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

Although many sources report the pk_a value as somewhere around 1.30 these results are inconsistent. However a reproducable method was developed by Filippo et al. in which the pka 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 +ZnCl₂₂Zn + NaCO₃ +NaOH → 2 C₆H₅SO₂Na + ZnCO₃

A convienient method is the reduction of the sulfonyl chloride or sulfonyl fluoride with sodium sulfite

Sodium sulfite

Sodium sulfite is a soluble sodium salt of sulfurous acid. It is a product of sulfur dioxide scrubbing, a part of the flue gas desulfurization process...

, producing the acid instead of a salt:

C₆H₅SO₂Cl + NaSO₃ + 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. Thus, they are prone to oxidization to sulphonic acids as well as reduction via sulphenic acids (+2) to thiols.

Sulphinic acid derivatives disproportionate

Disproportionation

Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....

 in the presence of acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

:

2PhSO₂H → 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 have been components for electroplating of palladium alloys.