[26.2] identifying aldehydes and ketones

Question 1

how can carbonyls be identified with 2,4-DNP?

Answer 1

a yellow / orange precipitate is formed

Question 2

how can you distinguish an aldehyde from a ketone?

Answer 2

• acidified potassium dichromate (VI)
• tollens’ reagent

Question 3

distinguishing an aldehyde from a ketone: acidified potassium dichromate (VI)

Answer 3

• aldehydes can be oxidised but ketones can’t
• for aldehydes, the colour will change from orange to green
• for ketones, it will stay orange

Question 4

what is tollens’ reagent?

Answer 4

• ammoniacal silver nitrate
• mild oxidising agent which will oxidise aldehydes but not ketones
• contains the diammine silver (I) ion: [Ag(NH₃)₂]⁺

Question 5

what is the equation for the silver mirror test?

Answer 5

• silver (I) ion is reduced to silver
• Ag⁺ (aq) + e⁻ ➞ Ag (s)

Question 6

how do you make tollens’ reagent?

Answer 6

1. in a clean test tube, add 3ml depth of AgNO₃ (aq)
2. add NaOH (aq) to the silver nitrate until a brown precipitate of Ag₂O is formed
3. add dilute ammonia solution dropwise until the brown precipitate just dissolves to form a clear colourless solution

Question 7

how do you test for an aldehyde group with tollens’ reagent?

Answer 7

1. pour 2ml depth of unknown solution into a clean test tube
2. add an equal volume of freshly prepared tollens’ reagent
3. leave the test tube to stand in a beaker of warm water at about 50°C for 10-15 mins
4. silver mirror indicates presence of aldehyde

Question 8

what is the method for testing for carbonyls with 2,4-DNP?

Answer 8

1. add 5ml of a solution of 2,4-DNP to a clean test tube. this is in excess
2. using a dropping pipete, add 3 drops of the unknown compound. leave to stand
3. if no crystals form, add a few drops of sulfuric acid
4. a yellow / orange precipitate indicates the presence of an aldehyde or ketone

Question 9

how can you distinguish an aldehyde from a ketone using 2,4-DNP?

Answer 9

1. filter precipitate (impure 2,4-DNP derivative)
2. recrystallise by dissolving impure precipitate in a minimum amount of hot solvent, cooling quickly and crystallising the pure solid
3. measure the melting point of the purified solid derivative using a capillary tube and a thiele tube
4. compare to data values