Aryl chlorides and bromides are prepared by electrophilic substitution of arenes with chlorine and bromine, respectively, in the presence of Lewis acid catalysts like iron or iron(III) chloride. The ortho and para isomers can be easily separated based on their melting points.
How are aryl chlorides and bromides prepared from arenes, and what role do Lewis acid catalysts play in this electrophilic substitution process?
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Aryl chlorides and bromides are often prepared from arenes through electrophilic aromatic substitution. In the presence of a Lewis acid catalyst, such as aluminum chloride (AlCl3) for chlorination or iron(III) bromide (FeBr3) for bromination, the arene reacts with a halogen (Cl2 or Br2). The Lewis acid catalyst serves to generate the electrophile, which is essential for the attack on the aromatic ring. The catalyst facilitates the formation of the electrophilic species by accepting an electron pair from the halogen, promoting the substitution of a hydrogen atom on the arene with a chlorine or bromine atom.