ABSTRACT
Quinoline and isoquinoline are stable; the former is a high-boiling liquid the latter a low-melting solid, each with a sweetish odour. The positional selectivity for proton exchange is partly mirrored in nitrations, quinoline gives approximately equal amounts of 5- and 8-nitroquinolines whereas isoquinoline produces almost exclusively the 5-nitro-isomer; mechanistically, the substitutions involve nitronium ion attack on the N-protonated heterocycles. Sulfonation of quinoline gives largely the 8-isomer whereas isoquinoline affords the 5-sulfonic acid. Reactions at higher temperatures produce other isomers, under thermodynamic control, for example both quinoline 8-sulfonic acid and quinoline 5-sulfonic acid are isomerised to the 6-acid. Introduction of halogen to the hetero-rings occurs under remarkably mild conditions in which the nitrogen lone pair initiates a sequence by interaction with an electrophile. The main principle here is that halogen on the homocyclic rings of quinoline and isoquinoline, and at the quinoline-3- and the isoquinoline-4-positions behave as would halobenzenes.
