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 acidor alkali, yielding benzoic acid or its salt.
P.1
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 themixture for about 20 minutes.
Test the gas evolved at the head of the glass tubing when themixture 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 a250 cm3 beaker. Add an excess of 1 M hydrochloric acidwith 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 crystalsthoroughly with minimum amount of ice cold water. (What is thepurpose of washing?)
e. Recrystallize the crude solid from boiling water, filter toobtain pure but damp crystals of the product.
f. Dry these crystals thoroughly between filter paper untilno 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 tablebelow.
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 pointof your product.
i. Base on the mass of the crystals you obtained, calculate thepercentage 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 thereaction 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 isthe amount of water used kept at a minimum and kept cold?
6. The melting point obtained by students is usually lower thanthe 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