26.2 Identifying Aldehydes And Ketones
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How is the presence of carbonyl group tested?
Use of 2,4 DNP (Brady’s reagent)
Positive result for carbonyl group
When 2,4 DNP is added, yellow/orange precipitate is formed
What is the compound formed in a positive 2,4 DNP test?
2,4 - dinitrophenylhhydrazone
How is 2,4 DNP made to test for carbonyls?
Dissolved in methanol and sulfuric acid Forms pale orange solution called Brady’s reagent
Why is 2,4 DNP not used as a solid?
It’s hazardous, friction or sudden blow can cause an explosion
How to distinguish between aldehydes and ketones
Using Tollen’s reagent If aldehyde is present, a silver mirror is formed
How to make Tollen’s reagent?
It has a short shelf life Add AgNO3 and NaOH to form a brown ppt of Ag2O Add dilute ammonia until brown precipitate dissolves to become colourless
Testing for aldehydes with Tollen’s Reagent
Use 2cm3 of unknown solution Add equal volume of Tollen’s reagent Leave the test tube in 50oC for 10-15 minutes and observe if a silver mirror is formed
Why does Tollen’s Reagent work?
Ag + ions act as an oxidising agent