Patent GB-721905-A (GB)
Title: Improvements in process for preparing cyanopyridines
Filed: 1952-04-10
Abstract: Cyanopyridines are prepared by heating an ammonium salt or acid amide of a pyridine carboxylic acid, the acid being one that is not decarboxylated at the temperature employed in the molten state at an elevated temperature in the presence of an anionic catalyst which is defined as meaning non-oxidizing mineral acids and their salts, other than those which catalyse complete decarboxylation of pyridine dicarboxylic acids at temperatures suitable for use in the present process, and sulphamic acid.  Thus, boric acid and silicic acid and sulphuric acid, hydrochloric acid and phosphoric acid and their ammonium, organic base, alkali metal, alkaline earth metal and certain of their heavy metal salts, e.g. diammonium hydrogen phosphate and ferric sulphate, may be used as catalysts.  In general, the pyridine carboxylic acid is mixed with sufficient aqueous ammonia to form the ammonium salt, whereupon the mixture is heated with stirring to distil off water and meet the salt, whereupon the catalyst is added, preferably in amount not substantially in excess of 10 per cent of the weight of pyridine carboxylic acid.  The stirred mixture is then further heated and the water and nitrile, formed at about 270 DEG  C., are distilled off; the distillate is extracted with a suitable solvent, e.g. benzene, to recover the nitrile.  The nitrile may be formed directly from the ammonium salt or acid amide of the acid, in which case the initial ammonia treatment is not necessary. When a pyridine dicarboxylic acid is used which is decarboxylated to the mono-carboxylic acid on heating in the molten state, then this decarboxylation is effected initially, e.g. at about 200 DEG  C., the ammonia treatment preferably being made after evolution of carbon dioxide has ceased and the addition of catalyst is made thereafter; in this manner, nicotinonitrile is obtained from quinoline, cinchomeronic and isocinchomeronic acids. Other dibasic acids yield dicyano-pyridines.  The process may be used in purifying very crude niacin, its ammonium salt or its amide, the formed nitrile being distilled from a residue of impurities, and hydrolysed to the amide as described in U.S.A. Specification 2,471,518 or to the acid.  Crude isonicotinic acid or acid amide may be converted to the nitrile as above and the nitrile hydrolysed to yield pure isonicotinic acid.  In typical examples: (1) nicotinic acid in aqueous ammonia is heated to remove water and then ammonia gas passed into the resulting melt; thereafter, diammonium hydrogen phosphate, sulphuric acid or ferric sulphate is added as catalyst and nicotinonitrile distilled off at 275-300 DEG  C.; (2) a mixture of crude nicotinic and isocinchomeronic acids is heated to 200 DEG  C. until evolution of CO2 ceases, ammonia is then passed into the melt which is finally heated as in (1) in presence of diammonium hydrogen phosphate; crude quinolinic acid is similarly treated; (3) nicotinamide is heated to 270-285 DEG  C. in presence of zinc, nickel, calcium, sodium or nicotinic acid sulphate or ammonium chloride; (4) a crude mixture of picolinamide, nicotinamide and isonicotinamide from the mother liquor obtained in manufacture of nicotinamide by the sulphonation of pyridine, is heated at 270-300 DEG  C. in presence of diammonium hydrogen phosphate and the resulting distillate of mixed nitriles is fractionated to isolate the isomeric nitriles; (5) isonicotinamide is heated as in (4) to yield isonicotinonitrile.