5- Isomerism and Carbonyl Compounds

Question 1

Describe the test for aldehydes and ketones using Tollens’ reagent

Answer 1

Aldehyde- a silver mirror will form as the aldehyde is oxidised and the diaminesilver ions are reduced
Ketone- no reaction

Question 2

What is Tollens’ reagent made from?

Answer 2

A solution of silver nitrate dissolved in aqueous ammonia

Question 3

What is Fehling’s solution made from?

Answer 3

Copper (II) dissolved in NaOH

Question 4

Describe the test for aldehydes and ketones using Fehling’s solution

Answer 4

Aldehyde- the copper ions are reduced to form a brick red precipitate
Ketone- no reaction and the solution stays blue

Question 5

What is the reducing agent that it used to reduce an aldehyde/ ketone into a primary/secondary alcohol?

Answer 5

NaBH4

Question 6

Name the mechanism that is used to turn an aldehyde back into a primary alcohol

Answer 6

Nucleophilic addition

Question 7

Describe the conditions needed for nucleophilic addition of an aldehyde

Answer 7

NaBH4 in water with methanol

Question 8

What is the difference between an aldehyde and a ketone?

Answer 8

The carbonyl functional group is a the end of the carbon chain in an aldehyde, but it is in the middle of the chain in the ketone

Question 9

What type of compound is produced when an aldehyde is oxidised?

Answer 9

A carboxylic acid

Question 10

Describe the structure of an hydroxynitrile

Answer 10

Organic compounds that contain both an hydroxyl group (OH) and a nitrile group (CN)

Question 11

How can hydroxynitriles be formed?

Answer 11

From reacting aldehydes or ketones with KCN and a dilute acid at 25 degrees c

Question 12

What mechanism is used to make an hydroxynitrile from an aldehyde?

Answer 12

Nucleophilic addition- elimination

Question 13

Why does the nucleophilic addition- elimination of an aldehyde with KCN and a dilute acid make more than one type of product?

Answer 13

The C=O double bond is planar
So when nucleophiles attack, they can attack above or below the plane of the bond
Depending on which way the nucleophile attacks from, a different enantiomer will form
Usually, attack from both sides is equally likely, so a racemic mixture is formed

Question 14

What are the hazards associated with KCN? and what safety precautions should be taken?

Answer 14

KCN is toxic. Any reaction using KCN should be done in a fume cupboard. If heating KCN, this should be done in a water bath rather than an open flame.