Darzens halogenation

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