Aromatic acids are compounds in which one or more carboxyl groups (-COOH) are attached directly to the aromatic ring. They are named by the common system or as derivatives of the parent benzoic acid.

1. Aromatic Acids
Prepared by
Dr. Girish Kashid
Asst. Prof.
Sanjivani College of Pharmaceutical
Education & Research
Prepared by Dr. Girish Kashid

2. Aromatic acids are compounds in which one or more carboxyl groups (-COOH)
are attached directly to the aromatic ring.
They are named by the common system or as derivatives of the parent benzoic
acid.
For example

3. Phenylacetic acid (C6H5CH2COOH) and other similar compounds in which the
carboxyl group is not attached directly to the aromatic ring are called side-chain
aromatic acids

4. (1) By oxidation of alkylbenzenes with acidic potassium permanganate or
sodium dichromate.
Preparation

5. (2) By reaction of phenylmagnesium bromide with carbon dioxide followed by
acid-hydrolysis of the addition product
(3) By acid-hydrolysis of phenyl cyanide (benzonitrile).

6. (4) By basic-hydrolysis of benzotrichloride.
Benzotrichloride is obtained by chlorination of toluene.

7. Properties. (Physical).
❖ Benzoic acid is a colourless solid, mp 122°C.
❖ It is soluble in hot water, diethyl ether, ethanol, and benzene.
❖ It is steam-volatile.
❖ Benzoic acid is a slightly stronger acid than acetic acid (Ka=6.3x10-5;
1.75x10-5).
❖ The characteristic carbonyl IR stretching absorption for benzoic acid is
found at 1710 – 1720 cm-1

8. A. REACTIONS OF COOH GROUP
(1) Salt formation.
Benzoic acid reacts with sodium hydroxide or sodium bicarbonate to form
sodium benzoate

9. (2) Ester formation. It reacts with alcohols in the presence of
concentrated sulfuric acid catalyst to form ester
(3) Acyl halide formation.
Benzoic acid reacts with phosphorus pentachloride or thionyl chloride to form
benzoyl chloride.

10. (4) Reduction to Benzyl alcohol.
Benzoic acid undergoes reduction with lithium aluminium hydride to give
benzyl alcohol.
(5) Decarboxylation.
When heated with soda-lime (CaO/NaOH), benzoic acid loses a molecule of
carbon dioxide to yield benzene.

11. B. REACTIONS OF BENZENE RING
(6) Electrophilic Substitution.
The benzene ring of benzoic acid undergoes the usual electrophilic substitution
reactions.
-COOH group is a meta director and deactivating.
For example

12. Halogenation

13. Sulphonation

14. Uses.
Benzoic acid is used:
(1) as germicide-in medicine for urinary infection and in vapor form for
disinfecting bronchial tubes and;
(2) as food preservative-sodium benzoate is used for preserving pickles,
tomato ketchup, and fruit juices

15. It might be expected that resonance-stabilization by the aromatic molecular
orbital would play a large role in the relative acidity of benzoic acid and
substituted benzoic acids.
However, this is not so. The negative charge of the carboxylate ion is shared by
the two carboxylate oxygen atoms but cannot be effectively delocalized by the
aromatic ring.
This is because, the oxygens of the carboxylate anion are not directly adjacent to
the aromatic ring: resonance structures cannot be drawn.
ACIDITY OF BENZENE AND SUBSTITUTED BENZOIC ACIDS

17. Because the benzene ring does not participate in resonance-stabilization of the
carboxylate group, substituents on a benzene ring influence acidity primarily by
the inductive effect.
Regardless of the position of substitution, an electron-withdrawing group
usually increases the acidity of a benzoic acid.

19. An electronegative substituent increases the acid strength are again
destabilization of the acid and stabilization of the anion
An electron-releasing alkyl substituent that is p- or m- to the carboxyl group
decreases the acidity of a benzoic acid.

20. All ortho-substituents (whether electron-withdrawing or electron-
releasing) increase acidity of a benzoic acid.
The reasons for this are not entirely known, although there may be
steric effect from having two substituents ortho to one another.