5.0 Halogenation

  Aromatic Compounds

Halogenation of an aromatic ring is a synthetically important reaction. It takes place in the presence of varied reaction condition depending on the reactivity of the aromatic ring. For very reactive aromatic compounds in polar solvents, the molecular halogens themselves may acts as electrophiles. In the case of nonpolar solvents,halogenation is catalysed by a Lewis acid like $AlC{l_3}$ , or $FeC{l_3}$ , Reactivity of halogens has the following order,

$${I_2}\; < \;B{r_2}\; < \;C{l_2}$$

Let us take chlorination as a representative reaction to understand the mechanism of halogenaton. Chlorine, in the presence of $AlC{l_3}$ or $FeC{l_3}$ forms a complex, $C{l_2}-AlC{l_3}$ . This complex can itself be the reactive electrophile or it may dissociate to give $C{l^ + }$.

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