Synthesis of Aryl Halides ArX

Electrophilic addition of HCl or HBr to an alkene normally occurs with Mar-kovnikov regiochemistry, resulting in an alkyl halide with the halogen located at the more substituted carbon of the starting alkene. Since the presence of peroxides (ROOR) initiates a radical mechanism, HBr can also be added to an alkene with anti-Markovnikov orientation. Anti addition of bromine or chlorine to an alkene gives a trans-dibromo or dichloro product. 

The retrosynthesis of an alkyl halide TM involves an FGI to a suitable alkane, alcohol, or alkene starting material. 

The methods described above for alkyl halides (on sp hybridized carbons) cannot be applied to the synthesis of aryl halides (on sp hybridized carbons). The introduction of a halogen to benzene (or a benzene derivative) can be accomplished either by an electrophilic aromatic substitution reaction (Cy FeCls or Br2/FeBr3) or by treatment of a diazonium salt (ArNaO with a halide nucleophile (Cul, CuBr, HBF4, or KI). 

Since electrophilic aromatic substitution begins with benzene (or a benzene derivative), the retrosynthesis of an aryl chloride or bromide simply involves the replacement of the halide with a hydrogen atom. Another possible retro-synthesis that can be applied to any aryl halide is an FGI that replaces the halide with a nitro group, since the nitro group can be converted to any halide via the diazonium salt. 

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