Identifying Aldehydes And Ketones Flashcards
What solution is used to detect the presence of the carbonyl functional group in both aldehydes and ketones?
2,4-dinitrophenylhydrazine (2,4-DNPH).
What precipitate is formed when a carbonyl group is present with 2,4-DNPH?
A yellow or orange precipitate.
How is 2,4-DNPH usually used in practical work? Why?
Used dissolved in methanol and sulfuric acid as a pale orange solution called Brady’s reagent. This is because solid 2,4-DNPH can be very hazardous because friction or a sudden blow can cause it to explode.
What is the method for testing for a carbonyl group in aldehydes and ketones?
- Add 2,4-DNPH solution to a clean test tube. This is in excess.
- Using a dropping pipette, add three drops of the unknown compound and leave to stand.
- If no crystals form, add a few drops of sulfuric acid.
- A yellow/orange precipitate indicates the presence of an aldehyde or a ketone.
How do carbonyl groups react with 2,4-DNPH?
In a nucleophillic addition in an elimination reaction. It is a condensation reaction as water is eliminated.
What is the equation of reaction a carbonyl group with 2,4- DNPH?
2,4-DNPH + ketone/aldehyde = product + water.
Once establishing that a carbonyl group is present using 2,4-DNPH, what can be used to test for whether it is an aldehyde or a ketone?
A fresh sample of the compound can be tested using Tollens’ reagent.
What is Tollens’ reagent?
A solution of silver nitrate in aqueous ammonia.
What is produced if Tollens’ reagent is mixed with an aldehyde?
A silver mirror.
What is the method for testing for the aldehyde group with Tollens’ reagent?
MAKING TOLLENS’ REAGENT:
1. In a clean test tube, add aqueous silver nitrate.
2. Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide is formed.
3. Add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution. This is Tollens’ reagent.
TEST FOR AN ALDEHYDE:
1. Pour unknown solution into a clean test tube.
2. Add an equal volume of freshly prepared Tollens’ reagent.
3. Leave the test tube to stand in a beaker of warm water for about 50C for about 10 to 15 minutes and then observe whether any silver mirror is formed.
Why does Tollens’ reagent need to be prepared just before use?
Because it has a short-half life.
How does oxidation and reduction happen in the reaction between an aldehyde and silver ions?
Tollens’ reagent contains silver ions which act as an oxidising agent in the presence of ammonia so silver ions are reduced to silver and the aldehyde is oxidising to a carboxylic acid.
What are the reactions when Tollens’ reagent is added to an aldehyde? (2)?
Ag+ (aq) + e- = Ag(s) (silver mirror).
Aldehyde + [O] = Carboxylic Acid.
How else can you identify an aldehyde or ketone?
By their melting points.
What is the method of identifying an aldehyde or a ketone by their melting points?
- The impure yellow/orange solid, from the reaction between 2,4-DNPH and a carbonyl group, is filtered to separate the solid precipitate from the solution.
- The solid is then recrystallised to produce a pure sample of crystals.
- The melting point of the purified 2,4-DNPH is measured and recorded.
- The melting point is then compared to a database or a data table of melting points to identify the original carbonyl compound.
What happens when a ketone reacts with Tollens’ reagent?
Nothing happens.
