Numerous substitution products of quinoline are known, and the positions in the molecule are generally designated in accordance with the scheme shown in the inset formula: the letters o, m, p, a, standing for ortho-, meta-, Para-, and ana-.
Bamberger's observations on reduced quinoline derivatives point to the same conclusion, that condensed nuclei are not benzenoid, but possess an individual character, which breaks down, however, when the molecule is reduced.
Similarly a CH group may be replaced by a nitrogen atom with the production of compounds of similar stability; thus benzene gives pyridine, naphthalene gives quinoline and isoquinoline; anthracene gives acridine and a and 3 anthrapyridines.
This view has been fairly well supported by later discoveries; but, in addition to pyridine and quinoline nuclei, alkaloids derived from isoquinoline are known.
It may be separated from the quinoline which accompanies it by means of the difference in the solubility of the sulphates of the two compounds, isoquinoline sulphate being much less soluble than quinoline sulphate.
His perception of the analogy between it and ammonia led to his famous work on the amines and ammonium bases and the allied organic phosphorus compounds, while his researches on rosaniline, which he first prepared in 1858, formed the first of a series of investigations on colouring matters which only ended with quinoline red in 1887.
Naphthyridines and naphthinolines result from the condensation of two pryridine and two quinoline nuclei respectively; and quino-quinolines are unsymmetrical naphthyridine nuclei condensed with a benzene nucleus.
The reaction is a perfectly general one, for the aniline may be replaced by other aromatic amines and the aldehyde by other aldehydes, and so a large number of quinoline homologues may be prepared in this way.
Limpach, Ber., 1887, 20, P 947) Numerous carboxylic acids of quinoline are known, the most important of which are quinaldic, cinchoninic and acridinic acids.
The oxy derivatives of the quinoline homologues are best obtained from the aniline derivatives of (3-ketonic acids.