Ch_322b_15.02

Ch_322b_15.02 - Chapter 15.2 lecture note

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Halogenation of Benzene The halogenation of benzene (and other simple arenes) is one of the most common ways to functionalize aromatics. The reaction of benzene with Cl 2 or Br 2 occurs in the presence of Lewis acids . The reaction generally will not proceed with only X 2 . For example, benzene will not decolorize a solution of Br 2 /CCl 4 . Examples: Chlorination and Bromination of Benzene in the Presence of Fe The chlorination or bromination of benzene is carried out with a small amount of added iron. A redox reaction between the Fe and X 2 immediately occurs forming the Lewis acid catalyst, FeX 3 . 2 Fe + 3 X 2 2 FeX 3 Ferric chloride and ferric bromide are electrophilic reagents that act as catalysts for the halogenation reactions.
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+ Cl 2 Cl + HCl FeCl 3 chlorobenzene (~90%) + Br 2 Br + HBr FeBr 3 bromobenzene (~75%) Chlorination and Bromination of Benzene
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Role of the Catalyst The Lewis acid acts to produce a more reactive electrophile capable of disrupting the rather stable aromatic
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Ch_322b_15.02 - Chapter 15.2 lecture note

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