26.2 Identifying Aldehydes And Ketones

Question 1

How is the presence of carbonyl group tested?

Answer 1

Use of 2,4 DNP (Brady’s reagent)

Question 2

Positive result for carbonyl group

Answer 2

When 2,4 DNP is added, yellow/orange precipitate is formed

Question 3

What is the compound formed in a positive 2,4 DNP test?

Answer 3

2,4 - dinitrophenylhhydrazone

Question 4

How is 2,4 DNP made to test for carbonyls?

Answer 4

Dissolved in methanol and sulfuric acid
Forms pale orange solution called Brady’s reagent

Question 5

Why is 2,4 DNP not used as a solid?

Answer 5

It’s hazardous, friction or sudden blow can cause an explosion

Question 6

How to distinguish between aldehydes and ketones

Answer 6

Using Tollen’s reagent
If aldehyde is present, a silver mirror is formed

Question 7

How to make Tollen’s reagent?

Answer 7

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

Question 8

Testing for aldehydes with Tollen’s Reagent

Answer 8

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

Question 9

Why does Tollen’s Reagent work?

Answer 9

Ag + ions act as an oxidising agent