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.

Phenylsulfinic acid
Structural formula of phenylsulfinic acid
Ball-and-stick model of the phenylsulfinic acid molecule
Names
Preferred IUPAC name
Benzenesulfinic acid
Other names
  • Phenylsulfinic acid
  • Phenyl sulfinic acid
  • Benzene sulfinic acid
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.009.591 Edit this at Wikidata
UNII
  • InChI=1S/C6H6O2S/c7-9(8)6-4-2-1-3-5-6/h1-5H,(H,7,8) ☒N
    Key: JEHKKBHWRAXMCH-UHFFFAOYSA-N ☒N
  • InChI=1/C6H6O2S/c7-9(8)6-4-2-1-3-5-6/h1-5H,(H,7,8)
    Key: JEHKKBHWRAXMCH-UHFFFAOYAY
  • C1=CC=C(C=C1)S(=O)O
Properties
C6H6O2S
Molar mass 142.17 g·mol−1
Appearance Colorless prisms
Density 1.45 g/cm3
Melting point 83 to 84 °C (181 to 183 °F; 356 to 357 K)
Acidity (pKa) 2.76 (H2O)[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Acidity

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There is a large range of pKa values in the literature, with most authors giving a value of around 1.30.[2][3][4] This inconsistency can be explained by differences in ionic strength between quoted values. By measuring the pKa at various ionic strengths and extrapolating to zero ionic strength, the pKa of phenylsulfinic acid was determined to be 2.76.[1] This makes phenylsulfinic acid a stronger acid than its corresponding carboxylic acid, benzoic acid (pKa = 4.2), but weaker than its corresponding sulfonic acid, benzenesulfonic acid (pKa = −6.5).[5]

Preparation

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Phenylsulfinic acid can be prepared in several ways, most easily through reduction of sulfonyl chlorides with zinc dust or iron.[2] 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:[3]

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.[4] The preparation of sulfinic acids by the oxidation of thiols is difficult due to overoxidation.

Properties

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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.[2]

Sulphinic acid derivatives disproportionate in the presence of acid:[2]

2 PhSO2H → PhSO2SOPh + H2O
PhSO2SOPh → PhSO2• + PhSO → PhSO3SPh
PhSO3SPh + PhSO2H → PhSO3H + PhSO2SPh

When phenylsulfinic acid reacts with sulfur to give thiosulfinates and thiosulfinic acids.[6]

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.[7]

References

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  1. ^ a b De Filippo, D.; Momicchioli, F. (1969). "A study of benzenesulfinic and seleninic acids". Tetrahedron. 25 (23): 5733. doi:10.1016/S0040-4020(01)83080-5.
  2. ^ a b c d S. Patai (1990). The Chemistry of Sulphinic Acids, Esters and Their Derivatives. New York: J. Wiley and Sons. ISBN 0-471-91918-7.
  3. ^ a b 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).
  4. ^ a b R. J. Cremlyn (1996). An Introduction to Organosulfur Chemistry. New York: J. Wiley and Sons. ISBN 0-471-95512-4.
  5. ^ E. P. Serjeant, B. Dempsey. "Ionization Constants of Organic Acids in Solution" IUPAC Data, Series No. 23 (Pergamon Press, Oxford)
  6. ^ B. Zwanenburg; A.J. H. Klunder (1987). Perspectives in the Organic Chemistry of Sulfur. New York: Elsevier. ISBN 0-444-42739-2.
  7. ^ Chiang, Yunn Hui; Luloff, Jerome S.; Schipper, Edgar (1969). "Aminolyses of sulfinic acid derivatives". The Journal of Organic Chemistry. 34 (8): 2397. doi:10.1021/jo01260a031.