Dehydrohalogenation

Dehydrohalogenation is an organic chemistry reaction from which an alkene is obtained from an alkyl halide. It is also called the β-Elimination reaction. Ethanolic potassium hydroxide when reacted with alkyl halide gives alkene.

In this reaction, the halogen on the carbon in the alkyl halide reacts with hydrogen present on the β-carbon releasing hydrogen chloride. Hence an alkene is formed due to formation of a double bond between the α- and β-carbon.

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Reactions

Here ethyl chloride reacts with potassium hydroxide dissolved in ethanol, giving ethene. Similarly 1-chloropropane and 2-chloropropane give propene.

Chlorobenzene does not react with potassium hydroxide due to the presence of the benzene ring which provides too strong of a substrate for conventional elimination.

Promoting elimination

In general, the above reaction of haloalkane with potassium hydroxide would compete with an Sn2 nucleophilic substitution reaction (minor product) because OH^- is a strong, sterically unhindered nucleophile. Perhaps a better reagent would be a deprotonated alcohol such as potassium tert-Butoxide ([CH₃]₃CO⁺ K^-) or because it is an oxide anion and thus a strong base and nucleophile, potassium ethoxide (CH₃CH₂O⁺ K^-) and because of steric hindrance therefore promoting an elimination mechanism instead of a substitution mechanism. Also, providing heat for the reaction to progress would further promote elimination.

See also

• Hydrohalogenation
• Zaitsev's rule

References

Dehydrohalogenation