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Aldehydes and ketones can be structural isomers of each other. Aldehydes are produced by the oxidation of a primary alcohol and have the C=O on the terminal (end) carbon. Ketones are produced by the oxidation of a secondary alcohol and have the C=O on a carbon atom in the middle of the carbon chain. Aldehydes can be further oxidised to carboxylic acids, while ketones are not oxidised further.
The tests we use to distinguish between aldehydes and ketones all involve oxidising the aldehyde but not the ketone. • While acidified dichromate or permanganate will distinguish between aldehydes and ketones, they are strong oxidising agents which will also change colour in the presence of alcohol or other reagents. • Oxidising agents which oxidise aldehydes but not ketones are: • Tollen’s reagent • Benedict solution • Fehling’s solution
Tollen’s reagent Tollen’s reagent is [Ag(NH3)2]+ which, when reduced, forms Ag(s). It must be freshly prepared. Silver nitrate solution A few drops of NaOH to form a precipitate. Add ammonia solution till the precipitate dissolves.
Add a few drops of the aldehyde or ketone, shake, and warm gently. The ketone remains colourless, the aldehyde will react. If you are lucky you will get a ‘silver mirror’ as elemental silver forms on the inside of the test tube. Less spectacular, but just as valid, is the formation of a grey or black precipitate, also of elemental silver. A grey precipitate of silver.
The ‘silver mirror’. You’re more likely to get a mirror with a very clean test tube.
Benedict solution You will have used blue Benedict solution in Y9 or 10 when you tested food for sugar. Benedict solution is an alkaline solution of Cu2+, complexed with citrate ions to keep it in solution. It is a mild oxidising agent which is reduced to Cu+. In the alkaline solution the Cu+ is in the form of Cu2O which is a brick-red precipitate.
Take about 2 mL of Benedict solution in each of two test tubes. Add a few drops of aldehyde to one tube, and ketone to the other tube, and shake to mix.
Heat the mixture by putting the tubes in hot water. Shake several times to mix.
A reaction has occurred in the left hand (aldehyde) tube, but not in the right hand (ketone) tube.
If you wait long enough you will see the red-brown precipitate of Cu2O form.
Fehling’s solution Like Benedict solution, Fehling’s contains alkaline Cu2+, but Fehling’s uses potassium tartrate to complex the copper. The mixture is freshly prepared: Pour a little Fehling’s A solution into each test tube. Add the ‘B’ solution until a precipitate forms.
Keep adding ‘B’ solution until the precipitate has redissolved and the solution is a clear, dark blue.
Add a few drops of aldehyde and ketone to separate tubes, shake, and heat in a beaker of hot water.
A reaction occurs in the aldehyde tube as Cu2O forms. No reaction occurs in the ketone tube.
In all of these reactions (Tollen’s, Benedict and Fehling’s), the aldehyde is oxidised to the carboxylic acid while no reaction occurs to the ketone.
How does glucose fit in? Glucose, C6H12O6, is the building-block molecule for starches and cellulose. It is often represented in diagrams as a simple hexagon, but actually, one of the carbon atoms is not part of that hexagon: Glucose
In aqueous solution, this ‘ring’ form of glucose exists in equilibrium with the ‘straight-chain’ form of glucose, which is an aldehyde: It is this straight-chain form of glucose which reacts with Benedict solution (to form the carboxylic acid). Most of the glucose is in the ring form, which is why the reaction with Benedict solution is slow.
Glucose molecules combine in a condensation reaction, losing one molecule of water for every join. Sucrose (table sugar) is made from two molecules of glucose: 2C6H12O6 → C12H22O11 + H2O Once the glucose molecules are joined, they are unable to open to form the straight-chain aldehyde form, which is why table sugar does not react with Benedict or Fehling’s solution.
Starch is composed of several hundred glucose units joined together. Cellulose is composed of several thousand glucose units joined in more complex chains with crosslinks between them. Sucrose, starch and cellulose are hydrolised by aqueous acid or enzymes. The water lost in the polymerisation reaction is returned and glucose is reformed.
