Quinolines are the class of organic compound of the heteroaromatic series characterized through a double-ring structure comprised of benzene and a pyridine ring fused at two adjacent carbon atoms (figure shown below). The benzene ring includes six carbon atoms, whereas the pyridine ring includes five carbon atoms and a nitrogen atom (compare by the structure of Naphthalene below). The simplest member of the quinoline family is quinoline itself, a compound having molecular structure C9H7N.
Fig: Structures of Quinoline and Naphthalene (Hydrocarbon Analogue)
General Physical and Chemical Properties of Quinoline:
Quinoline is a colourless hygroscopic liquid having a strong odor. It becomes yellow on exposure to light and afterward turns brown. Quinoline is only slightly soluble in cold water however dissolves readily in hot water and most organic solvents. Quinoline is a slightly weaker base (pKa 4.9) as compare to pyridine (pKa 5.2).
The general properties are mainly based on those of the individual ring structures making that system. This can be predicted that the benzene ring in quinoline would experience electrophilic substitution by electrophiles however be resistant to oxidation and reduction even as the pyridine ring would act as a base, experience nucleophilic substitution and reduction however be resistant to oxidation and electrophilic substitution as electrophilic substitution of the benzene ring is easier. Moreover, by comparison by its aromatic analogue naphthalene, one would expect position 3 to be comparatively unreactive.
In practice, quinoline is a weak base (pKa 4.9). It generally undergoes electrophilic substitution at positions 5 and 8 of the benzene ring. Nucleophilic substitution takes place mostly at positions 2 and 4 of the pyridine ring. Reduction of the pyridine ring is fairly easy however reduction of the benzene ring is comparatively difficult. The oxidation of both rings is difficult.
Fig: General types of Reaction of Quinoline
Acids and Lewis acids react by quinoline at the basic nitrogen atom to form quinolinium salts, and there is a question over the nature of the substrate for electrophilic attack, that is, is it quinoline or the quinolinium substrate for electrophilic attack. The answer is not a simple one and appears to depend on the reagents and reaction conditions. Therefore while acetyl nitrate at 20ºC provides 3-nitroquinoline, fuming nitric acid in concentrated sulphuric acid having sulphur trioxide at 15 to 20ºC results a mixture of 5-nitroquinoline (35%) and 8-nitroquinoline (43% - Scheme 3). In case of acetyl nitrate, the reaction might carry on by the 1,4-addition of the reagent to quinoline, followed via electrophilic attack on the 1,4-dihydro derivative.
Fig: Reaction of Quinoline with Acetyl Nitrate
Fig: Reaction of Quinoline with Fuming Nitric Acid
Though, the rate of nitration of quinoline in 80 to 99% sulphuric acid is of the similar order as that of N-methylquinolinium salts, recommending that here the quinolinium cation might be the target for attack.
Sulphonation with oleum at approx 90ºC affords mostly the 8-sulphonic acid, however as this product is sterically hindered, at higher temperatures it rearranges to the 6-sulphonic acid (figure above). This rearrangement is identical to that illustrated by naphthalene-1-sulphonic acid, the kinetic sulphonation product of naphthalene that isomerizes on heating to the thermodynamically favored (less hindered) 2-isomer.
Alkyl and acyl halides react directly by quinoline to provide N-alkyl or N-acylquinoliniumm salts (figure shown below), while the N-alkyl salts are stable and can often be isolated as crystalline solids; the N-acyl analogues are unstable and experience rapid hydrolysis in moist air or in the aqueous solution.
Fig: Reactions of Quinoline with Acyl and Alkyl Halides
N-Acyl- or N-sulphonylquinolinium salts can be trapped via cyanide ion to form what are known usually as Reissert adducts. The simple elimination of the N-substituent in a subsequent reaction with a base gives access to 2-cyanoquinoline.
Fig: Reaction of Quinoline with Potassium Cyanide
There is a strong similarity between the reactions of the pyridines and quinolines towards nucleophiles. Addition takes place mostly at C-2 giving 1,2-dihydroquinolines, however the locus of the reaction can be diverted to C-4, specifically if there is a good leaving group placed at this position.
In a Chichibabin-kind reaction quinoline reacts by potassamide (KNH2) in liquid ammonia at 70ºC to provide 2-amino-1,2-dihydroquinoline and this is oxidized by potassium permanganate [manganate(VII)] at similar temperature to result 2-aminoquinoline (figure shown below). Whenever the temperature is allowed to increase to -45ºC the adduct rearranges to 4-amino-3,4-dihydroquinoline and on oxidation this product provides 4-aminoquinoline.
Fig: Reaction of Quinoline with Potassamide
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