Bromine is a strong electrophile and has a characteristic red-brown color. These properties make bromine a useful reagent for probing the reactivity of colorless nucleophilic organic compounds, such as alkenes and arenes. Two kinds of reaction are common: addition & substitution. In an addition reaction to an alkene two covalent C-Br bonds are formed, and the color of the bromine fades as the colorless product is generated. In a substitution reaction of a hydrocarbon a C-Br covalent bond is also formed, but this product is accompanied by release of of an equal molar quantity of hydrogen bromide gas. HBr is a colorless gas, having a nasty sharp, choking odor. The easiest way of detecting this gas is through its rapid reaction with ammonia (also a gas), which generates a cloud of ammonium bromide particles. The ammonia is conveniently introduced by way of an adjacent beaker containing ammonium hydroxide solution.
The following unit provides a virtual demonstration of the reactions (or lack of reaction) of bromine with a selected group of compounds. Cyclohexane has no pi-unsaturation and is therefore not nucleophilic. It does not react with bromine unless energy in the form of light or heat is applied. In such a case a free-radical substitution reaction occurs. Cyclohexene is a typical alkene, and benzene and anisole are aromatic compounds. The methoxy substituent present in anisole increases the nucleophilicity of the aromatic ring, and greatly enhances the reactivity of the ring toward electrophilic attack. A small amount of the ortho substituted product is also obtained from the bromination of anisole.
This page was scripted by William Reusch, Department of Chemistry, Michigan State University
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