Halohydrin

What is Halohydrin?

Halohydrin is a functional group in which hydroxyl and halogen groups are bonded to adjacent carbon atoms.

• Halohydrins are saturated compounds.
• 2-bromophenol is not regarded as a halohydrin even though it has halide and a hydroxy group on the adjacent carbon.

Halohydrins are categorised as fluorohydrins, chlorohydrins, bromohydrins or iodohydrins, depending on the halogen present.

Table of Content

• Synthesis of Halohydrin
• Regioselectivity of Halohydrin
• Halohydrin to Epoxide
• Applications of Halohydrins
• Frequently Asked Questions – FAQs

Synthesis of Halohydrin

From Alkene

• Halohydrin can be prepared by adding a halogen like fluorine, chlorine, bromine, and iodine to an alkene in the presence of water.
• The reaction is favoured in the presence of solvents like DMSO, NBS and THF.

From Epoxide

• Halohydrins can be prepared from the reaction of epoxide with hydrohalic acid like HCl HBr or HI.
• Halohydrins can be prepared from the reaction of epoxide with a metal halide NaCl, NaBr, KCl, KBr or KI.

From 2-halo acids

• Halohydrin can be prepared by reduction of 2-halo acids with lithium aluminium hydride.
• 2-halo acids can be prepared by:
• Hell–Volhard–Zelinsky halogenation
• Halogenation of propionyl halide followed by hydrolysis of the propionyl halide.
• 2-halopropionic acids can be prepared from amino acids via diazotisation.

Regioselectivity of Halohydrins

• The formation of halohydrin from an alkene in the presence of water is regioselective.
• This reaction proceeds with anti stereospecificity.
• The reaction follows the Markovnikov pathway as
• The electrophile is attached to less substituted carbon.
• The nucleophile is attached to the more substituted carbon.

Here,

Bromine acts as an electrophile, and the OH group from water acts as a nucleophile.

Halohydrins to Epoxide

• Halohydrins reacts with a strong base (such as NaOH) to form epoxides through an intramolecular SN2 reaction.
• This reaction is the reverse of the formation of halohydrin from an epoxide.

Applications of Halohydrins

• Halohydrins help in the degradation of various halogenated environmental pollutants.
• It can be used as biocatalysts for the kinetic resolution of an epoxide.
• It can be used for the synthesis of various substituted alcohols.
• Halohydrins can catalyse the reverse reaction in the presence of nucleophiles, such as cyanide, azide, and nitrite ions.