Darzens halogenation

Not to be confused with Darzens reaction.

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 ${\displaystyle \xrightarrow {\text{pyridine}} }$ CH₃CH₂X + SO₂ + HX

References

1. Darzens, George (1911). "Nouvelle methode d'etherification des alcools par les hydracides" [New method of etherification of alcohols with hydracids]. Compt. Rend. (in French). 152: 1314–1317.
2. Darzens, George (1911). "Action du Chlorure de Thionyle en Presence d'une Base Tertiare sur quelques Ether d'Acides Alcohols" [Action of Thionyl Chloride in the Presence of a Tertiary Base on some Aether of Alcohols]. Compt. Rend. (in French). 152: 1601–1603.
3. "Darzens Halogenation". Comprehensive Organic Name Reactions and Reagents. 2010. pp. 846–847. doi:10.1002/9780470638859.conrr179. ISBN 9780471704508.
4. Libermann, D. (27 December 1947). "Interaction of Thionyl Chloride and Hydroxy Compounds". Nature. 160 (4078): 903–904. Bibcode:1947Natur.160..903L. doi:10.1038/160903a0. PMID 18917309. S2CID 4138548.