Halogenation of aromatic rings

Removal of halogen from aromatic rings can also be accomplished by various... 

When a heteroatom, such as N, O, or a halogen, is present in a molecule containing an aromatic ring or a double bond, lithiation is usually quite regio-selective. The lithium usually bonds with the sp carbon closest to the hetero atom, probably because the attacking species coordinates with the hetero atom. Such reactions with compounds such as anisole are often called directed metala-tions. In the case of aromatic rings, this means attack at the ortho position.Two examples are... 

The introduction of the halogens onto aromatic rings by electrophilic substitution is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, but Lewis acid catalysts are normally needed to achieve desirable rates. Elemental fluorine reacts very exothermically and careful control of conditions is required. Molecular iodine can effect substitution only on very reactive aromatics, but a number of more reactive iodination reagents have been developed. 

Despite the general belief that halogens deactivate aromatic rings in electrophilic substitutions, the major product of chlorination of o-fluorotolu-ene is 5-chloro-2-fluorotoluene, the result of replacement of the hydrogen para to the fluorine (O), not para to the methyl group. Only a smaller fraction results from chlorination ortho to the methyl group (P). 

Polymers aren t really that different from other o antc molecules. They re much larger, of course, but their chemistry Is similar to that of analc ous small molecules. Thus, the alkane chains of polyethylene undergo radical-initiated halogenation the aromatic rings of pelyalyrone undergo typical electrophilic aromatic substitution reactions and the amide linkages of a nylon are hydrolysed by (wise. 

Substitution in general of halogens on aromatic ring will decrease compound p/C . Example o-chloroaniline pKa = 2.6, aniline pK = 4.6. 

Treatment of the Cr complex with MeLi or BuLi usually leads to deprotonation as a consequence of the acidity of the ring. The lithiated species reacts with electrophiles in the usual manner (see Section 10.1.3) (Scheme 10.28). This route is another means of deprotonation of aromatic rings and may sometimes be more convenient than directed metallation or halogen-metal exchange, especially if the precursors to such species are not readily accessible. 

Removal of halogen from aromatic rings can also be accomplished by various reducing agents, among them BusSnH, " catalytic hydrogenolysis, " catalytic transfer hydrogenolysis, ... 

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