Aromatic acids

1. Aromatic Acids
Mr.P.S.Kore
Assistant Professor(Research Scholar)
Pharmaceutical Chemistry Department
RCP, Kasegaon

2. Aromatic acid
• The compounds in which one or more carboxyl groups are attached
directly to the aromatic ring.
COOH
benzoic acid
COOH
OH
COOH
NH2
COOH
COOH
phthalic acid
COOH
CH3
2-methylbenzoic acid
(toluic acid)
COOH
NO2
3-nitrobenzoic acid
2-aminobenzoic acid
(anthranilic acid)
2-hydroxybenzoic acid
(Salicylic acid)

3. Acidity of aromatic acids
• The π electron plays important role in acidity of carboxylic acid.
• The negative charge of the carboxylate ion is shared by the two
carboxylate oxygen atoms cannot be effectively delocalized by
aromatic ring.
• Acidity influence by inductive effect
• According to lowery-bronsted theory, it is acid because it proton
donor.
• According Arrhenius theory it is acid because it give H+ ion in water
COOH C
O
O
+ H
I II
C
III
O
O

4. Factor affecting on acidity of benzoic acid
• The electron withdrawing group increases the acidity of a benzoic acid.
• The electron deficient atom stabilizes anion
• Ortho substitution by electrophilic group increases acidity due to steric
effect.
COOH
HNO3
benzoic acid
H2SO4
COO
NO2
H
More acidic

5. Effect of withdrawing group on acidity
of benzoic acid
COOH
NO2 NO2
C
O O
H
NO2
C
O O
NO2
C
O O
NO2
C
O O
NO2
C
O O
Inductive effect
N
C O
O
O O
I
II
III
IV
•Electron withdrawing group increases resonance structure and stability.
•Reduces increases electron density on the hence increases acidity of benzoic acid

6. Factor affecting on acidity of benzoic acid
• Electron donating group adds electron density on the
benzene.
• It increases electron density and decreases acidity of benzoic
acid NH2
CH3Cl
Aniline
AlCl3
NH2
CH3
NH2
COOH
K2Cr2O7
Less acidic
(Base)
(Base)

7. Effect of electron withdrawing group on acidity
of benzoic acid
COOH C
O O
H
C
O O
C
O O
C
O O
C
O O
Inductive effect
NH2 NH2
NH2 NH2
H2N
II
I
•Electron donating group reduces resonance structure and stability
• Increases electron density hence decreases acidity of benzoic acid

8. COOH COOH COOH
NH2
(Less acidic)
NO2
> >
Acid
(More acidic)
Benzoic acid
m-nitro benzoic acid
Para amino Benzoic acid
Decreases PE
Increases ionization
H
MORE ACIDIC
Electron withdrawing group withdraws
electron density from the benzene and hence lowers
electron density of benzene
Equilibrium shift to right side
Increases PE
Decreases Ionization
H
Less ACIDIC
Electron donating group donates
electron density to the benzene and hence
increases
electron density of benzene
Equilibrium shift to left side
Increases
decreases
Electron withdrawing group increases
acidity of benzoic acid
Electron donating Group decreases
acidity of Benzoic acid
Effect of
substituents on
acidity of benzoic
acid

9. Why Aromatic carboxylic acid is meta director?
• Ortho and Para positions in benzoic acid resonating structure carry
positive charge .
• Hence an electrophile can not attack these positions(repulsion).
• Thus the carboxyl group directs all electrophile to the meta positions.
C
HO
O
C
O-
HO
C
HO
O
CH
HO
O

10. Why carboxylic acid is called as deactivating?
• Benzoic acid undergoes electrophilic substitution is more slowly than
benzene because carboxyl group withdraws electron from the ring by
resonance.
• It decreases electron density of the ring and makes it less attractive to an
incoming electrophile. hence electrophilic substitution is slow
• It requires vigorous reagent and condition
COOH
HNO3
benzoic acid
H2SO4
COOH
NO2

11. Preparation of aromatic acid
1. Oxidation of benzyl chloride
2. Reaction of phenyl magnesium bromide with
carbon dioxide
3. Acid hydrolysis of benzonitrile
4. Basic hydrolysis of benzotrichloride
5. Hydrolysis of phenyl benzoate

12. 1.Oxidation of Benzyl chloride
CH2Cl
H2O
CH2OH
Oxidation of Benzyl chloride with acidic potassium permanganate or sodium dichromate
COOH
benzoic acid
KMnO4
CH2Cl
H OH
CH2OH
+
HCl
CH2OH
+ O O +H2O
C
O
OH

