Chemical tests on carbonyl compounds

Question 1

Explain a chemical test that can be used to detect the presence of a carbonyl group.

Answer 1

• Aldehydes and ketones can be detected using the reagent 2.4-dinitrophenylhydrazine (2,4-DNP/2,3-DNPH). - A solution of 2,4-DNP in a mixture of methanol ad sulfuric acid is known as Brady’s reagent. - When Brady’s reagent is added to an aldehyde or a ketone, a yellow or orange precipitate is formed. - This precipitate called 2,4-dinitrophenylhydrazone derivative, confirms the presence of the carbonyl functional group, C=O in an organic compound.

Question 2

What compounds does the chemical test of using 2,4-DNP come out as positive?

Answer 2

• Ketones/ Aldehydes - Carboxylic acids and esters don’t produce precipitates because they are not carbonyl compounds even though they contain a C=O bond, the reason is beyond the scope of the course.

Question 3

Write out the reaction of a carbonyl compound with 2,4- dinitrophenylhydrazine.

Answer 3

Question 4

How can a aldehydes and ketones be distinguished from each other?

Answer 4

• Tollens’reagent is a weak oxidising agent used to distinguish between aldehydes and ketones.
• Aldehydes are easily oxidised to carboxylic acids by Tollens’reagent. Ketones are not oxidised by Tollen’s reagent.

Question 5

Explain how Tollens’reagent can be made.

Answer 5

• aqueous sodium hydroxide is added to aqueous silver nitrate until a brown precipitate of silver oxide is formed.
• dilute aqeous ammonia is then added until the precipiate just dissolves.

The colourless soltuion is Tollen’reagent, sometimes called ‘ ammoniacal silver nitrate’.

Question 6

Explain the observation when an aldehyde is added to Tollens’reagent and when a ketone is added to Tollens’reagent.

Answer 6

Aldehyde - Silver-grey solid or ‘silver mirror’ is formed.

Ketone - No reaction.

Question 7

In Tollens’reagent what is the oxidising species?

Answer 7

The oxidising species is the aqueous silver(l) ion , Ag⁺(aq).

The silver ions are reduced to silver metal:

Ag⁺(aq) + e^- → Ag(s)

Question 8

What further experiments can be done to identify the actual structure of the aldehyde or ketone?

Answer 8

• It is possible to identify the carbonyl compound by carrying out further experiments on the 2,4-dinitrophenylhydrazone derivative.
• The yellow/orange solid 2,4-dinitrophenylhydrazone derivative is slightly impure.
• The impure product is filtered and recrystallised to produce a purified sample of yellow or orange crystals of the 2,4-DNP derivative - this is filtered and allowed to dry.
• The melting point of this purified derivative is measured and recorded.
• The melting point is then compared to a database or data table to identify the original aldehyde or ketone.
• It is very difficult to destinguish the boiling points of the actual organic compounds on there own, because their boiling points are very similar.