Halohydrin

General structure of a halohydrin, where X = I, Br, F, or Cl

Structure of the halohydrin 2-chloroethanol

In organic chemistry a halohydrin (also a haloalcohol or β-halo alcohol) is a functional group in which a halogen and a hydroxyl are bonded to adjacent carbon atoms, which otherwise bear only hydrogen or hydrocarbyl groups (e.g. 2-chloroethanol, 3-chloropropane-1,2-diol).^[1] The term only applies to saturated motifs, as such compounds like 2-chlorophenol would not normally be considered halohydrins.

Halohydrins may be categorized as chlorohydrins, bromohydrins, flurohydrins or iodohydrins depending on the halogen present.

Synthesis

Halohydrins are usually prepared by treatment of an alkene with a halogen (often bromine), in the presence of water.^[2] The reaction is a form of electrophilic addition, similar to the halogen addition reaction and proceeds with anti addition, leaving the newly added X and OH groups in a trans configuration. 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).

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

Halohydrins may also be prepared from the reaction of an epoxide with a hydrohalic acid, or a metal halide.^[3]

Reactions

In presence of a base, such as potassium hydroxide, a halohydrin may undergo internal S_N2 reaction to form an epoxide. This is the reverse of the formation reaction from an epoxide and can be considered a variant of the Williamson ether synthesis. Most of the world's supply of propylene oxide arises via this route.^[4]

Such reactions can form the basis of more complicated processes, for example epoxide formation is one of the key steps in the Darzens reaction.

Of halogenated halohydrins

2,2,2-trichloroethanol

Compounds such as 2,2,2-trichloroethanol, which contain several geminal halogens adjacent to a hydroxyl group may be considered halohydrins (although, strictly speaking, they fail the IUPAC definition) as they possess similar chemistry. In particular they also undergo intramolecular cyclisation to form dihaloepoxy groups. These species are both highly reactive and synthetically useful, forming the basis of the Jocic-Reeve, Bargellini and Corey–Link reactions.^[5]

Safety

Owing to their general reactivity halohydrins often toxic with several low molecular weight compounds having been identified as probable or likely human carcinogen. Additionally, they are often sensitizing agents which can cause individuals handling them to develop an allergic reaction to their presence.

See also

• Cyanohydrin

Misnomers

Despite their rather suggestive names epichlorohydrin and sulfuric chlorohydrin are not halohydrins.

References