aldehydes and ketones

Question 1

How are aldehydes prepared?

Answer 1

Aldehydes are made by oxidising primary alcohols using acidified potassium dichromate. They are prepared using distillation, where the oxidation product is immediately removed through evaporation and condensation.

Question 2

What happens during reflux of primary alcohols?

Answer 2

Repeated cycles of evaporation and condensation through reflux would further oxidise the aldehyde into a carboxylic acid.

Question 3

How are ketones prepared?

Answer 3

Ketones are prepared by refluxing secondary alcohols with acidified potassium dichromate.

Question 4

What is the reverse reaction of aldehydes and ketones?

Answer 4

The reverse of the oxidation reaction can be carried out using a reducing agent such as sodium borohydride (NaBH4).

Question 5

What do aldehydes and ketones reduce to?

Answer 5

Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols.

Question 6

What type of reaction occurs during the reduction of carbonyl compounds?

Answer 6

This is a nucleophilic addition reaction with the hydride ion acting as a nucleophile.

Question 7

What is a nucleophile?

Answer 7

Nucleophiles are species with a lone pair of electrons that are attracted to positive charges, such as the slightly positive carbon on the carbonyl bond.

Question 8

What are the steps in the reduction of aldehydes?

Answer 8

1. The hydride nucleophile attacks the slightly positive carbon atom on the carbonyl functional group.
2. The double bond of the C=O group breaks, forming a negatively charged intermediate.
3. The lone pair of electrons on the oxygen attacks a water molecule, forming the alcohol and a hydroxide ion.

Question 9

How does hydrogen cyanide react with carbonyl compounds?

Answer 9

Hydrogen cyanide reacts with aldehydes and ketones where the cyanide ion acts as the nucleophile, forming a hydroxynitrile.

Question 10

What are the steps in the reaction with cyanide?

Answer 10

1. The cyanide nucleophile attacks the slightly positive carbon atom on the C=O group.
2. The double bond breaks, forming a negatively charged intermediate.
3. The lone pair of electrons on the oxygen donates to a hydrogen ion to form a hydroxyl group, resulting in a hydroxynitrile.

Question 11

What is used in the lab instead of hydrogen cyanide?

Answer 11

In the lab, sodium cyanide is used instead of hydrogen cyanide as it is less toxic.

Question 12

What test is used for the presence of carbonyl compounds?

Answer 12

Test for the carbonyl functional group (C=O) using 2,4-dinitrophenylhydrazine (2,4-DNP).

Question 13

How is the test for carbonyl compounds performed?

Answer 13

2,4-DNP is dissolved in methanol and concentrated sulfuric acid is added to the unknown sample. An orange precipitate indicates the presence of a ketone or aldehyde.

Question 14

How can you identify the carbonyl compound?

Answer 14

Different aldehydes and ketones have established crystalline melting points. Crystallise the compound and record its melting point to compare with published data.

Question 15

What test distinguishes between aldehydes and ketones?

Answer 15

Add Tollens’ reagent to the unknown sample. It forms a silver mirror if an aldehyde is present; nothing will be seen if the sample is a ketone.

Question 16

What does Tollens’ reagent contain?

Answer 16

Tollens’ reagent contains silver nitrate dissolved in ammonia solution, which oxidises aldehydes into carboxylic acids.

Question 17

Why do ketones not react with Tollens’ reagent?

Answer 17

Ketones cannot be further oxidised, which is why they do not give a result in this test.