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Sulfinic acids are of sulfur with the structure RSO(OH). In these organosulfur compounds, sulfur is pyramidal. (1981). 9780470772270, John Wiley & Sons. ISBN 9780470772270
Alkylation of sulfinic acids can give either 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. Sulfinates react with to give , and undergo a variant of the Claisen condensation towards the same end.
Cobalt(III) salts can oxidize sulfinic acids to , although yields are only 30–50%.
An alternative route is the reaction of 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.
may eliminate in base, particularly if a strong nucleophile is present; thus for example sodium cyanide causes bis(2butanone-4yl) sulfone to split into levulinic acid and 3oxobutane 1sulfinic acid:
Friedel-Crafts addition of thionyl chloride to an alkene gives an αchloro 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. Sulfinyl chlorides attack sulfinates to give sulfinyl sulfones (sulfinic anhydrides).
Unsubstituted sulfinic acid, when R is the hydrogen atom, is a higher energy isomer of sulfoxylic acid, both of which are unstable.
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.
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, although also reduce thiosulfonates to a sulfinate and a disulfide.
The nitrile presumably forms through conjugate addition of cyanide to the corresponding enone.
Examples
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.D. Schubart "Sulfinic Acids and Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH, Weinheim.
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 .
See also
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