Sulfinic acid

Short description: Class of chemical compounds

The general structure of a sulfinic acid

Sulfinic acids are oxoacids of sulfur with the structure RSO(OH). In these organosulfur compounds, sulfur is pyramidal.[1]

## Contents

* 1 Structure and properties
* 2 Preparation
* 3 Examples
* 4 Sulfinates
* 5 See also
* 6 References
* 7 External links

## Structure and properties

Sulfinic acids RSO2H are typically more acidic than the corresponding carboxylic acid RCO2H.[2][3] Sulfur is pyramidal, consequently sulfinic acids are chiral. The free acids are typically unstable, disproportionating to the sulfonic acid RSO3H and thiosulfonate RSSO2R.[4]: 679 The formal anhydride of a sulfinic acid has no oxygen atom bridge, but is instead a sulfinyl sulfone (R-S+(-O−)-S2+(-O−)2-R′),[5] and disproportionation is believed to occur through the free-radical fission of this intermediate.[6]

Alkylation of sulfinic acids can give either sulfones or sulfinate esters, depending on the solvent and reagent. Strongly polarized reactants (e.g. trimethyloxonium tetrafluoroborate) give esters, whereas relatively unpolarized reactants (e.g. an alkyl halide or enone) give sulfones.[4]: 682 Sulfinates react with Grignard reagents to give sulfoxides, and undergo a variant of the Claisen condensation towards the same end.[4]: 686

Cobalt(III) salts can oxidize sulfinic acids to disulfones, although yields are only 30-50%.[7]

## Preparation

Sulfinic acids are often prepared in situ by acidification of the corresponding sulfinate salts, which are typically more robust than the acid. These salts are generated by reduction of sulfonyl chlorides with metals,[8] although thiolates also reduce thiosulfonates to a sulfinate and a disulfide.[4]: 681

An alternative route is the reaction of Grignard reagents with sulfur dioxide. Transition metal sulfinates are also generated by insertion of sulfur dioxide into metal alkyls, a reaction that may proceed via a metal sulfur dioxide complex.

Sulfones may eliminate in base, particularly if a strong nucleophile is present; thus for example sodium cyanide causes bis(2‑butanone-4‑yl) sulfone to split into levulinonitrile and 3‑oxobutane 1‑sulfinic acid:[4]: 681

SO2((CH2)2Ac)2 \+ NaCN → NaSO2(CH2)2Ac \+ NC(CH2)2Ac

The nitrile presumably forms through conjugate addition of cyanide to the corresponding enone.

Friedel-Crafts addition of thionyl chloride to an alkene gives an α‑chloro sulfinyl chloride, typically complexed to a Lewis acid. Likewise a carbanion can attack thionyl chloride to give a sulfinyl chloride. Careful hydrolysis then gives a sulfinic acid.[4]: 682, 684 Sulfinyl chlorides attack sulfinates to give sulfinyl sulfones (sulfinic anhydrides).[5]

Unsubstituted sulfinic acid, when R is the hydrogen atom, is a higher energy isomer of sulfoxylic acid, both of which are unstable.

## Examples

Sulfinic Acids and Derivatives

* thiourea dioxide, a reducing agent used in textiles

* hypotaurine, a biosynthetic intermediate

* Rongalite, a source of "SO22−"

An example of a simple, well-studied sulfinic acid is phenylsulfinic acid. A commercially important sulfinic acid is thiourea dioxide, which is prepared by the oxidation of thiourea with hydrogen peroxide.[4]

(NH2)2CS + 2H2O2 → (NH)(NH2)CSO2H + 2H2O

Another commercially important sulfinic acid is hydroxymethyl sulfinic acid, which is usually employed as its sodium salt (HOCH2SO2Na). Called Rongalite, this anion is also commercially useful as a reducing agent.

## Sulfinates

The conjugate base of a sulfinic acid is a sulfinate anion. The enzyme cysteine dioxygenase converts cysteine into the corresponding sulfinate. One product of this catabolic reaction is the sulfinic acid hypotaurine. Sulfinite also describes esters of sulfinic acid. Cyclic sulfinite esters are called sultines.

