And halogenation of aromatic compounds

Such long ago known in organic chemistry reactions as nitration, sulfonation, and halogenation of aromatic compounds are reactions of electrophilic substitution. 

Dolfing J (1998) Halogenation of Aromatic Compounds Thermodynamic, Mechanistic and Ecological Aspects. FEMS Microbiol Lett 167 271... 

Halogenation of aromatic compounds differs from the halogenation of alkenes, which do not require a Lewis Acid catalyst. The formation of the arenium ion results in the temporary loss of aromaticity, which has a higher activation energy compared to carbocation formation in alkenes. In other words, alkenes are more reactive and do not need to have the Br-Br or Cl-Cl bond weakened. Also, if the ring contains a strongly activating substituent such as -OH, -OR or amines, a catalyst is not necessary, however if a catalyst is used with excess bromine then a tribromide will be formed. 

The sp -hybridized domains on the surface exist in parts as bent, condensed aromatic structures, and in parts as isolated double bonds. Therefore, a possible strategy might comprise the application of typical reachons of olefins or aromatic compounds like, for instance, the Diels-Alder reachon or other cycloadditions, the alkylation, or the halogenation of aromatic compounds. However, there is a certain drawback to this approach usually it is only van der Waals forces connecting the graphitic structures situated on the surface to the particle s actual core. Hence, the functionality is only attached to a kind of shell, and stability problems may arise, especially if the respective material is to be employed in mechanically demanding applications. 

More progress has been reported on the halogenation of aromatic compounds over zeolite catalysts. Jang et al. [55] studied the vapor phase catalytic chlorination of chlorobenzene using solid-acid catalysts such as silica-alumina, alumina, zeolites and modified clay (bentonite) impregnated with FeClj Dichlorobenzene selectivity was higher over the zeolite catalysts. 

Halogenations of aromatic compounds have been investigated using different types of midostructured readors. A good overview comprising the gas-liquid processes of fluorinations and chlorinations and also liquid-phase brominations has been presented [26]. However, in this section only liquid-phase brominations and iodinations of aromatic compounds are considered. 

Halogenation and dehalogenation of aromatic compounds are summarized in Scheme 12. 

The early work on the sulfonation and chlorosulfonation of aromatic compounds has been extensively documented in Suter s book and for aromatic hydrocarbons and their halogen derivatives by Suter and Weston. More recently, the chlorosulfonation of aromatic and heteroaromatic systems was reviewed by... 

The halogenation of aromatic compounds in acidic media or in the presence of Lewis acids proceeds via the Se2 mechanism. The following species can be electrophilic agents in chlorination (arranged in decreasing their activity) Cl, ClOH, CI2, and CIOH. In nonaqueous solvents, CI2 acts most often as the main agent. In aqueous acidic solutions, it is Cr, which is formed ftom CIOH by the reactions... 

Like Friedel-Crafts reactions, halogenation such as bromination and iodination of aromatic compounds are classified as electrophilic aromatic substitution reactions. Bromine and iodine substituents are weakly electron-withdrawing groups, and introduction of such substituents causes a decrease in reactivity, and therefore, the first halogenation is faster than the second halogenation. However, the reactions in batch macroreactors suffer from the problem of disguised chemical selectivity, i.e, the formation of dihalogenated products. 

---