bromination

Bromination is a class of chemical reactions in which bromine is introduced into a molecule, forming carbon–bromine bonds. It encompasses several distinct reaction modes depending on the substrate: electrophilic addition to unsaturated compounds, electrophilic aromatic substitution on activated rings, and radical bromination of C–H bonds. Reactions often employ catalysts or initiators to polarize Br2 or generate bromine radicals.

In alkenes, bromination proceeds via a bromonium ion, leading to anti addition and vicinal dibromides. With alkynes, two equivalents of Br2 add across the multiple bond to give tetrabromides. Using excess bromine or heat, di- or polybrominated products can form depending on the substrate and conditions.

Aromatic bromination occurs by electrophilic aromatic substitution. Br2, often with a Lewis acid catalyst such as FeBr3, generates a strong brominating electrophile that substitutes a ring hydrogen. The direction of substitution follows substituent effects: halogens are ortho/para-directing but deactivating overall.

Radical bromination targets C–H bonds via initiation, propagation, and termination steps. N-bromosuccinimide (NBS) is commonly used for allylic and benzylic bromination, often under light or radical initiators. Substitution tends to occur at sites that form relatively stable radicals and can be more selective than ionic methods.

Bromination is widely used to prepare intermediates for pharmaceuticals, agrochemicals, and polymers, and to modify substrates for further transformations. Safety considerations include the corrosive and toxic nature of bromine and the corrosive hydrogen bromide byproducts; proper handling and waste treatment are essential.