## See also

* Sulfonic acid
* Carboxylic acid
* Sulfurous acid

## References

1. ↑ Saul Patai, ed (1981). Sulphinic Acids, Esters and Derivatives. PATAI'S Chemistry of Functional Groups. John Wiley & Sons. doi:10.1002/9780470772270. ISBN 9780470772270.
2. ↑ Ballatore, Carlo; Huryn, Donna M.; Smith III, Amos B. (2013). "Carboxylic Acid (Bio)Isosteres in Drug Design". ChemMedChem 8 (3): 385-395. doi:10.1002/cmdc.201200585.
3. ↑ Ripin, D.H.; Evans, D.A.. "Chem 206: pKa's of Inorganic and Oxo-Acids". http://ccc.chem.pitt.edu/wipf/MechOMs/evans_pKa_table.pdf.
4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 D. Schubart "Sulfinic Acids and Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH, Weinheim. doi:10.1002/14356007.a25_461
5. ↑ 5.0 5.1 Bredereck, Hellmut; Wagner, Adolf; Beck, Heinz; Klein, Rainer-Joachim (1960). "Die Struktur der Sulfinsäureanhydride". Chemische Berichte 93 (11): 2736–2742. doi:10.1002/cber.19600931145.
6. ↑ Kice, John L.; Guaraldi, Giancarlo; Venier, Clifford G. (Nov 1966). "The Mechanism of the Disproportionation of Sulfinic Acids. Rate and Equilibrium Constants for the Sulfinic Acid-Sulfinyl Sulfone (Sulfinic Anhydride) Equilibrium" (in en). The Journal of Organic Chemistry 31 (11): 3561–3567. doi:10.1021/jo01349a021. ISSN 0022-3263. https://pubs.acs.org/doi/abs/10.1021/jo01349a021.
7. ↑ Denzer, George C., Jr.; Allen, Paul, Jr.; Conway, Patrick; van der Veen, James M. (Oct 1966). "The preparation of α-disulfones by cobalt(III) oxidation". J. Org. Chem. 31 (10): 3418–3419. doi:10.1021/jo01348a514.
8. ↑ Whitmore, F. C.; Hamilton, F. H. (1922). "Sodium p-Toluenesulfinic Acid". Organic Syntheses 2: 89. doi:10.15227/orgsyn.002.0089.

## External links

* Sulfinic+Acids at the US National Library of Medicine Medical Subject Headings (MeSH)
* Diagram at ucalgary.ca
* Diagram at acdlabs.com

* v
* t
* e

Functional groups

| Only carbon,
hydrogen,
and oxygen|

| Hydrocarbons|

* Allene
* Alkene (Allyl group

|

Other|

* Acetoxy
* Acetyl
* Acryloyl
* Acyl
* Aldehyde
* Alkoxy (Methoxy)
* Benzoyl
* Carbonyl
* Carboxyl
* Carboxylic anhydride
* Dioxirane
* Epoxide
* Ester
* Ether
* Ethylenedioxy
* Hydroxy
* Ketone
* Methylenedioxy
* Peroxide (Organic)
* Ynone

Only one
element
apart from
C, H, O|

| Nitrogen|

* Amine
* Azo compound
* Cyanate
* Hydrazone
* Imide
* Imine
* Isocyanate
* Isonitrile
* Nitrene
* Nitrile
* Nitro compound
* Nitroso compound
* Organic amide
* Oxime

|

Phosphorus|

* Phosphonate
* Phosphonous

Sulfur|

* Disulfide
* Sulfone
* Sulfonic acid
* Sulfoxide
* Thial
* Thioester
* Thioether
* Thioketone
* Thiol

Selenium|

* Selenol
* Selenonic acid
* Seleninic acid
* Selenenic acid
* Selone

Tellurium|

* Tellurol
* Telluroketone

Other|

* Isothiocyanate
* Phosphoramide
* Sulfenyl chloride
* Sulfonamide
* Thiocyanate

See also chemical classification, chemical nomenclature ([[Chemistry:IUPAC nomenclature of inorganic, IUPAC nomenclature of organic chemistry|organic]])

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