26.2 Identifying aldehydes and ketones

Question 1

What is used to detect the presence of a carbonyl group (ketone or aldehyde)

Answer 1

2,4-DNP

Question 2

What is a positive result for the test for carbonyl group with 2,4-DNP?

Answer 2

yellow/orange precipitate formed

Question 3

What is a reagents and conditions used to test for carbonyl group with 2,4-DNP?

Answer 3

2,4-DNP dissolved in methanol and sulfuric acid to make pale orange solution called Bradys reagent

Question 4

Why is 2,4-DNP unsafe to use?

Answer 4

hazardous, friction or a sudden blow can cause it to explode

Question 5

Steps for the test of a carbonyl group (ketone or aldehyde)

Answer 5

1. add 5cm depth of 2,4-DNP to a test tube
2. add three drops of compound using a dropping pipettes
3. if no crystals formed, add a few drops of sulfuric acid
4. a yellow/orange precipitate will form in presence of a carbonyl group

Question 6

What is used to distinguish between carbonyl compounds

Answer 6

-tollens reagent

Question 7

What is the positive test for the tollens reagent test

Answer 7

aldehyde - silver mirror formed
ketone - no silver mirror

Question 8

What are the reagents and conditions required for the tollens reagent test

Answer 8

-silver nitrate
-aqueous ammonia

Question 9

What are the steps for the tollens reagent test

Answer 9

1. pour 2cm of unknown solution into clean test tube
2. add an equal volume of freshly prepared tollens reagent
3. 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

Question 10

How can you identify an aldehyde or ketones by melting point?

Answer 10

1. filter the yellow/orange precipitate from 2,4-DNP test to separate the solid precipitate from the solution
2. solid is then recrystallised to produce a pure sample of crystals
3. the melting point of the purified 2,4-DNP is measured and compared to the data base

Question 11

What is tollens reagent?

Answer 11

A solution of silver nitrate in aqueous ammonia

Question 12

What is a positive result of tollens reagent if an aldehyde is present?

Answer 12

Silver mirror produced

Question 14

What are the steps for making tollens reagent?

Answer 14

1. add 3cm^3 depth of aqueous silver nitrate (AgNO3)
2. add aqueous sodium hydroxide to the silver nitrate until a brown precipitate is formed
3. add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution (this is tollens reagent)

Question 15

How does tollens reagents work?

Answer 15

-Contain silver ions (Ag+) which act as an oxidising agent in the presence of ammonia
-Silver ions are reduced to silver as the carbonyl group is oxidised

Question 16

What is the equation for the oxidation and reduction that takes place in the reaction between an aldehyde and silver ions?

Answer 16