Synthesis of benzocaine from p-amino benzoic acid

1. Experiment No.-2
Synthesis of Benzocaine
Mrs. Pradnya Mane
Assistant Professor
Sarojini College of Pharmacy

2. • Aim: To synthesize Benzocaine from p-Amino
Benzoic Acid
• Requirements –
• Glasswares – Round bottom flask, reflux condenser,
measuring cylinder.
• Chemical – P-amino benzoic acid, Ethanol, conc.
sulphuric acid, Sodium carbonate.

3. • Principle –
Synthesis of ester by Esterification: - Carboxylic acid reacts with
alcohol to form esters through condensation reaction known as
Esterification. Acid catalysed reaction proceed very slowly in absence of
strong acid but they react within few hours. When acid and alcohol are
refluxed with a small amount of conc. sulphuric acid / hydrochloric
acid, since the position of control the amount of ester used to absorbs
excess –COOH and alcohol increases yield of synthesis of Benzocaine is
typical acid catalysed nucleophilic addition reaction.

5. Procedure –
1. To 2.5g P-amino benzoic acid, add 15ml
ethanol and add 5ml conc. sulphuric acid. Reflux
to 2 to 3 hours. Pour hot solution to 100ml water
and dissolve ppt. in water and separate residue.
2. Add sodium carbonate to clear solution until
it is neutral to litmus, filter off precipitated
Benzocaine.

8. Benzocaine is the trade name for the local anesthetic known
chemically as ethyl p-aminobenzoate, which is found in
many creams and ointments used in the treatment of pain
due to sunburn, minor burns, cuts, scrapes and insect bites.

9. Calculation:
• Molecular formula of p-amino benzoic acid= C7H7NO2
• Molecular formula of Benzocaine= C9H11NO2
• Molecular weight of PABA=137 g/mol
• Molecular weight of Benzocaine=165 g/mol

10. Theoretical yield:
• 137 g/mole of p-aminobenzoic acid contains 165 g/mol
of Benzocaine
• 2.5 g of PABA will contain …….. (X)g of Benzocaine
• X = 2.5×165/137
• X= 3 g
• Theoretical yield = 3 g

11. • Practical yield= gm
% Yield = (Practical Yield)
---------------------- × 100
( Therotical Yield)
= %

12. • Result:
• Therotical yield= gm
• Practical yield= gm
• % practical yield= %
• Melting point= °C

13. Physical Appearances-
After ethanol and sulfuric acid added to p-
aminobenzoic acid, solution became clumpy and
then turned into a white foam
After refluxing, solution became more solid
Upon filtration, white benzocaine crystals formed

14. Rf values=
Solution movement/Solvent movement= Rf value
Thin Layer Chromatography
Solvent front- 5.8 cm
Solvent- 1:1 hexane/diethyl acetate
p-aminobenzoic acid Rf value: 0.19
benzocaine Rf value: 0.34
Because the product had a different Rf value than the starting product, the
benzocaine is obviously not still p-aminobenzoic acid. Also, as the dots were fairly
distinct, it seems to lead to the conclusion that there is no evidence that there is
presence of the starting material in the reaction product.

15. References
1.Vogel's Text Book of Practical organic Chemistry by Brian S. Furniss, Antony
J.Hannaford, Peter W. G. Smith & Austin R. Tatchell; Fifth edition; Page No.
896.