Phenylsulfinic acid

Acidity

Although many sources report the pK_a value as somewhere around 1.30, these results are inconsistent. However, a reproducible method was developed by Filippo et al. in which the pK_a was determined to be 2.764. This is a strong acid compared to its corresponding carboxylic acid, benzoic acid (pK_a = 4.2), but weak when compared to its corresponding sulfonic acid (pK_a = -6.5).

Preparation

Phenylsulfinic acid can be prepared in several ways, most easily through reduction of sulfonyl chlorides with zinc dust or iron. 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 +NaOH → 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:

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. 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.

Sulphinic acid derivatives disproportionate in the presence of acid:

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

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

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.