26.2 identifying aldehydes and ketones

Question 1

how do you make Tollens’ reagent?

Answer 1

add aq silver nitrate
add aq sodium hydroxide until brown precipitate of silver oxide is formed
add dilute ammonia solution until brown precipitate dissolves

Question 2

Tollens’ reagent test

Answer 2

add equal volume of freshly prepared Tollens’ reagent to unknown solution
leave test tube to stand in a beaker of warm water at 50 degrees for 10-15 mins

Question 3

what is a positive test for aldehyde for Tollens’ reagent test?

Answer 3

silver mirror

Question 4

why is a silver mirror formed when Tollens’ reagent is reacted with aldehyde?

Answer 4

silver is reduced
aldehyde oxidised

Question 5

2,4- DNP test

Answer 5

add excess of 2,4-DNP
add unknown solution
if no crystals form, add a few drops of sulphuric acid
yellow/ orange precipitate= aldehyde or ketone

Question 6

2,4-DNP melting point test

Answer 6

yellow/orange precipitate is filtered to separate solid from solution
solid recrystallised to produce a pure sample of crystals
melting point of purified 2,4-DNP is measured and recorded
melting point is compared to a database to identify original carbonyl compounds

Question 7

Fehling’s solution test

Answer 7

add a few drops of unknown solution
warm the mixture in a hot bath
ketone= no change in blue solution
aldehyde= blue solution produces a dark red precipitate of copper (I) oxide