An organic acid is an organic compound that is an acid. The most common example are the carboxylic acids whose acidity derives from a carboxyl group -COOH. Other groups can also cause a weak acidity: hydroxyl group -OH, -SH, enol group, -OSO₃H, phenol group. Each organic acid is formed by a an oxygen atom forming a double covalent bond with one -OH group. This carbon attaches to an oxygen and one other carbon. When organic acids are reacted with alcohols, they form esters.