Chlorination of aromatic compounds

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. ... 

Chlorination of aromatic compounds under irradiation has been studied extensively (Wagner, 1969). With benzene, the product is a mixture of stereoisomeric hexachlorocyclohexanes with yields 104pmol.J 1. This certainly points to chain reaction with the initiation either from a dissociation, Cl2 2C1, or from the participation of the first excited singlet state of benzene 0B2u) giving... 

Predict the products of the Birch reduction, hydrogenation, and chlorination of aromatic compounds, and use these reactions in syntheses. Problems 17-45, 51, 53, and 67... 

YIELD AND PRODUCT DISTRIBUTIONS FROM THE CHLORINATION OF AROMATIC COMPOUNDS IN TRIFLUOROACETIC ACID ... 

The use of sulfuryl chloride instead of chlorine as a chlorinating agent eliminates the danger of explosion. It reacts readily with paraffins in a manner similar to that of chlorine. With unsaturated compounds, it forms the chlorine derivatives. It is also used for the chlorination of aromatic compounds but does not react readily in the absence of a catalyst. Halogen carriers are used as catalysts. In the presence of organic peroxides, sulfuryl chloride selectively chlorinates the side chain. Several reviews on the use of sulfuryl chloride as a chlorinating agent are available 18, 22, 66. ... 

Cyclodextrins exhibit remarkable ortho-para selectivity in the chlorination of aromatic compounds by hypochlorous acid (HOCl) [22] (Scheme 5). Chlorination takes place via formation of a covalent intermediate, a hypochlorite ester of cyclodextrin. In the chlorination of anisole by hypochlorous acid, pura-chlorination occurs almost exclusively in the presence of sufficient cyclodextrin, although in control experiments maltose had no effect on the product ratio. For example, selectivity for para-chlorination in the presence of 9.4 X 10 M a-cyclodextrin is 96%, which is much larger than that in the absence of a-cyclodextrin (60%). In the proposed mechanism, one of the secondary hydroxyl groups reacts with HOCl to form a hypochlorite ester, which attacks the sterically favorable para position of the anisole molecule included in the cyclodextrin cavity in an intracomplex reaction. The participation of one of the secondary hydroxyl groups at the C-3 position in the catalysis was shown by the fact that dodecamethyl-a-cyclodextrin, in which all the primary hydroxyl groups and all the secondary hydroxyl groups at the C-2 positions are methylated, exhibited equal or larger ortho-para specificity than native a-cyclo-... 

Chlorination of Aromatic Compounds. NCS has also been used for the chlorination of pyrroles and indoles however, the reaction is less straightforward than when NBS and N-Iodosuccinimide are used. In the chlorination of 1-methylpyrrole, it has been demonstrated that basic conditions (NaHCOs, CHCI3) lead to the formation of 1-methyl-2-succinimidylpyrrole (eq 11). In the presence of catalytic amounts of perchloric acid, thiophenes and other electron-rich aromatic compounds have been chlorinated with NCS. (N-Chlorosuccinimide-Dimethyl Sulfide is used for the selective -substitution of phenols.)... 

Dichloroiodo)arenes can also be used for the chlorination of aromatic compounds. Aminoacetophenone 73 is selectively chlorinated with (dichloroiodo)benzene to give product 74 in good yield (Scheme 3.25). This process can be scaled up to afford 24.8 kg of product 74 with 94% purity [58]. 

Table 4.10 Mechanochemical Bromination and Chlorination of Aromatic Compounds ...

Iron (III) chloride has been employed as a catalyst in such reactions as the Friedel-Crafts synthesis, the preparation of alkyl chlorides from olefins (other than ethylene), and the hydrogenation and chlorination of aromatic compounds. ... 

Various systems have been developed for the shape-selective chlorination of aromatic compounds. Molecular chlorine has been used in only a few cases, as it gives poor selectivity and there are diflBculties in handling. 

Gas-liquid reactions are used in several industrial processes. In the synthesis of chemical compounds, gas-liquid reactions are used in, for example, the oxidation of hydrocarbons. For a synthesis reaction, it is typical that one organic compound is transformed into another organic compoimd in the presence of a homogeneous catalyst. Typical reactions are, for example, chlorination of aromatic compounds in the production of chlorinated hydrocarbons, chlorination of carboxylic acids (mainly acetic acid), and oxidation of toluene and xylene in the production of benzoic acid and phthalic acid. In the production of hydrogen peroxide (H2O2), an oxidation process can also be used, namely oxidation of anthraquinole to anthraquinone. 

Chlorination of Aromatic Compounds. NCS can be used for the chlorination of electron-rich aromatic compounds, such as phenol and xylene derivatives under the microwave conditions. Palladium-catalyzed directing group-assisted chlorination of arenes has also been reported (eq 48). ... 

---