Addition of halogen to aromatic hydrocarbons

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115 

When pure naphthalene is treated in CC14 with bromine at room temperature, addition of bromine is greatly favored by light and peroxides 117 the same is true for pure phenan-threne and bromine.118 

Addition of fluorine to aromatic hydrocarbons is usually associated with simultaneous substitution. On fluorination of hexachlorobenzene in CC14 at 0° two crystalline adducts were isolated, namely, hexachlorotetrafluorocyclo-hexene and hexachlorohexafluorocyclohexane.119 

Addition of 3 moles of chlorine to benzene leads to 1,2,3,4,5,6-hexachloro-cyclohexanes. If the cyclohexane ring is in its more stable chair conformation eight isomers are possible, of which five have so far been isolated.120,121 One of these, y-hexachlorocyclohexane, has become a large-scale commercial insecticide.122 

The addition is accelerated by irradiation (500-w lamp, UV) and radical-formers. Catalysts such as iron, iron chloride, antimony chloride, and iodine favor substitution by chlorine and so must be excluded. The reaction is exothermic and the temperature must be kept at 50-60° by cooling. The /7-isomer is the predominant product when it crystallizes out the reaction is broken off, unchanged benzene is removed, and the mixture of isomers is treated with about its own weight of methanol. The a- and /7-isomer are relatively insoluble in methanol, and the y-isomer crystallizes when the extract is concentrated. 

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