13. Mechanism
CH2Cl CH2
Cl
H2O
H OH
CH2OH
HCl
KMnO4
COOH

14. 2.Reaction of phenyl magnesium bromide with
carbon dioxide
• The reaction of phenyl magnesium bromide with carbon dioxide followed
by acid hydrolysis.
MgBr
phenylmagnesium bromide
CO2
COO-
magnesium benzoate bromide
H2O
COOH
benzoic acid

15. Mechanism
MgBr
phenylmagnesium bromide
C O
O
C
O
O Mg Br
H OH
COOH
Mg Br
OH

16. 3.Acid hydrolysis of benzonitrile
CN
benzonitrile
H2O
COOH
benzoic acid

17. Mechanism
C C
H2O
OH H
COOH
NH3
N
OH
NH
O
H H

18. 4.Basic hydrolysis of benzotrichloride
CH3
toluene
CCl3
NaOH
H2O
COONa
H
H2O
COOH
benzoic acid
3Cl2
-3HCl

19. Mechanism
CCl3
C
NaOH
Na OH
COOH
OH
Cl
O
H H
Cl
-2HCl
-NaCl

20. 5. Hydrolysis of Phenyl benzoate
C
O
O
H2O
C
O
OH OH
Phenol
Benzoic acid
Phenyl benzoate

21. Aromatic acid derivatives
• Benzoyl chloride
• Benzamide
• Benzoic anhydride
• Benzoyl peroxide
• Benzonitrile
• Toluic acid
• Anthranilic acid
• Salicylic acid
• Benzene dicarboxylic acid
• Phthalic acid
• Phthalic anhydride
• Phenyl acetic acid
• Cinnamic acid

22. Reactions of aromatic acid
1. Salt formation
2. Ester formation
3. Acyl halide formation
4. Reduction to benzyl alcohol
5. Decarboxylation
6. Electrophilic substitution

23. 1.Salt formation
COOH
benzoic acid
NaOH
COONa
sodium benzoate
Benzoic acid reacts with base to form salt

24. 2.Ester formation
COOH
benzoic acid
C2H5OH
H2SO4
COOC2H5
ethyl benzoate
Benzoic acid undergoes esterification with ethanol and conc.
Sulfuric acid to form ethyl benzoate

25. Mechanism
COOH
benzoic acid
COO
benzoic acid
+ H
C2H5 OH
C2H5 + OH
COOC2H5
+ H2O

26. 3.Acyl halide formation
COOH
benzoic acid
PCl5
C
O
Cl
benzoyl chloride
POCl3 HCl
Benzoic acid treated with phosphorous penta chloride to give benzoyl chloride

27. 4.Reduction to benzyl alcohol
COOH
benzoic acid
LiAlH4
CH2OH
Benzylalcohol
Benzoic acid reduces to benzyl alcohol

28. 5. Decarboxylation
COOH
benzoic acid
CaO
NaOH CaCO3
When benzoic acid treated with soda lime and undergoes
decarboxylation to give benzene

29. 6. Electrophilic substitution
COOH
benzoic acid
HNO3
H2SO4
COOH
NO2
3-nitrobenzoic acid
a) Nitration: When benzoic acid treated with nitric acid and sulfuric
acid to give m-nitro benzoic acid

30. Mechanism
Step-I- Formation of Electrophile
HNO3 + H2SO4 NO2 + H2O + HSO4
COOH
Step-II- Formation of arenium ion
NO2
+
COOH
NO2
H
Step-III- Deprotonation
COOH
NO2
H
HSO4
COOH
NO2
+ H2SO4

31. 6. Electrophilic substitution
b) halogenation: When benzoic acid treated with bromine in acetic
acid to give 3-bromo benzoic acid
COOH
+ Br2
Acetic acid
COOH
Br
benzoic acid 3-bromobenzoic acid

32. Mechanism
Step-I- Formation of Electrophile
Br2 + CH3COOH
COOH
Step-II- Formation of arenium ion
Br
+
COOH
Br
H
Step-III- Deprotonation
COOH
Br
H
CH3COO
COOH
Br
+ CH3COOH
Br + CH3COO + HBr

33. 6. Electrophilic substitution
c) Sulfonation: When benzoic acid treated with concentrated
sulfuric acid to give 3-sulfo benzoic acid
COOH
+ H2SO4
COOH
SO3H
benzoic acid 3-sulfobenzoic acid
+ H2O

34. Mechanism
Step-I- Formation of Electrophile
H2SO4 + H2SO4
COOH
Step-II- Formation of arenium ion
+
COOH
SO3H
H
Step-III- Deprotonation
COOH
SO3H
H
COOH
SO3H
+ H2SO4
SO3H H2O
+ + HSO4
SO3H
HSO4

35. Uses
• Germicide-UTI(urinary tract infection)
• Food preservatives