Experiment Worksheet: Interactive Laboratory
Subject:
Level:
TAS:
Chemistry
F6
Experiment 2 : The Carbonyl Compounds
Aim :
The purpose of this experiment is investigate some of the properties of carbonyl
compounds by carrying out some of the reactions of the aldehydes and ketones.
Introduction :
The similarities in chemical properties of the carbonyl compounds are mainly due
to the presence of the carbonyl group, but the aldehydes and ketones may react
quite differently. These similarities and differences in chemical properties are
investigated in this experiment.
Requirements:
methanal
bench sodium
ammonia
ethanal
hydroxide
silver(I) nitrate solution
propanone
conc. sodium hydroxide
hot water
Fehling’s solutions A & B
potassium hydroxide
test-tubes w / stopper
Brady’s solution
dilute hydrochloric acid
beakers
phenolphthalein
conc. sulphuric acid
centrifuge
iodine
pH paper
Procedures
1. The iodoform reaction
Dissolve a few crystals of iodine in about 1 cm3 of ethanal, then add sodium
hydroxide solution until the brown colour of the solution just disappears and a
precipitate appears. Centrifuge, decant the supernatant liquid, and describe the
colour and smell of the precipitate.
Repeat the experiment but replacing ethanal with propanone. Do you get the
same result, and what conclusion can you draw? Write equations to show the
reactions taking place where appropriate.
Experiment 2, Page 1
2. The silver mirror test
You need three very clean test tubes for this experiment. Before using the tubes,
place 2 cm3 of potassium dichromate and a few drops of conc. sulphuric acid in
each tube, and warm in a hot water bath, then (CAREFUL!) rinse the sides of the
inside of the tubes with this hot liquid, then wash with de-ionized water.
To about 3 cm3 of silver nitrate solution in a clean test tube, add a few drops of
sodium hydroxide. A brown precipitate of silver oxide forms. Then add
ammonia solution drop by drop until the brown precipitate just disappears.
Divide this solution into three portions using the other two clean test-tubes.
To one of the test tubes, add a few drops of ethanal and warm in a hot water bath.
Observe.
Write an equation to show the reaction taking place.
To another tube add a few drops of methanal, and warm in a hot water bath.
Observe.
Write an equation to show the reaction taking place.
3. Reaction with Fehling’s solution
Mix 0.5 cm3 of Fehling’s solution A (copper(II) sulphate solution) and 0.5 cm3 of
Fehling’s solution B (a solution of sodium hydroxide and sodium potassium
tartrate) in a test tube.
Observe.
Add a few drops of ethanal and heat in a hot water bath.
Repeat the experiment with propanone in place of ethanal.
Observe.
What conclusion can you draw?
Write an equation to show the reaction taking place where appropriate.
Experiment 2, Page 2
4. Qualitative test for the carbonyl group
Add 1 cm3 of Brady’s solution (acidified 2,4-dinitrophenylhydrazine in methanol)
to a test tube, and then add a few drops of ethanal. Warm if necessary, and
observe.
Repeat the experiment with propanone in place of ethanal, warm and observe.
What conclusion can you draw?
Write equations to show the reactions.
5. The Cannizzaro reaction
To a test tube, add two drops of potassium hydroxide solution and a drop of
phenolphthalein. Note the colour of the solution. Then add 10 cm3 of methanal
solution and warm in a hot water bath. Look for any colour change.
What has happened?
Write an equation to show the reaction taking place.
6. The aldol reaction
Mix 1 cm3 of ethanal and 1 cm3 of concentrated sodium hydroxide solution in a
test tube, and warm in a hot water bath. Observe, and smell.
Explain the reaction taking place.
Experiment 2, Page 3
Write an equation to show the initial reaction that has taken place.
7. Addition to sodium hydrogensulphite
In a test tube, mix 1 cm3 of benaldehyde with 3 cm3 of sodium hydrogensulphite
solution (made by bubbling sulphur dioxide gas through a saturated solution of
sodium sulphite), shake, and observe. Centrifuge and decant off the supernatant
liquid, and add dilute hydrochloric acid to the solid remaining inside the tube.
Look for any gas bubbles formed, and try to identify the gas evolved.
Write equations to show the reactions taking place.
End of Experiment
Experiment 2, Page 4