Halohydrin formation reaction

The halohydrin formation reaction is a chemical reaction in which a halogen is added to an alkene in aqueous solution to form a halohydrin. The reaction is a form of electrophilic addition; it is similar to the halogen addition reaction.

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The basic chemical equation for this reaction is as follows:

C=C + X₂ + H₂O → X-C-C-OH

(X represents a halogen, either Cl or Br).

The reaction occurs with anti addition, leaving the newly added X and OH groups in a trans configuration.

When bromination is desired, N-bromosuccinimide is preferable to bromine because fewer side-products are produced.

Reaction mechanism

In the first step, the pi bond of the alkene attacks the halogen, forming a carbocation. For the case where bromine is the halogen, the carbocation comprises a three-membered C, C, Br ring; this is known as a bromonium ion.

The addition of water, which provides the hydroxide anion, gives the desired halohydrin with high anti stereoselectivity in order to minimize the repulsion between the halogen and the hydroxide approaching group. Markovnikov's rule applies: the positive charge resides on the more-substituted carbon, hence the hydroxide anion adds here.

References

• William Reusch. Addition Reactions of Alkenes. Virtual Textbook of Organic Chemistry.