Aldehydes And Ketones

Question 1

What does Tollens’ reagent test for, and what is the positive result?

Answer 1

= Aldehyde

+ve Silver mirror (slight heating of the aldehyde and reagent is required)

Question 2

How is Fehling’s reagent made?

Answer 2

Mix a copper (II) containing solution with sodium hydroxide

Question 3

What does Fehling’s reagent test for, and what is a positive result?

Answer 3

= Aldehyde

+ve Dark blue solution forms a red-brown precipitate

Question 4

Why are aldehydes able to react with Tollens’ and Fehling’s but ketones are not?

Answer 4

The hydrogen atom of the aldehyde group is quite reactive and allows aldehydes to act as reducing agents.

Question 5

How do you make Tollens’ reagent?

Answer 5

Add aqueous ammonia to an aqueous solution of silver nitrate until brown precipitate dissolves to give a colourless solution.

Question 6

Is it possible to distinguish between aldehydes and ketones using acidified potassium dichromate (VI)?

Answer 6

An excess of aldehyde will turn acidified potassium dichromate solution from orange to green.
Even in excess, a ketone will not change the colour of the solution; it will remain orange.
This also applies for acidified potassium manganate (VII)

Question 7

Explain why the reaction between 2,4-dinitrophenylhydrazine and aldehydes/ketones is described as a condensation reaction and give the alternative name.

Answer 7

An addition occurs, followed by the elimination of a water molecule, hence the name condensation. Alternative name; Nucleophilic addition-elimination reaction

Question 8

What is Brady’s reagent?

Answer 8

2,4-DNPH dissolved in methanol and concentrated sulphuric acid, this is how it is used in reactions.

Question 9

Describe the test to determine specific aldehydes and ketones using 2,4-DNPH.

Answer 9

1. Filter and wash precipitate with methanol
2. Recrystallise from a suitable solvent (a mixture of water and ethanol is suitable for smaller compounds)
3. Dissolve crystals in a minimum volume of hot solvent
4. Cool and re-filter to remove any soluble impurities
5. Find melting temperatures and compare with book values

Question 10

What is the positive result given with 2,4-DNPH?

Answer 10

Orange-red precipitate with aldehyde or ketone

Question 11

What does the triiodomethane reaction detect?

Answer 11

CH3C=O group found in ketones and ethanal

Positive result is also given by CH3CH(OH) group as this is oxidised during the reaction

Question 12

Describe one method of carrying out the triiodomethane reaction.

Answer 12

1. React iodine with enough sodium/potassium hydroxide to remove the iodine colour
2. Add a small amount of the test solution
   (Heating may be required)
   OR
3. Add a small amount of the test solution to an aqueous mixture of sodium (I) chlorate and potassium iodide
   (Heating may be required)

Question 13

What is observed for a positive result with the iodoform reaction?

Answer 13

Yellow precipitate of CHI3

Question 14

How would you increase the rate of reaction between carbonyl compounds and HCN?

Answer 14

Add cyanide ions, or alkali which will neutralise HCN to produce more cyanide ions.

Question 15

How would you ascend hydrogen cyanide up its homologus series? Note, in the lab hydrogen cyanide is not used as it is a poisonous gas.

Answer 15

React with an aldehyde or ketone; this will produce a hydroxynitrile.

Question 16

In the lab, what may be used instead of hydrogen cyanide to produce a hydroxynitrile when added to a carbonyl compound?

Answer 16

Sodium/ potassium cyanide dissolved in water to which a little bit of sulphuric acid has been added.

Question 17

Suggest how you would make an amino acid from a hydroxynitrile.

Answer 17

1. Replace the OH group with a chlorine; by adding the hydroxynitrile to a solution of HCl and shaking at room temp.
2. Convert the chlorine atom to an amine group, by reaction with ammonia.
3. Hydrolyse the nitrile group to a carboxylic acid.