Experiment M02
Preparation of benzoic acid from benzamide
Chemicals:
Benzamide (3 g),
0.5 M NaOH (70 cm3),
2 M HCl (~10 cm3),
litmus paper (red and blue),
ice,
forcept.
Apparatus:
100 cm3 conical flask fitted with a cork carrying a glass tubing,
250 cm3 beaker,
apparatus for suction filtration,
melting point apparatus,
hand len,
filter paper (3),
sample tube,
dropper.
Principle:
Benzamide C6H5CONH2 may be hydrolysed by aqueous acid or alkali, yielding
benzoic acid or its salt.
O
C
NH2
O
ONa
C
+
NaOH
+
NH3
benzamide
O
OH
O
ONa
C
C
+
+
HCl
benzoic acid
P.1
NaCl
Experiment M02
Preparation of benzoic acid from benzamide
Procedure:
a.
Weigh out 3.00 g of benzamide. Place it in a 100 cm3 conical flask and add to it 70 cm3 of
0.5 M sodium hydroxide solution.
b.
Fit the flask with a cork carrying a glass tubing and boil the mixture for about 20 minutes.
Test the gas evolved at the head of the glass tubing when the mixture begins to boil. (What
gas would you expect to get? What should be used for testing the gas?)
c.
Allow the reaction mixture to cool and then transfer it to a 250 cm3 beaker. Add an excess
of 1 M hydrochloric acid with a dropper, swirling continuously until a copious precipitate is
obtained. (How can you tell when an excess amount of HCl has been added?)
d.
Filter the resulting solution under suction, wash the crystals thoroughly with minimum
amount of ice cold water. (What is the purpose of washing?)
e.
Recrystallize the crude solid from boiling water, filter to obtain pure but damp crystals of
the product. 
f.
Dry these crystals thoroughly between filter paper until no further sign of dampness is seen
on the paper. Inspect the shape of crystals using a hand lens.
g.
Measure the mass of this product, and record it in the table below.
Put this product into a sample tube and hand in after determination of its melting point.
On the label put down
Your Name:
Name of product:
Mass of product:
m.p.:
h.
Using the melting point apparatus, determine the melting point of your product.
i.
Base on the mass of the crystals you obtained, calculate the percentage yield.
P.2
Experiment M02
Preparation of benzoic acid from benzamide
Name:
Seat No.:
Date:
Grade:
Results
Colour of crystal product obtained:
Shape of crystal product:
Melting point of the crystal product:
Mass of benzamide used:
Mass of crystal product obtained:
Calculate your percentage yield.
Molecular mass of benzamide (C6H5CONH2) =
Molecular mass of benzoic acid (C6H5COOH) =
P.3
Experiment M02
Preparation of benzoic acid from benzamide
Interpretations
1.
Is benzamide soluble in (i) water,
(ii) aqueous NaOH?
(i)
(ii)
2.
Write equations for the reactions involved in procedure b.
3.
After boiling for 20 minutes, what is the appearance of the reaction mixture?
4.
In step c, how can you tell when the acid has been added in excess?
5.
What is the purpose of washing the crystals in step d? Why is the amount of water used kept
at a minimum and kept cold?
6.
The melting point obtained by students is usually lower than the true value. What may be
the reason(s) for this ?
What improvements could be made?
7.
What criteria are usually used to judge whether a preparation method is good.
P.4