26.2 Identifying aldehydes and ketones Flashcards
What is used to detect the presence of a carbonyl group (ketone or aldehyde)
2,4-DNP
What is a positive result for the test for carbonyl group with 2,4-DNP?
yellow/orange precipitate formed
What is a reagents and conditions used to test for carbonyl group with 2,4-DNP?
2,4-DNP dissolved in methanol and sulfuric acid to make pale orange solution called Bradys reagent
Why is 2,4-DNP unsafe to use?
hazardous, friction or a sudden blow can cause it to explode
Steps for the test of a carbonyl group (ketone or aldehyde)
- add 5cm depth of 2,4-DNP to a test tube
- add three drops of compound using a dropping pipettes
- if no crystals formed, add a few drops of sulfuric acid
- a yellow/orange precipitate will form in presence of a carbonyl group
What is used to distinguish between carbonyl compounds
-tollens reagent
What is the positive test for the tollens reagent test
aldehyde - silver mirror formed
ketone - no silver mirror
What are the reagents and conditions required for the tollens reagent test
-silver nitrate
-aqueous ammonia
What are the steps for the tollens reagent test
- add 3cm^3 depth of aqueous silver nitrate (AgNO3)
- add aqueous sodium hydroxide to the silver nitrate until a brown precipitate is formed
- add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution (this is tollens reagent)
- pour 2cm of unknown solution into clean test tube
- add an equal volume of freshly prepared tollens reagent
- leave the test tube to stand in a warm water bath of about 50°C for 10 to 15 minutes and observe whether a silver mirror is formed
How can you use the 2,4-DNP test to distinguish between aldehydes and ketones
- filter the yellow/orange precipitate to separate the solid precipitate from the solution
- solid is then recrystallised to produce a pure sample of crystals
- the melting point of the purified 2,4-DNP is measured and compared to the data base
