Detecting carbonyl compounds

Question 1

How can you detect the presence of a carbonyl compound?

Answer 1

• using 2,4 dinitrophenylhydrazine (2,4-DNP) also known as Bradys reagent can be used to detect the presence of aldehydes and ketones
• In the presence of a carbonyl compound, a yellow/orange precipitate is formed

Question 2

How would identify a carbonyl compound after using 2,4 - DNP?

Answer 2

• after getting an orange precipitate, you can recrystallise the solution and measure the melting point and compare it to a known database of melting points of carbonyls

Question 3

How do you distinguish between an aldehyde and a ketone?

Answer 3

• After using 2,4 -DNP, you can use tollens reagent which is a solution containing silver nitrate in aqueous ammonia
• In the presence of an aldehyde group, a silver mirror is produced
• This is because tollens reagent contains silver ions, Ag+ which acts as an oxidising agent in the presence of ammonia
• The aldehyde is oxidised and silver is reduced forming Ag(S) which leaves a thin film called a silver mirror
• Ag+(aq) = Ag(S)