Quinoline and Isoquinoline

Quinoline and isoquinoline are benzopyridines. They behave by showing the reactivity associated with either the benzene or the pyridine rings. 

Quinoline is basic with a p Ta of 4.9, similar to that of pyridine (pATa 5.2). As with pyridine, 

Quinoline is much more reactive towards electrophilic substitution than pyridine, but this is because substitution occurs on the benzene ring, not on the pyridine. We have already seen that pyridine carbons are unreactive towards electrophilic reagents, with strongly acidic systems protonating the nitrogen 

This may be rationalized by considering the stability of intermediate addition cations. When the electrophile attacks at C-5 or C-8, the intermediate cation is stabilized by resonance, each having two favourable forms that do not perturb the aromaticity of the pyridinium system. In contrast, for attack at C-6 or C-7 there is only one such resonance form. We used similar reasoning to explain why naphthalene 

Nucleophilic substitution occurs at C-2, and to a lesser extent C-4, as might be predicted from similar reactions with pyridine. Chichibabin amination occurs rather more readily than with pyridine, giving 2-aminoquinoline. A typical hydride abstraction process occurs when qninoline is heated with sodinm 

Quinoline was discovered in coal tar by Friedlieb Ferdinand Runge in 1834 it is present in concentrations of approximately 0.3%. Quinoline is recovered by extraction with sulfuric acid from the methylnaphthalene fraction of coal tar, followed by springing with ammonia and rectification of the crude base mixture. Quinoline can be synthesized by the Skraup method, by the reaction of aniline with glycerol (or acrolein produced from glycerol) and catalytic gas-phase reaction of aniline with acetaldehyde. Since the supply of the tar-derived material has been adequate for a long time, synthetic production is not warrented. 

The main application for quinoline is the production of 8-hydroxyquinoline (oxine), which is obtained by alkali fusion of quinoline-8-sulfonic acid. 

Sulfonation of quinoline to produce quinoline-8-sulfonic acid is carried out at 180 to 200 °C. Alkali fusion is performed at 250 °C and 45 bar final purification of the crude 8-hydroxyquinoline is effected by distillation. 

Apart from the production of 8-hydroxyquinoline (and its copper salt), which is mainly used as a fungicide and disinfectant, quinoline serves as a feedstock for production of pyridine-2,3-dicarboxylic acid (quinolinic acid), by oxidation. Quinolinic acid is used to manufacture the plant protection agent imazapyr (American Cyanamid). 

Condensation of aniline with ketones, such as acetone or methyl isobutyl ketone, produces trimethyl-1,2-dihydroquinolines, such as 2,2,4-trimethyl-l,2-di-hydroquinoline, which is used as an important anti-oxidant in rubber processing. West European production is around 10,000 tpa. 

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The first quantitative studies of the nitration of quinoline, isoquinoline, and cinnoline were made by Dewar and Maitlis, who measured isomer proportions and also, by competition, the relative rates of nitration of quinoline and isoquinoline (1 24-5). Subsequently, extensive kinetic studies were reported for all three of these heterocycles and their methyl quaternary derivatives (table 10.3). The usual criteria established that over the range 77-99 % sulphuric acid at 25 °C quinoline reacts as its cation (i), and the same is true for isoquinoline in 71-84% sulphuric acid at 25 °C and 67-73 % sulphuric acid at 80 °C ( 8.2 tables 8.1, 8.3). Cinnoline reacts as the 2-cinnolinium cation (nia) in 76-83% sulphuric acid at 80 °C (see table 8.1). All of these cations are strongly deactivated. Approximate partial rate factors of /j = 9-ox io and /g = i-o X io have been estimated for isoquinolinium. The unproto-nated nitrogen atom of the 2-cinnolinium (ina) and 2-methylcinno-linium (iiiA) cations causes them to react 287 and 200 more slowly than the related 2-isoquinolinium (iia) and 2-methylisoquinolinium (iii)... 

Unlike quinoline and isoquinoline which are of comparable stability the compounds indole and isoindole are quite different from each other Which one is more stable Explain the reason for your choice... 

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