Darzens halogenation

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]

Not to be confused with Darzens reaction.

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.[3] The reaction takes place through an SN2 mechanism but is also often used in the description of SNi mechanisms.

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

CH3CH2OH + SOX2 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. 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.
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