Darzens halogenation is the chemical synthesis of alkyl halides from alcohols via the treatment upon reflux of a large excess of thionyl chloride or thionyl bromide (SOX₂) in the presence of a small amount of a nitrogen base, such as a tertiary amine or pyridine or its corresponding hydrochloride or hydrobromide 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 as cyclohexanol, which normally decomposes to form cyclohexene if reacted with only SOCl₂.^[4] The reaction takes place through an S_N2 mechanism but is also often used in the description of S_Ni mechanisms.

For example, ethanol can be converted into chloroethane (X=Cl) or bromoethane (X=Br) as follows:

CH₃CH₂OH + SOX₂ ${\displaystyle {\xrightarrow {pyridine}}}$ CH₃CH₂X + SO₂ + HX