Sulfonic Acids - Properties

Properties

Sulfonic acids are much stronger acids than the corresponding carboxylic acids. p-Toluenesulfonic acid, with a pK_a of -2.8, is about a million times stronger acid than benzoic acid, with a pK_a of 4.2. Similarly, methanesulfonic acid, pK_a = -1.9, is also about one million times stronger acid than acetic acid. Because of their polarity, sulfonic acids tend to be crystalline solids. They are also usually colourless and nonoxidizing, which is convenient. Because of their high acidity, sulfonic acids are often soluble in water or exhibit detergent-like properties.

The structure of sulfonic acids is illustrated by the prototype, methanesulfonic acid. The sulfonic acid group, RSO₂OH features a tetrahedral sulfur centre, meaning that sulfur is at the center of four atoms: three oxygens and one carbon. The overall geometry of the sulfur centre is reminiscent of the shape of sulfuric acid.

• Representative sulfonic acids and derivatives.

• Taurine, a bile acid, and one of the few naturally occurring sulfonic acids (shown in uncommon tautomer).

• PFOS, a surfactant and a controversial pollutant.

• Toluenesulfonic acid, a widely used reagent in organic synthesis.

• Nafion, a polymeric sulfonic acid useful in fuel cells.

• Sodium dodecylbenzenesulfonate, an alkylbenzenesulfonate surfactant used in laundry detergents.