The document describes a two-step process to prepare benzocaine from p-nitrobenzoic acid. The first step involves reducing p-nitrobenzoic acid to p-aminobenzoic acid using tin and hydrochloric acid. The second step is a Fischer esterification of p-aminobenzoic acid with ethanol in the presence of sulfuric acid to form benzocaine. The procedure provides details of the reaction conditions and calculations to determine the theoretical and percent yield of benzocaine produced.

1. “Preparation of Benzocaine”
Prepared by
Dr. Krishna swamy
Faculty
DOS & R in Organic Chemistry
Tumkur University

2. Experiment No: 1
Preparation of Benzocaine
AIM:
To prepare benzocaine from p-nitrobenzoic acid through two step
reaction.
PRINCIPLE:
Ethyl ester of para-aminobenzoic acid is called "Benzocaine" has
anaesthetic properties and is used extensively as a topical pain reliever.
The preparation of benzocaine has two steps. First step involves
reduction of p-nitrobenzoic acid to p-aminobenzoic acid using tin in
presence of hydrochloric acid. Second step involves Fischer
esterification of p-aminobenzoic acid by ethanol in presence of sulphuric
acid as a catalyst.
CHEMICALS REQUIRED:
p-nitrobenzoic acid, tin, Hydrochloric acid, p-aminobenzoic acid,
Ethanol, sulphuric acid.
APPARATUS:
Conical flask, Reflux condenser, Thermometer, Beaker, Funnel,
Measuring cylinder, Filter paper.
PROCEDURE:
Step-1: Reduction of p-nitrobenzoic acid
Place 3 g of 4-nitro benzoic acid in conical flask or round bottom flask
fitted with reflux condenser containing 15 mL of hydrochloric acid.

3. Weigh 10.6 g of tin granules and add to the above reaction mixture
Heat the resulting mixture on the water bath until tin dissolves (about
40 minute) at 100oC with constant stirring.
Cool the reaction mixture to lab temperature and decant the
supernatant into a beaker and wash the residual tin with water.
Add concentrate ammonia solution until the solution is alkaline to red
litmus and hydrated tin oxide separates out. Filter the content on filter
paper and collect the filtrate.
Concentrate the filtrate to reduce the volume, filter off any solids
separates.
Acidify the concentrated liquid with glacial acetic acid until the
solution is just acidic to litmus and evaporate the content on water bath
until crystals commence to separate, cool in ice, filter the crystals and
dry it.
Step-2: Fischer esterification of 4-amino benzoic acid
Place 1.2 g of 4-amino benzoic acid in conical flask or round bottom
flask fitted with reflux condenser containing 15 mL of Ethanol. Heat
the mixture until all the solid was dissolved (104oC).
Prepare ice bath, cool the above mixture and add 2.5 mL of
concentrated sulphuric acid dropwise. Precipitate will be formed.
Reflux the resulting mixture on the water bath for 60-70 minutes at
80-85oC with constant stirring.
Cool the reaction mixture to lab temperature and pour into 50 mL of
cold water.

4. Add saturated sodium bicarbonate until effervescence ceases and check
the pH with red litmus until alkaline solution is reached, benzocaine
will precipitate.
Collect the precipitated product on a filter paper, wash it with cold
water and allow it to air dry on the filter paper.
Weigh the product, calculate the yield and obtain melting point.
REACTION:
OH
O
Sn
HCl
EtOH
H2SO4
O2N
OH
O
H2N
OEt
O
H2N
CALCULATIONS:
Compound Amount
Molecular
Weight
Moles Equivalent
4-Nitrobenzoic acid 3 g 167 0.018 1
Tin granules 10.6 g 118.71 0.089 5
Hydrochloric acid 15 mL - - -
4-Aminobenzoic acid 1.2 g
Ethanol 15 mL -
Sulphuric acid 2.5 mL -
THEORETICAL CALCULATION:
167 g of p-Nitrobenzoic acid gives 165 g of Benzocaine
3 g of p-Nitrobenzoic acid will forms X g of Benzocaine
Theoretical yield =
Practical yield =
% Yield = (Practical yield) / (Theoretical yield) X 100

5. MECHANISM:
Step-1: Reduction of 4-nitrobenzoic acid to 4-amino benzoic acid
Sn
OH
O
N
O
O
H OH
O
N
OH
O
H OH
O
N
OH
HO
-H2O
OH
O
N
O
Sn
H
H
OH
O
HN
OH
H
OH
O
HN
OH2
H
OH
O
H2N
Step-2: Fischer esterification of 4-amino benzoic acid
OH
O
H2N
H+
OH
OH
H2N
HO
OH
OH
H2N
O
OH2
OH
H2N
O
O
H2N
O
H
O
H2N
O
-H+ -H2O

6. REPORT
Structure and
name of the
compound
Theoretical
yield
Practical
yield
Percentage
yield
Melting
point