3.3.8 Aldehydes and Ketones

Question 1

Testing for aldehydes and ketones - oxidation

Answer 1

Aldehydes are readily oxidised to carboxylic acids whilst ketones are not

Question 2

Testing for aldehydes and ketones - Tollen’s reagent (4)

Answer 2

Colourless solution of silver nitrate dissolved in aqueous ammonia
Aldehyde - forms silver mirror if heated
Ketones - no reaction
Ag(NH3)2+ + e- > Ag + 2NH3

Question 3

Testing for aldehydes and ketones - Fehling’s solution (4)

Answer 3

Blue solution of copper(II) ions dissolved in sodium hydroxide
Aldehyde - forms brick-red precipitate if heated
Ketone - no reaction
Cu2+ + e- > Cu+

Question 4

Reducing aldehydes and ketones (4)

Answer 4

Use NaBH4 (aq) as the reducing agent
Aldehydes reduce to a primary alcohol
Ketones reduce to a secondary alcohol
Nucleophilic addition - H- ion from reducing agent attacks polar carbon, lone pair on oxygen attacks H+ ion

Question 5

What is a hydroxynitrile?

Answer 5

Molecules containing an alcohol group and a nitrile group

Question 6

How are hydroxynitriles produced? (2)

Answer 6

Nucleophilic addition

React aldehyde/ketone with KCN and dilute acid

Question 7

Why is a racemic mixture of hydroxynitriles produced in nucleophilic addition? (3)

Answer 7

The C=O bond is planar so the nucleophiles can attack above or below
Attack from either side produces a different enantiomer
If attack from both sides is equally likely a racemic mixture forms

Question 8

What are the hazards of using KCN? (2)

Answer 8

Toxic

Can release HCN gas when reacting with a carbonyl compound