Sulfonyl halide groups occur when a sulfonyl functional group is singly bonded to a halogen atom. They have the general formula RSO2X where X is a halogen. The stability of sulfonyl halides decreases in the order fluorides > chlorides > bromides > iodides, all four types being well known. The sulfonyl chlorides are of dominant importance in this series.
Sulfonyl halides have tetrahedral sulfur centres attached to two oxygen atoms, an organic radical, and a halide. In a representative example, methanesulfonyl chloride, the S=O, S−C, and S−Cl bond distances are respectively 1.424, 1.763, and 2.046 Å.
Sulfonic acid chlorides, or sulfonyl chlorides, are a sulfonyl halide with the general formula RSO2Cl. They are generally colourless compounds that are sensitive to water.
The intermediate benzenesulfonic acid can be chlorinated with thionyl chloride as well. Benzenesulfonyl chloride, the most important sulfonyl halide, can also be produced by treating sodium benzenesulfonate with phosphorus(V) chlorides.
- [C6H5N2]Cl + SO2 → C6H5SO2Cl + N2
For alkylsulfonyl chlorides, one synthetic procedure is the Reed reaction:
- RH + SO2 + Cl2 → RSO2Cl + HCl
The most obvious reaction is their tendency to hydrolyse to the corresponding sulfonic acid:
- C6H5SO2Cl + H2O → C6H5SO3H + HCl
These compounds react readily with nucleophiles other than water, like alcohols and amines (see Hinsberg reaction). If the nucleophile is an alcohol the product is a sulfonate ester, if it is an amine the product is a sulfonamide. Using sodium sulfite as the nucleophilic reagent, sulfonyl chlorides convert to the sulfinate salts, such as C6H5SO2Na.
- RSO2Cl + C6H6 → RSO2C6H5 + HCl
The desulfonation of arylsulfonyl chlorides provides a way to make aryl chlorides:
- ArSO2Cl → ArCl + SO2
1,2,4-Trichlorobenzene is prepared industrially in this way.
Treatment of alkanesulfonyl chlorides having α-hydrogens with amine bases can give sulfenes, highly unstable species that can be trapped:
- RCH2SO2Cl → RCH=SO2
Sulfonyl fluorides have the general formula RSO2F and can be important sulfonyl halide synthetic precursors. For example, "most, if not all" industrially synthesized perfluorooctanesulfonyl derivatives, such as PFOS, have the sulfonyl fluoride as their precursor. Sulfonyl fluorides are used in molecular biology as reactive probes. They specifically react with residues based on serine, threonine, tyrosine, lysine, cysteine and histidine. The fluorides are more resistant than the corresponding chlorides and are therefore better suited to this task  A facile GST fusion protein immobilization approach utilizing sulfonyl fluoride containing small molecule was recently developed.
Sulfonyl bromides have the general formula RSO2Br. In contrast to sulfonyl chlorides, sulfonyl bromides readily undergo light-induced homolysis affording sulfonyl radicals, which can add to alkenes, as illustrated by the use of bromomethanesulfonyl bromide, BrCH2SO2Br in Ramberg–Bäcklund reaction syntheses.
Sulfonyl iodides, having the general formula RSO2I, are quite light-sensitive. Perfluoroalkanesulfonyl iodides, prepared by reaction between silver perfluoroalkanesulfinates and iodine in dichloromethane at −30 °C, react with olefins to form the normal adducts, RFSO2CH2CHIR and the adducts resulting from loss of SO2, RFCH2CHIR. Arenesulfonyl iodides, prepared from reaction of arenesulfinates or arenehydrazides with iodine, can be used as initiators to facilitate the synthesis of poly(methyl methacrylate) containing C–I, C–Br and C–Cl chain ends.
In popular culture
In the episode "Encyclopedia Galactica" of his TV series Cosmos: A Personal Voyage, Carl Sagan speculates that some intelligent extraterrestrial beings might have a genetic code based on polyaromatic sulfonyl halides instead of DNA.
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