Identifying aldehydes and ketones 26.2

Question 1

What chemical is used to detect the presence of the carbonyl functional group?

Answer 1

2-4 DNP, also known as Brady’s reagent

Question 2

What would you see if 2-4 DNP was in the presence of a carbonyl group?

Answer 2

A yellow/orange precipitate is produced

Question 3

How is 2-4 DNPH used in practicals?

Answer 3

1. Dissolved in methanol and sulfuric acid

2. To form pale orange solution known as Brady’s reagent

Question 4

Why is solid 2-4 DNP not used?

Answer 4

It can be very hazardous

Friction or a sudden blow can cause it to explode

Question 5

What is tollens reagent ?

Answer 5

A solution of silver nitrate in aqueous ammonia

Question 6

What is tollens reagent used for ?

Answer 6

To help classify unknown carbonyls as either aldehyde or ketone.

Question 7

Why should tollens be made immediately before the test?

Answer 7

It has a short shelf life

Question 8

In presence of which type of carbonyl is a silver mirror produced?

Answer 8

An aldehyde

Question 9

How do you make tollens reagent?

Answer 9

1. In a test tube add aqueous silver nitrate
2. Then add sodium hydroxide (aq)
3. Until brown precipitate forms (Ag2O)
4. Add dilute ammonia solution until brown ppt dissolves

Question 10

What changes would you observe if you added tollens reagent to a solution containing a ketone?

Answer 10

None, ketones do not react

Question 11

How is the silver mirror produced?

Answer 11

T Reagent contains silver ions which act as an oxidising agent in the presence of ammonia.
In the reaction silver ions are reduced to silver as aldehyde is oxidised to carboxylic acid