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卤代醇合成反应是卤素与烯烃在水溶液中反应，以合成卤代醇的一种化学反应。该反应类似于卤素加成反应，是一种亲电加成反应。

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. For the case where bromine is the halogen, a three-membered C, C, Br ring, known as a bromonium ion, is formed.

The addition of water, which provides the hydroxide anion, gives the desired halohydrin with high anti stereospecificity as a consequence of the S_N2 opening of the bromonium ion by hydroxide or water. Markovnikov's rule applies: the positive charge resides on the more-substituted carbon, hence the hydroxide anion adds here.

However, in the case of bromine water reacting with cyclohexene, the solubility of bromine in water is about 0.21 moles per litre, and the concentration of HOBr formed in the equilibrating reaction between bromine and water is about 1.15 x 10⁻³ moles per litre. So it is thousands of times more likely that Br₂ will be the species that attacks the C=C bond.

A couple of other factors have some bearing too:

Br₂ is much more soluble in the non-polar alkene than either water or HOBr.
When the Br⁺ ion is attacked by the C=C bond electrons, the Br^- is right on the spot to add to the other side of the bond. In this manner, steric issues are minimized.

So while there can be no doubt that some halohydrin is formed when a halogen-water mixture is mixed with an alkene, the main product will be the dihalo compound. For example, when bromine water reacts with cyclohexene, the main product is 1,2-dibromocyclohexane.

References

William Reusch. Addition Reactions of Alkenes. Virtual Textbook of Organic Chemistry.
Cotton and Wilkinson: Advanced Inorganic Chemistry. John Wiley 1972. p 476.