Module 6

Question 1

Test to detect carbonyl functional group

Answer 1

2,4-DNP

Question 2

Test to identify specific compounds (of carbonyls)

Answer 2

Melting point of 2,4-DNP derivative

Question 3

Test to distinguish between aldehydes and ketones

Answer 3

Tollens’ Reagent

Question 4

Testing for carbonyl group with 2,4-DNP

Answer 4

2,4-DNP dissolved in methanol and sulfuric acid combined with few drops of unknown compound. Yellow/orange ppt is a positive result, indicates presence of aldehyde or ketone.

Add 5cm depth of 2,4-DNP to test tube. This is in excess
Add 3 drops of unknown compound using dropping Pipette
Leave to stand
If no crystals form, add sulfuric acid
Yellow/orange ppt indicates presence of aldehyde or ketone

Question 5

How to determine exact carbonyl compound in an experiment?

Answer 5

Filter and recrystallise 2,4-DNP derivative
Measure and record melting point
Compare to database of melting pints to identify the compound

Question 6

How to use tollens reagent to distinguish between aldehydes and ketones

Answer 6

Tollens reagent= solution of silver nitrate in aqueous ammonia.
Silver ions reduced to silver metal, hence silver mirror formed in presence of aldehyde.

Tollens’ reagent contains Ag+ ions, which act as an oxidising agent in presence of ammonia. Reduced to silver metal as the aldehyde is oxidised to a carboxylic acid.

Method:
Add equal volume of tollens reagent and unknown compound into test tube.
Leave to stand for 10-15 mins in water bath at 50 degrees
Observe if silver mirror formed