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Photochlorination is a chlorination reaction that is initiated by light. Usually a C-H bond is converted to a C-Cl bond. Photochlorination is carried out on an industrial scale. The process is exothermic and proceeds as a chain reaction initiated by the homolytic cleavage of molecular chlorine into chlorine radicals by ultraviolet radiation ...
For example, consider radical bromination of toluene: [5] bromination of toluene with hydrobromic acid and hydrogen peroxide in water. This reaction takes place on water instead of an organic solvent and the bromine is obtained from oxidation of hydrobromic acid with hydrogen peroxide. An incandescent light bulb suffices to radicalize.
The light is absorbed by chlorine molecules, the low energy of this transition being indicated by the yellowish color of the gas. The photon induces homolysis of the Cl-Cl bond, and the resulting chlorine radical converts toluene to the benzyl radical: Cl 2 + hν → 2 Cl· C 6 H 5 CH 3 + Cl· → C 6 H 5 CH 2 · + HCl C 6 H 5 CH 2 · + Cl ...
For example, when heated with N-bromosuccinimide (NBS) in the presence of AIBN, toluene converts to benzyl bromide. The same conversion can be effected with elemental bromine in the presence of UV light or even sunlight. Toluene may also be brominated by treating it with HBr and H 2 O 2 in the presence of light. [31] C 6 H 5 CH 3 + Br 2 → C 6 ...
Benzyl chloride is prepared industrially by the gas-phase photochemical reaction of toluene with chlorine: [3] C 6 H 5 CH 3 + Cl 2 → C 6 H 5 CH 2 Cl + HCl. In this way, approximately 100,000 tonnes are produced annually. The reaction proceeds by the free radical process, involving the intermediacy of free chlorine atoms. [4]
Industrially, the diazonium method is reserved for 3-chlorotoluene. The industrial route to 2- and 4-chlorotoluene entails direct reaction of toluene with chlorine. The more valuable 4-chlorotoluene is separated from 2-chlorotoluene by distillation. Distillation cannot be applied to separating 3-chlorotoluene from 4-chlorotoluene. [2]
Benzal chloride is produced by the free radical chlorination of toluene, being preceded in the process by benzyl chloride (C 6 H 5 CH 2 Cl) and followed by benzotrichloride (C 6 H 5 CCl 3):
Production capacity of benzotrichloride was estimated at 80,000 tonnes for the year 2000. It is produced by the free radical chlorination of toluene, catalysed by light or radical initiators such as dibenzoyl peroxide. Mono- and di-chlorinated intermediates are observed as benzyl chloride and benzal chloride: [2] C 6 H 5 CH 3 + Cl 2 → C 6 H 5 ...