Carbonyls

Question 1

Explain the test to identify carbonyls

Answer 1

Brady’s reagent

2,4 DNPH detects carbonyl compounds. This is shown by the formation of a yellow/ orange precipitate

Question 3

Explain the oxidation of aldehydes

Answer 3

Aldehydes can be oxidised into carboxylic acids by refluxing it with acidified potassium dichromate; has an orange to green colour change:

Aldehyde + [O] —> Carboxylic acid

Question 4

Explain the test to distinguish between carbonyls and how it works

Answer 4

Tokens reagent (ammonical silver nitrate) oxidises aldehydes as they can be further oxidised but ketones cannot. Therefore there is a visible change as the aldehydes reduce the silver ions to form a silver precipitate

Question 5

Explain the mechanisms for nucleophilic addition reactions for aldehydes and ketones

Answer 5

NaBH4 introduces the hydride ion (H-) which selectively reduces carbonyls, forming a primary alcohol from an aldehyde and a secondary alcohol from a ketone. The ketones formed can have a chiral centre and if so, a racemic solution is formed.

When using KCN, hydroxyl nitriles are formed instead;
Aldehydes —> secondary hydroxy nitrile
Ketone —> tertiary hydroxy nitrile