Organic 2 Review Problem 191

Organic 2 Review Problem 191 - P REPARATIONS 175 and s...

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PREPARATIONS 175 and should always be stored damp. Bromobenzene. C 6 H 5 Br . If cold benzene is treated with bromine in the absence of sunlight, very little reaction occurs; if, however, a "halogen carrier," such as iron, iodine, pyridine, etc., is also present, a rapid reaction by substitution occurs, forming first C 6 H. + Br, = C 8 H 5 Br + HBr bromobenzene, and then mainly p-dibromobenzene. The reaction must be carried out in the absence of direct sunlight, since sunlight causes the bromine to add directly on to benzene, particularly if the C 6 H, + 3Br 2 = C 6 H 6 Br 6 latter is warm, to give benzene hexabromide. Bromination of the aromatic nucleus is now regarded as replacement of a hydrogen atom of the intact nucleus as a result of an attack by a polarised complex with a positive end. Iron acts as a "carrier" by forming FeBr 3 , which as a Lewis acid forms a polarised complex with one mol. of Br 2 : FeBr 3 -> Br - v - - Br - -_- FeBr3 ^ ' . + FeBr 4 I Br + HBr + FeBr 3 Required: Purified benzene, 34 ml.; pyridine, 0-5 ml.; bromine,
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