2. Aromatic acid
benzoic acid
OH
• The compounds in which one or more carboxyl groups are attached
directly to the aromatic ring.
COOH COOH COOH
NH2
COOH
COOH
phthalic acid
CH3
2-hydroxybenzoic acid
(Salicylic acid)
COOH
2-methylbenzoic acid
(toluic acid)
NO2
3-nitrobenzoic acid
2-aminobenzoic acid
(anthranilic acid)
COOH
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
NO2 NO2
COOH C
O O
H
NO2
O C 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 electron density on the BENZENE 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
AlCl3
NH2 NH2
COOH
Less acidic
Aniline
(Base) CH3
(Base)
K2Cr2O7
7. Effect of electron withdrawing group on acidity
of benzoic acid
COOH C
O O
H
O C O
C
O O
C
O O
C
O O
Inductive effect
NH2
II
NH2
NH2 NH2
H2N
I
•Electron donating group reduces resonance structure and stability
• Increases electron density hence decreases acidity of benzoic acid
8. COOH COOH COOH
NH2
Para amino Benzoic acid
(Less acidic)
Electron donating group donates
electron density to the benzene and hence
increases
electron density of benzene
NO2
m-nitro benzoic acid
(More acidic)
> >
Acid
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
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
H2SO4
benzoic acid
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. Reactions of aromatic acid
1. Salt formation
2. Ester formation
3. Acyl halide formation
4. Reduction to benzyl alcohol
5. Decarboxylation
6. Electrophilic substitution
19. 6. Electrophilic substitution
benzoic acid
NO2
3-nitrobenzoic acid
a) Nitration: When benzoic acid treated with nitric acid and sulfuric
acid to give m-nitro benzoic acid
COOH
COOH
HNO3
H2SO4
20. Mechanism
Step-I- Formation of Electrophile
+
HNO3 H2SO4 NO2 + H2O + HSO4
COOH
Step-II- Formation of arenium ion
NO2
+
COOH
H
NO2
Step-III- Deprotonation
COOH
H
NO2
HSO4
COOH
NO2
+ H2SO4
21. 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
3-bromobenzoic acid
benzoic acid
22. Mechanism
Step-I- Formation of Electrophile
Br2 + CH3COOH
COOH
Step-II- Formation of arenium ion
Br
+
COOH
H
Br
Step-III- Deprotonation
COOH
H
Br
CH3COO
COOH
Br
+ CH3COOH
Br + CH3COO + HBr
23. 6. Electrophilic substitution
c) Sulfonation: When benzoic acid treated with concentrated
sulfuric acid to give 3-sulfo benzoic acid
COOH
+ H2SO4
COOH
SO3H
3-sulfobenzoic acid
benzoic acid
+ H2O
24. Mechanism
Step-I- Formation of Electrophile
+
H2SO4 H2SO4
COOH
Step-II- Formation of arenium ion
+
COOH
H
SO3H
Step-III- Deprotonation
COOH
H
SO3H
COOH
SO3H
+ H2SO4
SO3H H2O
+ + HSO4
SO3H
HSO4
