Sulfonic acids are organic sulfur compounds with the general structure R-SO2-OH, where R is an organic radical. Their salts and esters with the general structure R-SO2-O-X are called sulphonates, where X can be an organic radical or a cation.
In contrast to the esters of sulphuric acid, sulphonates have R-S and not R-O-S bonds as sulphates do. They also differ from the sulfones (R-SO2-R'), which do not have a hydroxyl group. Sulphonic acids are strong acids. Acids and sulphonates are usually water-soluble or easily miscible with water.
Sulfonic acids in which the residual group R is aliphatic are called alkanesulfonic acids; their salts and esters are called alkylsulfonates or also alkanesulfonates.