Sulfinic acid

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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.

They 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.[1] 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. Unsubstituted sulfinic acid, when R is the hydrogen atom, is a higher energy isomer of sulfoxylic acid, both of which are unstable.


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.[2]

(NH2)2CS + 2H2O2 → (NH)(NH2)CSO2H + 2H2O
Structure of "thiourea dioxide", a commercially useful sulfinic acid.

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.


The conjugate base of a sulfinic acid is a sulfinite anion. The enzyme cysteine dioxygenase converts cysteine into the corresponding sulfinite. One product of this catabolic reaction is the sulfinic acid derivative hypotaurine.


  1. ^ Whitmore, F. C.; Hamilton, F. H. (1941). "Sodium p-Toluenesulfinic acid". Org. Synth. ; Coll. Vol., 1, p. 492 
  2. ^ D. Schubart "Sulfinic Acids and Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH, Weinheim. doi:10.1002/14356007.a25_461

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