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Electrophilic addition of bromine to an alkene using the example of a triglyceride in an oil with a blue- marked saturated fatty acid residue, a green- marked monounsaturated fatty acid residue and a red- marked triunsaturated fatty acid residue. Arrows point to the four carbon-carbon double bonds. Below the bromination product after the addition of four bromine molecules (Br 2 ), corresponding to eight bromine atoms (drawn without taking the stereochemistry into account ).
Photochemical side chain bromination of toluene - SSS rule.

The bromination is a special case of the halogenation, and refers to a chemical reaction to introduce one or more bromine atoms in an organic compound . This can be an addition or a substitution reaction . The reaction mechanism can be ionic or radical .

Numerous compounds are possible as substrates for a bromination:

In the chemical industry, chlorination is often used as an alternative , since chlorine is cheaper and has a lower molar mass . In the laboratory, on the other hand, bromination is usually preferred because bromine is easier to handle ( liquid at room temperature) and has greater selectivity due to its lower reactivity .

Bromination of alkanes

The radical substitution is a difficult-to-control reaction and usually produces all sorts of products and isomers . It is therefore only suitable for organic synthesis in exceptional cases.

Bromination of alkenes / alkynes

The electrophilic bromination of alkenes / alkynes takes place at the double / triple bond and can also be carried out with complex substrates with several functional groups. The addition of bromine to alkenes / alkynes proceeds with high atom economy .

Bromination of aromatics

Depending on the reaction conditions, aromatics can be brominated either on the core or in the ( aliphatic ) side chain .

The bromination on the nucleus is an example of electrophilic substitution .

Substitution on the side chain takes place preferably in the benzylic position (ie in the direct vicinity of the ring), since benzylic radicals are stabilized by the aromatic ring.

When UV radiation and / or high temperatures {the bromination takes place in the side chain rule SSS : S onne (UV radiation), S iedehitze (high temperature), S eitenkette}.

With the addition of a catalyst (usually a Lewis acid , for example, FeBr 3 ) and low temperatures will nuclear substitution ( KKK rule : K atalysator, K älte, K ernsubstitution).

Bromination in the allyl or benzyl position

A special feature is the introduction of a bromine atom in the allylic or benzylic position. The bromination at this point is also known as the Wohl-Ziegler reaction . This is a radical substitution in which NBS ( N -bromosuccinimide ) is used as the reagent in order to keep the bromine concentration low and thus suppress electrophilic addition to the double bond or the aromatic.

Individual evidence

  1. ^ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry , Springer-Verlag, 1972, pp. 297–306, ISBN 3-211-81060-9 .