26.2 Identifying aldehydes and ketones

Question 1

what happens to 2,4-DNP when it reacts with a carbonyl group?

Answer 1

a yellow/orange precipitate is produced

Question 2

how can you find the identity of a carbonyl using 2,4 DNP?

Answer 2

react with 2,4 DNP to form a oranger precipitate

recrystallise precipitate and obtain the melting point

compare melting point with known values

Question 3

what is Tollen’s reagent?

Answer 3

silver nitrate in aqueous ammonia

Question 4

what happens when Tollen’s reagent reacts with an aldehyde?

Answer 4

a carboxylic acid is formed

a silver mirror is formed

Question 5

what is a silver mirror?

Answer 5

silver precipitate formed when Ag+ ions are reduced to form Ag

Question 6

what happens when Tollen’s reagent reacts with an ketone?

Answer 6

nothing

Question 7

what is acidified potassium dichromate?

Answer 7

K₂Cr₂O₇/H₂SO₄

Question 8

what happens to acidified potassium dichromate when it reacts with an aldehyde?

Answer 8

colour change of chromium ions from orange to green

formation of a carboxylic acid

Question 9

what happens to acidified potassium dichromate when it reacts with an ketone?

Answer 9

nothing as a ketone cannot be oxidised