Darzens halogenation is the chemical synthesis of

alkyl halide The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely us ...

s from alcohols via the treatment upon reflux of a large excess of

thionyl chloride Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...

or thionyl bromide (SOX₂) in the presence of a small amount of a nitrogen base, such as a

tertiary amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a ...

or its corresponding

hydrochloride In chemistry, a hydrochloride is an acid salt resulting, or regarded as resulting, from the reaction of hydrochloric acid with an organic base (e.g. an amine). An alternative name is chlorhydrate, which comes from French. An archaic alternative n ...

hydrobromide In chemistry, a hydrobromide is an acid salt resulting, or regarded as resulting, from the reaction of hydrobromic acid with an organic base (e.g. an amine). The compounds are similar to hydrochlorides. Some drugs are formulated as hydrobromides, ...

salt. The reaction is named after its creator,

Auguste Georges Darzens Auguste Georges Darzens (12 July 1867 in Moscow, Russia – 10 September 1954) was a Russian-born French organic chemist. Biography From 1886 he studied at the École Polytechnique in Paris under Louis Édouard Grimaux. In 1895 he received ...

, who first reported it in 1911. 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 Cyclohexanol is the organic compound with the formula HOCH(CH2)5. The molecule is related to cyclohexane by replacement of one hydrogen atom by a hydroxyl group. This compound exists as a deliquescent colorless solid with a camphor-like odor, w ...

, which normally decomposes to form

cyclohexene Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes. Cyclohexene is not very stable upon long term storage with exposure to light a ...

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 Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

can be converted into chloroethane (X=Cl) or bromoethane (X=Br) as follows: ::CH₃CH₂OH + SOX₂

\xrightarrow

CH₃CH₂X + SO₂ + HX

References

{{Organic reactions Halogenation reactions Substitution reactions Name reactions