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Halohydrin
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Halohydrin
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). The term only applies to saturated motifs, as such compounds like 2-chlorophenol would not normally be considered halohydrins. Megatons of some chlorohydrins, e.g. propylene chlorohydrin, are produced annually as precursors to polymers.
Halohydrins may be categorized as chlorohydrins, bromohydrins, fluorohydrins or iodohydrins depending on the halogen present.
Halohydrins are usually prepared by treatment of an alkene with a halogen, in the presence of water. The reaction is a form of electrophilic addition, with the halogen acting as electrophile. In that regard, it resembles the halogen addition reaction and proceeds with anti addition, leaving the newly added X and OH groups in a trans configuration. The chemical equation for the conversion of ethylene to ethylene chlorohydrin is:
When bromination is desired, N-bromosuccinimide (NBS) can be 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.
This reaction is produced on an industrial scale for the production of chlorohydrin precursors to two important epoxides, epichlorohydrin and propylene oxide[citation needed]. At one time, 2-chloroethanol was produced on a large scale as a precursor to ethylene oxide, but the latter is now prepared by the direct oxidation of ethylene.
2-Chlorocarboxylic acids can be reduced with lithium aluminium hydride to the 2-chloroalcohols. The required 2-chlorocarboxylic acids are obtained in a variety of ways, including the Hell–Volhard–Zelinsky halogenation. 2-Chloropropionic acid is produced by chlorination of propionyl chloride followed by hydrolysis of the 2-chloropropionyl chloride. Enantiomerically pure (S)-2-chloropropionic acid and several related compounds can be prepared from amino acids via diazotization.
In presence of a base halohydrins undergo internal SN2 reaction to form epoxides. Industrially, the base is calcium hydroxide, whereas in the laboratory, potassium hydroxide is often used.
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Halohydrin
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). The term only applies to saturated motifs, as such compounds like 2-chlorophenol would not normally be considered halohydrins. Megatons of some chlorohydrins, e.g. propylene chlorohydrin, are produced annually as precursors to polymers.
Halohydrins may be categorized as chlorohydrins, bromohydrins, fluorohydrins or iodohydrins depending on the halogen present.
Halohydrins are usually prepared by treatment of an alkene with a halogen, in the presence of water. The reaction is a form of electrophilic addition, with the halogen acting as electrophile. In that regard, it resembles the halogen addition reaction and proceeds with anti addition, leaving the newly added X and OH groups in a trans configuration. The chemical equation for the conversion of ethylene to ethylene chlorohydrin is:
When bromination is desired, N-bromosuccinimide (NBS) can be 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.
This reaction is produced on an industrial scale for the production of chlorohydrin precursors to two important epoxides, epichlorohydrin and propylene oxide[citation needed]. At one time, 2-chloroethanol was produced on a large scale as a precursor to ethylene oxide, but the latter is now prepared by the direct oxidation of ethylene.
2-Chlorocarboxylic acids can be reduced with lithium aluminium hydride to the 2-chloroalcohols. The required 2-chlorocarboxylic acids are obtained in a variety of ways, including the Hell–Volhard–Zelinsky halogenation. 2-Chloropropionic acid is produced by chlorination of propionyl chloride followed by hydrolysis of the 2-chloropropionyl chloride. Enantiomerically pure (S)-2-chloropropionic acid and several related compounds can be prepared from amino acids via diazotization.
In presence of a base halohydrins undergo internal SN2 reaction to form epoxides. Industrially, the base is calcium hydroxide, whereas in the laboratory, potassium hydroxide is often used.