Phenylsulfinic acid explained

Phenylsulfinic acid is an organosulfur compound with the formula C₆H₅SO₂H. It is a colorless or white crystalline solid that is usually stored in the form of its sodium salt. In aqueous solution it is strongly acidic and is easily oxidized in air. Phenylsulfinic acid and its esters are chiral.

Acidity

There is a large range of pK_a values in the literature, with most authors giving a value of around 1.30.^{[1] [2] [3]} This inconsistency can be explained by differences in ionic strength between quoted values. By measuring the pK_a at various ionic strengths and extrapolating to zero ionic strength, the pK_a of phenylsulfinic acid was determined to be 2.76. This makes phenylsulfinic acid a stronger acid than its corresponding carboxylic acid, benzoic acid (pK_a = 4.2), but weaker than its corresponding sulfonic acid, benzenesulfonic acid (pK_a = -6.5).^[4]

Preparation

Phenylsulfinic acid can be prepared in several ways, most easily through reduction of sulfonyl chlorides with zinc dust or iron.^[1] However other starting materials can be used. Due to the air sensitivity of this compound it is often formed as a salt.

2 C₆H₅SO₂Cl + 2 Zn → (C₆H₅SO₂)₂Zn + ZnCl₂

(C₆H₅SO₂)₂Zn + Na₂CO₃ → 2 C₆H₅SO₂Na + ZnCO₃

A convenient method is the reduction of the sulfonyl chloride or sulfonyl fluoride with sodium sulfite, producing the acid instead of a salt:^[2]

C₆H₅SO₂Cl + Na₂SO₃ + H₂O → C₆H₅SO₂H + NaCl + NaHSO₄

Many other methods have been reported for production of sulfinic acids such as the use tin(II) chloride, or the Grignard reagent with sulfur dioxide.^[3] The preparation of sulfinic acids by the oxidation of thiols is difficult due to overoxidation.

Properties

In sulfinic acids, sulfur has the +4 oxidation state. They are prone to oxidation to sulphonic acids as well as reduction via sulphenic acids (+2) to thiols.^[1]

Sulphinic acid derivatives disproportionate in the presence of acid:^[1]

2 PhSO₂H → PhSO₂SOPh + H₂O

PhSO₂SOPh → PhSO₂• + PhSO → PhSO₃SPh

PhSO₃SPh + PhSO2H → PhSO₃H + PhSO₂SPh

When phenylsulfinic acid reacts with sulfur to give thiosulfinates and thiosulfinic acids.^[5]

Use

The main use of phenylsulfinic acid is for the asymmetric synthesis of carbon-carbon bonds due to its ability to stabilize negative charges on an adjacent carbon atom. Phenylsulfinic acid has been a component for electroplating of palladium alloys.^[6]

Notes and References

1. Book: S. Patai . The Chemistry of Sulphinic Acids, Esters and Their Derivatives . J. Wiley and Sons . 1990 . New York . 0-471-91918-7.
2. a) A. T. Fuller, I. M. Tonkin and J. Walker, J. Chem. Soc., 1945, 636; b) S. Smiles and C. M. Bere, "Organic Syntheses," Coll. Vol. I, ed. by A. H. Blatt, John Wiley and Sons, Inc., New York, 1948, p. 7; c) E. Bader and H. D. Hermann, Chem. Ber., 88, 46 (1955); d) M. Kulka, Can. J. Chem., 32, 601 (1954).
3. Book: R. J. Cremlyn . An Introduction to Organosulfur Chemistry. J. Wiley and Sons . 1996 . New York . 0-471-95512-4.
4. E. P. Serjeant, B. Dempsey. "Ionization Constants of Organic Acids in Solution" IUPAC Data, Series No. 23 (Pergamon Press, Oxford)
5. Book: B. Zwanenburg . A.J. H. Klunder . Perspectives in the Organic Chemistry of Sulfur . Elsevier . 1987 . New York . 0-444-42739-2.
6. 10.1021/jo01260a031. Aminolyses of sulfinic acid derivatives. The Journal of Organic Chemistry. 34. 8. 2397. 1969. Chiang. Yunn Hui. Luloff. Jerome S.. Schipper. Edgar.