ADVANCED LEVEL CHEMISTRY PRACTICAL
ASSESSMENT
Candidate Number: Centre Number:71637
Name: Date:
PREPARATION OF METHYL 3-NITROBENZOATE
INTRODUCTION
The nitration
of benzene to nitrobenzene is a reaction of considerable synthetic
importance. Nitrobenzene and similar
nitro compounds provide important intermediates in the manufacture of many
important compounds from drugs to dyestuffs.
However benzene itself is too hazardous to use in the school
laboratory. An ester of benzoic acid
provides a suitable alternative to benzene for the preparation and purification
of a nitro-aromatic compund. The
compound used is methyl benzoate, a relatively simple derivative of benzene, of
low toxicity, which is used in commercial heat meters.
HAZARD WARNING
Concentrated
sulphuric acid is corrosive.
Concentrated
nitric acid is corrosive and gives off harmful vapour.It can react violently
with organic substances.
Ethanol
is highly flammable.
Therefore
you MUST:
WEAR
SAFETY SPECTACLES AND GLOVES.
KEEP
STOPPERS ON BOTTLES AS MUCH AS POSSIBLE.
KEEP
BOTTLES OF FLAMMABLE LIQUIDS AWAY FROM FLAMES.
PROCEDURE
Preparation
of methyl 3-nitrobenzoate.
1. Prepare a nitrating mixture by measuring 5 cm3
of concentrated nitric acid into a hard glass test tube. Add 5 cm3
of concentrated sulphuric acid dropwise to the nitric acid. Make sure the two acids are mixed. TAKE CARE!.
Cool the nitrating mixture in a beaker of crushed ice.
2.
Measure 10 cm3 of concentrated sulphuric acid into
another hard glass test tube. (This acid provides a solvent for the
ester). Place this tube into the beaker
of crushed ice. It is important that
the reaction mixture is kept below 10oC
throughout the reaction.
3. Weigh (to the nearest 0.1g) about 10 cm3
of methylbenzoate in a 100 cm3 conical flask. Record this mass.
4. Check that the concentrated acids have cooled to below 10oC. Add the 10 cm3
portion of the concentrated sulphuric acid to the ester in the flask and place
the flask in the beaker of ice.
5. Using a dropping pipette, add the nitrating mixture a few drops at a
time to the ester solution in the flask.
Stir the mixture gently with the thermometer keeping the temperature below 10oC. The addition of the nitrating mixture will
take 15 to 20 minutes.
6. When the addition is complete , allow the mixture to stand at room
temperature for a further 15 minutes.
7. After this time, pour the mixture on to about 25 g of crushed ice in
a 100 cm3 beaker.
Stir and leave until all the ice has melted and the crystalline methyl
3-nitrobenzoate has formed.
Purification of methyl 3-nitrobenzoate.
8. Filter the mixture using a
Buchner funnel. Wash the solid
thoroughly with at least three 10 cm3
portions of water. Using the filter
pump draw air over the solid to remove as much water as possible.
9.
Heat about 30 cm3 of ethanol (IMS) to about 50oC
in a conical flask on a water bath away from all naked flames.
10. Transfer your solid product to a clean, dry 100 cm3
conical flask. Add sufficient hot
ethanol to just dissolve the solid.
(Warm the solution in the hot water bath as you do this. You should obtain a clear liquid.)
11. Leave the solution to cool.
Crystals of methyl 3-nitrobenzoate should separate. Filter these crystals off when the solution
is quite cool. Dry the product in air,
covered with a filter paper.
12.
Weigh the dried
product and record the yield. Calculate
the percentage yield, based on the mass of ester used at the start. Place your product in a labelled sample
tube.
13. Determine the melting point of your sample
and record this.
QUESTIONS
1. What conditions employed during the preparation help to prevent
further nitration of the product to the dinitroderivative?
2. Give the names and structural formulae of two nitro-compounds that
are very likely to contaminate the impure crystals of methyl 3-nitrobenzoate.
3. How do the conditions for nitration of methyl
benzoate differ from those for the nitration of benzene?
4. Why were the crystals washed with water before
recrystallization?
5. What happens during the recrystallization to
a impurities,
b the main product?
6. How is the loss of product kept to a minimum
during recrystallization?
7. What are the usual stages in the purification
of an organic solid?
8. How many moles of methyl benzoate and nitric
acid were used in the preparation of methyl
3-nitrobenzoate? (assume that concentrated nitric acid is pure HNO3
and that its densituy is 1.5 g cm-3.) Which reactant is present in excess?
9. What is the theoretical yield of methyl
3-nitrobenzoate?
10. What is your percentage yield?
ADVANCED
LEVEL CHEMISTRY PRACTICAL ASSESSMENT CHECKLIST
Candidate
Number: Centre
Number: 71637
Name: Date:
PREPARATION
OF METHYL 3-NITROBENZOATE
SKILL
AREA 2 MANIPULATION Student
Mark
Safety
goggles and gloves
Transfer
of liquids without spills
Preparation
of nitrating mixture
Use
of balance
Temperature
control on addition of nitrating mixture
Buchner
filtration
Setting
up and using water bath
Recrystallisation
Drying
Use
of melting point apparatus
SKILL
AREA 3 MEASURING & RECORDING Student Mark
Volumes of
acids
Temperatures
Weighing to
required accuracy
Volume of
ethanol
Records all
masses and volumes
Melting point
value
SKILL
AREA 5 CONCLUDING & COMMUNICATING Student Mark
Q
1 Prevention of further nitration
Q
2 Structural formulae of
contaminants
Q
3 Nitration conditions
Q
4 Washing crystals
Q
5 Recrystallisation effects on
impurities and main product
Q
6 Minimising product loss
Q
7 Usual purification stages
Q
8 Excess reactant calculation
Q
9 Theoretical yield calculation
Q
10 Percentage yield calculation