B.
4-Pyridinesulfonic acid. A
115-g. (0.50 mole) quantity of N-(4-pyridyl)pyridinium chloride hydrochloride is dissolved in 750 ml. of water in a 2-l. round-bottomed flask, and
378 g. (1.50 moles) of solid sodium sulfite heptahydrate is added cautiously. After the evolution of
sulfur dioxide has ceased, the solution is gently heated under reflux in a
nitrogen atmosphere for 24 hours. After slight cooling,
20 g. of charcoal is added to the mixture, and it is heated under reflux for an additional hour. The resultant mixture is filtered through a fluted paper, the filtrate is evaporated to dryness on a
steam bath under reduced pressure, and the residue is air-dried at 100–110° (Note
6). This solid is now continuously extracted with absolute
ethanol for 24 hours in a Soxhlet apparatus. The alcohol is distilled from the extract on a steam bath, and the crude
sodium 4-pyridinesulfonate is dissolved in about 160 ml. of hot water. After
320 ml. of 12N hydrochloric acid has been added with mixing, the solution is cooled to room temperature. The precipitate of
sodium chloride is filtered, and the filtrate is evaporated to dryness under reduced pressure on a steam bath. Crystallization of the residue from
600 ml. of 70% aqueous ethanol yields
27–30 g. of colorless crystals of
4-pyridinesulfonic acid, m.p.
313–315° (dec.). Concentration of the mother liquor affords about
10 g. of additional product which is less pure. The total yield is
36–40 g. (
45–50%) (Note
7). Recrystallization from
70% aqueous ethanol affords a purer specimen, m.p.
317–318° (dec.). (Note
8).