p-Toluenesulfonic acid (PTSA) is an organic compound with the formula CH3C6H4SO3H. An aromatic sulfonic acid, often used as a strong acid catalyst. p-Toluenesulfonic acid monohydrate has been used as a reducing agent for the reductive amination of ketones and aldehydes. In the presence of p-Toluenesulfonic acid monohydrate novel deazaflavin-cholestane hybrid compounds have been synthesized in a condensation reaction. 2-Phenylethyl alpha-glucoside has also been synthesized in the presence of p-Toluenesulfonic acid monohydrate. p-toluenesulfonic acid esters, are a common class of reagents used in the pharmaceutical industry as alkylating agents, catalysts, and in purification steps of the chemical synthesis of a drug substance.