Sulfonic acids are similar to carboxylic acids and have the general structure of RSO3H. Sulfonic acids are very strong acids (pKa ~ -7). The most common sulfonic acid is p-toluenesulfonic acid.
The conjugate bases of sulfonic acids are called sulfonate anions and are resonance stabilized. Consequently, sulfonate anions make good leaving groups.
The sulfonate anion has three major resonance structures.