COAS Chemistry 2 Teacher Resources Original material © Cambridge University Press 2005, 2007, 2009 2
2 Practical 1
The nitration of methyl benzoate
Health and safety
Concentrated sulfuric(VI) acid and nitric acid are both corrosive; nitric(V) acid is also oxidising.
Wear protective nitrile gloves when using these chemicals or the nitrating mixture.
Make sure that all apparatus is dry before use.
Methyl benzoate is harmful. Do not inhale its vapour and avoid contact with skin.
Ethanol is highly flammable and harmful. Keep away from all sources of ignition.
In particular, use recently boiled water from an electric kettle to warm the ethanol for recrystallisation. Dispose of all ethanol safely before determining the melting point of the product.
Safety goggles must be worn at all times.
Introduction
In this experiment, you will be nitrating a benzene compound, methyl benzoate.
This reaction is a safer alternative to the nitration of benzene itself, which is a class 1 carcinogen.
The equation for the reaction is shown below.
methyl benzoate + nitric acid methyl 3-nitrobenzoate + water
The procedure consists of three stages:
A preparation of the crude product
B purification of the product by recrystallisation and filtration
C testing the purity of the product, by melting-point determination.
Apparatus per student or group
• concentrated sulfuric(VI) acid / • 400 cm3 beaker• concentrated nitric acid / • 250 cm3 beaker
• methyl benzoate / • three 100 cm3 conical flasks
• ice / • 25 cm3 measuring cylinder
• ethanol / • two 10 cm3 measuring cylinders
• distilled water / • two boiling tubes
• access to an electric kettle / • eight graduated droppers
• top-pan balance measuring to 2 decimal places / • Büchner funnel and flask
• melting-point apparatus / • filter pump or large syringe
• protective nitrile gloves / • filter papers
• safety goggles / • small thermometer
Procedure
A: Preparation of crude product
1 Prepare an ice bath in a small trough or 400 cm3 beaker.
2 Add 10 cm3 of concentrated sulfuric acid (corrosive) to a clean, dry boiling tube and place the tube in the ice bath. Leave for 5 minutes while you carry out the next part of the experiment.
3 In another clean, dry boiling tube, prepare a mixture of 3 cm3 of concentrated sulfuric acid and 3cm3 of concentrated nitric acid (corrosive, oxidising). Label the tube ‘nitrating mixture’ and cool it in the ice bath.
4 Place the 10 cm3 measuring cylinder on the top-pan balance and add 4 cm3 of methyl benzoate (harmful). Record the mass of the liquid.
5 Add the methyl benzoate to the concentrated sulfuric acid (not the nitrating mixture) and continue to cool it in the ice bath.
6 Using a clean, dry dropping pipette, add the nitrating mixture drop by drop to the acidified methyl benzoate. Carefully swirl the tube after each addition and make sure the temperature stays in the range 5–15°C.
7 After all the nitrating mixture has been added, leave the mixture to stand at room temperature for 10 minutes. Meanwhile cool 15 cm3 of distilled water and 10 cm3 of ethanol, ready for steps 11 and 12 respectively.
8 Crush up some ice and transfer about 40 g of it to a 250 cm2 beaker.
9 Add the reaction mixture to the ice and stir until the product solidifies.
10 Filter the product using a Büchner flask and suction filtration.
11 Wash the solid product with three 5cm3 portions of cold distilled water.
12 Change the collection flask for a clean, dry conical flask and wash the product with two 5 cm3 portions of cold ethanol.
B: Purification of the product
1 Transfer the product to a 100 cm3 conical flask, add 20 cm3 of ethanol (highly flammable, harmful) and place the flask in a 400 cm3 beaker of almost boiling water from an electric kettle.
2 When the solid dissolves, remove the flask from the hot water bath, filter its contents into a clean flask and allow to cool in the ice bath.
3 Measure and record the mass of a dry filter paper. Using this paper, filter off the solid and allow it to dry overnight.
4 Weigh the solid and filter paper. Calculate the mass of solid produced, and hence the percentage yield.
C: Checking the purity of the product
1 Using the melting-point apparatus, measure the melting point of the purified product.
(Pure methyl 3-nitrobenzoate melts at 77.5 °C.)
2 As an extension activity, carry out thin-layer chromatography on the purified solid and the ethanolic filtrate from step 12.
COAS Chemistry 2 Teacher Resources Original material © Cambridge University Press 2005, 2007, 2009 2