Darzens halogenation is the chemical synthesis ofalkyl halides fromalcohols via the treatment upon reflux of a large excess ofthionyl chloride orthionyl bromide (SOX₂) in the presence of a small amount of a nitrogen base, such as atertiary amine orpyridine or its correspondinghydrochloride orhydrobromide salt. The reaction is named after its creator,Auguste Georges Darzens, who first reported it in 1911.^[1][2][3]

The addition of the amine and use of a large excess of the thionyl halide as compared to the usual halogenation protocol makes this reaction effective for a wide range of alcohols including those that are difficult to halogenate, such ascyclohexanol, which normally decomposes to formcyclohexene if reacted with only SOCl₂.^[4] The reaction takes place through anS_N2 mechanism but is also often used in the description ofS_Ni mechanisms.

For example,ethanol can be converted intochloroethane (X=Cl) orbromoethane (X=Br) as follows:

CH₃CH₂OH + SOX₂${\displaystyle \stackrel{\text{pyridine}}{\to }}$ CH₃CH₂X + SO₂ + HX

• Halogenation reactions
• Substitution reactions
• Name reactions

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