The reactivity of pyridine towards sulphuric acid at elevated temperatures

Abstract When pyridine is heated with 100% sulphuric acid in the presence of mercuric sulphate as a catalyst at about 275°, pyridine‐3‐sulphonic acid is formed as the chief product, together with very small amounts of byproducts. The course of the reaction is more complicated at somewhat higher reaction temperatures. At 330° a mixture is obtained which contains, together with pyridine‐3‐sulphonic acid and the 4‐isomer, 4‐hydroxypyridine in considerable amount. The sulphonation of pyridine is a reversible process. When pyridine‐3‐sulphonic acid is heated with 100% sulphuric acid and mercuric sulphate at ∼ 330°, a mixture is formed containing chiefly pyridine, together with pyridine‐4‐sulphonic acid, 4‐hydroxypyridine and the starting substance.