1911 Encyclopædia Britannica/Cinnolin
CINNOLIN, C8H6N2, a compound isomeric with phthalazine, prepared by boiling dihydrocinnolin dissolved in benzene with freshly precipitated mercuric oxide. The solution is filtered and the hydrochloride of the base precipitated by alcoholic hydrochloric acid; the free base is obtained as an oil by adding caustic soda. It may be obtained in white silky needles, melting at 24-25°C. and containing a molecule of ether of crystallization by cooling the oil dissolved in ether. The free base melts at 39°C. It is a strong base, forming stable salts with mineral acids, and is easily soluble in water and in the ordinary organic solvents. It has a taste resembling that of chloral hydrate, and leaves a sharp irritation for some time on the tongue; it is also very poisonous (M. Busch and A. Rast, Berichte, 1897, 30, p. 521). Cinnolin derivatives are obtained from oxycinnolin carboxylic acid, which is formed by digesting orthophenyl propiolic acid diazo chloride with water. Oxycinnolin carboxylic acid on heating gives oxycinnolin, melting at 225°, which with phosphorus pentachloride gives chlorcinnolin. This substance is reduced by iron filings and sulphuric acid to dihydrocinnolin.