Experiment 1: Reactions of Aldehydes
and Ketones
Risk: Hazard category B:
Great care needed.
Read all hazard warnings carefully. Toxic, poisonous, corrosive and explosive substances
are all involved.
Wear goggles and lab coat. Mop up spills using water. Wash hands thoroughly
afterwards. Watch out for toxic and explosive substances and follow all instructions to the
letter.
You are provided with separate samples of A and B, one of which is an aldehyde and the
other a ketone. In pairs, carry out the following tests to identify them – divide up the work
between you!
Prepare a results table.
1. To confirm aldehydes or ketones, add 2 cm3 of 2, 4 dnph solution (Brady’s reagent
– highly corrosive and toxic) to each of three test tubes and then add 5 drops of
unknown liquid. A positive result is orange/red crystals.
2. Silver mirror test
To approximately 1 cm3 of a solution of silver nitrate add 1 drop of aqueous sodium
hydroxide to make a precipitate. Add aqueous ammonia drop-wise until the
precipitate just re-dissolves – this is called TOLLENS’ reagent and must be
disposed of immediately after use as it explodes on standing! Make up two
tubes of this reagent. Add 5 drops (separately) of the unknown liquids to your tubes
and warm it in a water bath of boiling water. A positive result is a silver mirror and
this means you have an aldehyde present.
3. Fehling’s test
Add Fehling’s B to 1 cm3 of Fehling’s A until the blue precipitate just redissolves to give a deep blue solution. Prepare four tubes of this
mixture. Then add 5 drops of unknown liquid to the tube and place in
the boiling water bath. A positive result is a brick red precipitate and this
means you have an aldehyde present – but it does not work for
benzaldehyde.