QUINOLINE is a colourless, highly refractive oil, of spe cific gravity 1.095, and b.p. 239° C. It has a peculiar character istic smell and is only slightly soluble in water, but freely sol uble in the usual organic solvents. It is alkaline to litmus. As a mono-acid base it forms crystalline salts such as the hydrochloride, which are very soluble in water. Quinoline, C,H,N, occurs with its homologues in the fraction of coal tar and bone-oil bases boil ing between 230--250° C. It was first obtained from coal tar by Runge in 1834, and later Gerhardt showed that it was produced when cinchonine, quinine and other alkaloids were distilled with potash. Crude quinoline is obtained from coal tar and bone-oil by a process similar to the extraction of pyridine (q.v.), but since it is difficult to separate from its homologues, it is usually pre pared synthetically by a method devised by Skraup, in which a mixture of aniline, glycerine, nitrobenzene and concentrated sul phuric acid is heated until reaction proceeds without further heat ing. Since this reaction is often very violent, arsenic acid is some times used as an oxidizing agent in place of nitrobenzene, in which case the reaction proceeds more smoothly and gives a cleaner product. During the reaction the glycerine is converted into acrolein which condenses with the aniline to form acrolein aniline, and oxidation converts this to quinoline.
Being a tertiary amine, quinoline gives quinolinium or quater nary ammonium salts with alkyl iodides or methyl sulphate. It forms double salts with inorganic salts, e.g., the platinichloride When potassium dichromate is added to a solution of quinoline hydrochloride, the sparingly soluble quino line dichromate is formed. Nitric and chromic acids have no ac tion on quinoline, but alkaline permanganate oxidizes it to quinolinic acid (aj3-dicarboxypyridine).
Constitutionally, quinoline bears the same relation to naph thalene that pyridine bears to benzene, for it may be considered as a compound formed by the fusion of a benzene ring with a pyridine ring :— And quinoline differs from pyridine just as naphthalene differs from benzene in chemical reactivity. The formation of quinolinic
acid on oxidation shows the presence and stability of the pyridine ring in quinoline. The existence of the benzene ring follows from the Skraup synthesis. The pyridine nucleus takes up hydrogen more readily than the benzenoid part of the molecule : reduction with tin and hydrochloric acid yields a tetrahydroquinoline, whilst the decahydride is obtained with greater difficulty. Numerous substitution products of quinoline are known, and the position of any substituent in the molecule is designated in accordance with the scheme shown in the formula, the letters o, m, p, and a, standing for the terms ortho-, meta-, para-, and ana-.
Derivatives with the substituents in the benzene nucleus are prepared by the methods in use for the introduction of such groups into benzene and naphthalene ; or the substituted benzene derivative can be subjected to the quinoline synthesis given above. It is more difficult to introduce halogen, nitro- or similar groups into the pyridine nucleus.
a-Hydroxyquinoline can be obtained in 8o% yield by strongly heating quinoline with potash. The hydroxyquinolines are impor tant because of their relationship to the alkaloids. Those with the hydroxyl in the benzene nucleus are conveniently prepared from aminophenols.
a-Hydroxydihydroquinolines are obtained by the action of alkalis on the alkyl quinolinium iodides. The hydroxide or am monium base first formed isomerizes into the less ionized a-hy droxydihydroquinoline, which is a pseudo-base. These compounds are distinguished by a remarkable reactivity of the hydroxyl group, forming alcoholates and condensing readily with aniline, phenylhydrazine and hydroxylamine. This great reactivity, to gether with the capacity that the a- and -y-methylquinolines pos sess for condensing with aldehydes and similar compounds, is utilized in the preparation of the magnificent blue and red dyes of the cyanines, isocyanines and apocyanines. These dyes are used as sensitizers in photography for rendering the plate ortho chromatic.