26.2 identifying aldehydes and ketones

what is used to detect the carbonyl functional group in ketones and aldehydes

• 2,4-dinitrophenylhydrazine / 2,4-DNP, brady’s reagant

• in the presence of a carbonyl group, a yellow or orange precipitate called a 2,4-dinitrilehydrazone is produced

how to carry out the test for a carbonyl group in aldehydes and ketones

1. add 5cm depth of a solution of 2,4-dinitrophenylhydrazine to a clean test tube

2. using a dropping pipette, add three drop of the unknown compound

3. if no crystals for, add a few drops of sulphuric acid

4. a yellow/orange precipitate indicates the presence of an aldehyde or ketone

5. the impure yellow/ orange solid is filtered to remove the solid precipitate from the solution

6. the solid is then recrystallised to produce a pure sample of crystals

7. the melting point of the purified 2,4-dinitrophenylhydrazone is measured and recorded

how to tell the difference between an aldehyde or a ketone

use tollens reagant which will produce a silver mirror if an aldehyde group is present

how to carry out the tollens test

1. pour 2cm³ of the unknown solution into a test tube

2. add an equal volume of tollens solution

3. leave the test tube to stand in warm water (about 50 degrees) for 10-15 minutes and observe whether any silver mirror is formed

the oxidation + reduction reactions that happen in the tollen’s test