituted benzaldehyde can be attributed to electronic effects due to the difference in the Hammett constants for these two positions. Steric effects were found to be responsible for the higher rate observed with the o-chlorobenzaldehyde. 

Kinetic results on the chlorination of aniline by A-chloro-3-methyl-2,6-diphenylpiperi-din-4-one (3) suggest that the protonated reagent is reactive and that the initial site of attack is at the amino nitrogen. The effects of substituents in the aniline have been analysed but product studies were not reported. Zinc bromide supported on acid-activated montmorillonite K-10 or mesoporous silica (100 A) has been demonstrated to be a fast, selective catalyst for the regioselective para-bromination of activated and mildly deactivated aromatics in hydrocarbon solvents at 25 °C. For example, bromobenzene yields around 90% of dibromobenzenes with an ortholpara ratio of 0.12. 

TJ. Mitchell, S.W. Benson, and S.B. Karra. Kinetic Model for Formation of Aromatics in the High Temperature Chlorination of Methane. Combust. Sci. Techn., 107 223-260,1995. 

The oxidation rates for bromoform were slower than the oxidation rates of unsaturated chlorinated aliphatic compounds, including the TCE. Because the hydroxylation rate constant of TCE is 109 Mr1 s 1 and the hydrogen abstraction of bromoform is 1.1 x 108 M 1 s aromatics and alkenes react more rapidly by hydroxyl addition to double bonds than does the more kinetically difficult hydrogen atom abstraction. No oxidative destruction of chloroform by Fenton s reagent was experimentally observed an explanation for this is that both H202 and Fe2+ have rate constants about one magnitude higher with respect to hydroxyl radicals than chloroform. 

As discussed earlier, the effects of the meta, para, and ortho positions of chlorine on the dechlorination kinetics of monochlorophenols, dichlorophenols, and trichlorophenols during Fenton oxidation were evaluated by comparing the rate constants of the kinetic model (Tang and Huang, 1995). This study proposed a pseudo first-order steady state with respect to organic concentration. The proposed reaction pathways considered that the hydroxyl radicals would attack unoccupied sites of the aromatic ring. 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

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