Dehydrohalogenation

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Dehalogenation

Dehydrohalogenation is an organic reaction in which alkyl halide when boiled with alcoholic alkali gives corresponding alkene. It is also called a β-Elimination reaction and is a type of elimination reaction.

In this reaction, the halogen on the carbon in the alkyl halide reacts with hydrogen present on the β-carbon releasing hydrogen halide. Hence an alkene is formed due to formation of a double bond between the α- and β-carbon. Zaitsev's rule helps to explain regioselectivity for this reaction type. The opposite transformation is called hydrohalogenation.

Reactions

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

Chlorobenzene does not react with potassium hydroxide due to the presence of the benzene ring, which, due to stabilization as a result of aromaticity, does not give conventional elimination, as it would lead to a very unstable benzyne intermediate.

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, thereby promoting an elimination mechanism instead of a substitution mechanism.

External links

• Dehydrohalogenation