Unit 4 Pharmaceutical Organic Chemisty 3 Quinoline

Quinoline

1. “
”
UNIT -4
HETEROCYCLIC COMPOUNDS

2. QUINOLINE
Quinoline is a heterocyclic aromatic organic
compound in which benzene ring is fused to a
pyridine ring at alpha and beta position .
Hence quinoline is also known as α,β
Benzopyridine.
It has been named, so because it was obtained from
distillation of anti-malarial drug quinine with alkyl.

3. STRUCTURE

4. STRUCTURE
Quinoline contains 9 carbons and 1 nitrogen
atom which undergoes SP2 hybridization .
Overlap of these SP2 hybridized orbitals with
each other and with S-orbital of hydrogen atom
results in the formation of C-C , C-N , C-H σbonds .
In addition the unhybridized P-orbitals of each C and N
atom overlap to form πbonds .

5. The πmolecular orbital of quinoline
contains 10 electrons. Thus it satisfies
Huckle’s rule 4n+2 , n=2 , and therefore
exhibits aromaticity like pyridine .
The lone pair of electrons on nitrogen
atom are not involved in the
delocalization of πorbital .

7. CANONICAL
STRUCTURE
Quinoline is
considered to be
a resonance
hybrid of the
following….…

8. METHODS OF PREPERATION
SKRAUB SYNTHESIS :
Heating Aniline and glycerol in the presence of H2SO4 and mild
oxidisingagent like nitrobenzene yield quinoline

9. MECHANISM OF ACTION
A) Dehydration of glycerol with H2S04 gives acrolein .

10. MECHANISM OF ACTION
B) 1,4 Addition of aniline to acrolein gives β-phenyl amino propionaldehyde .

11. MECHANISM OF ACTION
C) The carbonyl carbon of this aldehyde is deffficientin electron . Hence it
act as a electrophile and attack the benzene ring to form 1,2-dihydro
quinoline by inducing ring closure .

12. MECHANISM OF ACTION
D) Oxidation of 1,2 dihydroquinoline with nitrobenzene gives quinoline .

13. METHODS OF PREPERATION
FRIED –LAMDER SYNTHESIS :
When ortho amino benzaldehyde is condensed with acetaldehyde
in the presence of alkali it gives quinoline .

14. PHYSICAL PROPERTIES
It is a colourlessliquid .
Boiling point is 237°C .
It has pyridine like odour and turns yellow on
standing .
Soluble in water upto0.7% at room temperature and
soluble in most of the organic solvents .

15. CHEMICAL PROPERTIES
BASICITY :
1. It is a weak base with Pka= 4.94 . Its basicity is relatively
less than pyridine Pka= 5.2 .
2. It can be readily protonated and the N-H bond can be
formed because the lone pair of electron in nitrogen are not
involved in the delocalization of πmolecular orbitals.
When it is treated with one mole of alkyl halides or acids,
quinoline forms water soluble quaternary salts similar to pyridine
salts .

16. CHEMICAL PROPERTIES
ELECTROPHILIC SUBSTITUTION REACTION :
Quinoline undergoes electrophilic substitution reaction only under
extreme conditions which usually occur at carbon-8 position along
with the formation of small amount of 5-substituted product. Such
vigorous conditions are like that because the N-atom present in the
pyridine ring deactivates the ring for electrophilic substitution
reaction.
1. NITRATION
2. SULPHONATION

17. CHEMICAL PROPERTIES
1.NITRATION :
Quinoline treated with fumicHNO3 in the presence of H2SO4 gives a mixture of
5-Nitro and 8-Nitro quinoline .

18. CHEMICAL PROPERTIES
2. SULPHONATION :
Quinoline treated with fumicH2SO4 at 200°Cgives a mixture of Quinoline 5-
sulphonic acid and Quinoline 8-sulphonic acid.

19. CHEMICAL PROPERTIES
NUCLEOPHILIC SUBSTITUTION REACTION :
Nucleophilic substitution reaction similar to pyridine .
Quinoline undergoes Nucleophilic substitution reaction at C-2 (or)
C-4(if C-2 is blocked) but not at C-3 . This can be explained on the
basis of stabilities of the intermediate obtained when a nucleophile
attack at C-2 , C-3 , C-4 positions respectively . N-atom due to its
electronegativity withdraw the electrons which makes C-2 & C-4
positions electron deficient . These positions are more favourable
for nucleophilic attacKthan C-3

20. CHEMICAL PROPERTIES

21. CHEMICAL PROPERTIES

22. CHEMICAL PROPERTIES
Attack at C-3 :

23. CHEMICAL PROPERTIES
REACTION WITH SODAMIDE ( CHICHIBABIN REACTION ) :
Quinoline on treatment with soda amide in liquid
ammonia undergoes nucleophilic substitution reaction at C-
2 and gives 2-amino quinoline. However 2 methyl quinoline
under same condition undergoes nucleophilic substitution
reaction at C-4 and less C-2 is blocked and gives 2-methyl
4-amino quinidine .

24. CHEMICAL PROPERTIES

25. CHEMICAL PROPERTIES
REACTION WITH KOH :
Quinoline treated with KOH at 220°C gives 2-hydroxy quinoline .

26. CHEMICAL PROPERTIES
REACTION WITH N -BUTYL LITHIUM :
Quinoline treated with n-butyl lithium gives 2-n-butyl quinoline .

27. CHEMICAL PROPERTIES
OXIDATION :
A) Quinoline is resistant towards oxidizing agent but in
the presence of vigourousoxidizing condition and
potassium permanganate it gives pyridine 2,3
dicarboxylicacid(quionolinicacid). When this quinolinic
acid is decarboxylated it yields pyridine. This reaction
helps to indicate that quinoline contains a pyridine ring .

28. CHEMICAL PROPERTIES

29. OXIDATION
B) Quinoline undergoes peracetic acid to give quinoline n-oxide .

30. OXIDATION
C) Oxidation of quinoline with selenium in the presence of H2S04 to yield
picolinic acid .

31. CHEMICAL PROPERTIES
REDUCTION :
A) Quinoline on mild reduction with tin and hydrochloric acid gives 1,2,3,4
tetrahydro quinoline .

32. REDUCTION
B) Quinoline is reduced to decahydroquinoline when treated with hydrogen in
the presence of platinum .

33. Uses
As a high boiling basic solvent which is used to synthesis of
organic compounds .
In the synthesis of drugs.
In the preparation of insecticides .
In the manufacture of pharmaceuticals .
EXAMPLES OF DRUGS CONTAINING QUINOLINE AS A BASIC RING :
1.chloroquine –Antimalarial drug
2. Iodoquinol –Diagnostic agent