Sulfinic acids areoxoacids ofsulfur with the structure RSO(OH). In theseorganosulfur compounds, sulfur ispyramidal.[1]
Sulfinic acids RSO2H are typically more acidic than the correspondingcarboxylic acid RCO2H.[2][3] Although sulfur is pyramidal, sulfinic acids are achiral due to chemical exchange of the proton between the two oxygen atoms. The free acids are typically unstable, disproportionating to thesulfonic acid RSO3H andthiosulfonate RSSO2R.[4]: 679 The formal anhydride of a sulfinic acid has nooxygen 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 eithersulfones orsulfinate esters, depending on the solvent and reagent. Strongly polarized reactants (e.g.trimethyloxonium tetrafluoroborate) give esters, whereas relatively unpolarized reactants (e.g. analkyl halide orenone) give sulfones.[4]: 682 Sulfinates react withGrignard reagents to givesulfoxides, and undergo a variant of theClaisen condensation towards the same end.[4]: 686
Cobalt(III) salts can oxidize sulfinic acids todisulfones, although yields are only 30–50%.[7]
Sulfinic acids are often prepared in situ by acidification of the correspondingsulfinate salts, which are typically more robust than the acid. These salts are generated by reduction of sulfonyl chlorides with metals,[8] althoughthiolates also reduce thiosulfonates to a sulfinate and adisulfide.[4]: 681
An alternative route is the reaction ofGrignard reagents withsulfur dioxide. Transition metal sulfinates are also generated by insertion of sulfur dioxide into metal alkyls, a reaction that may proceed via ametal sulfur dioxide complex.
Sulfones may eliminate in base, particularly if a strongnucleophile is present; thus for examplesodium cyanide causes bis(2‑butanone-4‑yl) sulfone to split intolevulinonitrile and 3‑oxobutane 1‑sulfinic acid:[4]: 681
The nitrile presumably forms throughconjugate addition of cyanide to the correspondingenone.
Friedel-Crafts addition ofthionyl chloride to analkene gives an α‑chloro sulfinyl chloride, typically complexed to a Lewis acid. Likewise acarbanion 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 ofsulfoxylic acid, both of which are unstable.
An example of a simple, well-studied sulfinic acid isphenylsulfinic acid. A commercially important sulfinic acid isthiourea dioxide, which is prepared by theoxidation of thiourea withhydrogen peroxide.[4]
Another commercially important sulfinic acid is hydroxymethyl sulfinic acid, which is usually employed as its sodium salt (HOCH2SO2Na). CalledRongalite, this anion is also commercially useful as a reducing agent.
Theconjugate base of a sulfinic acid is asulfinate anion. The enzymecysteine dioxygenase convertscysteine into the corresponding sulfinate. One product of this catabolic reaction is the sulfinic acidhypotaurine. Sulfinite also describes esters of sulfinic acid. Cyclic sulfinite esters are calledsultines.
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