17 Carbonyl compounds (aldehydes and ketones)

Question 1

Production of aldehydes and ketones

Answer 1

1) The oxidation of primary alcohols using acidified K2Cr2O7 or acidified KMnO4 and distillation to produce aldehydes
2) The oxidation of secondary alcohols using acidified K2Cr2O7 or acidified KMnO4 and distillation to produce ketones

Question 2

Acidified potassium dichromate (K2Cr2O7)

Answer 2

• acidified with dilute sulfuric acid
• when alcohols are oxidised the orange dichromate ions (2-) are reduced to green Cr 3+ ions

Question 3

Acidified potassium permanganate (KMnO4)

Answer 3

• acidified with dilute sulfuric acid
• when alcohols are oxidised the purple manganate ions (4-) are reduced to colourless Mn2+ ions

Question 4

To make an aldehyde

Answer 4

• warm primary alcohol is slowly added to the oxidising agent
• formed aldehydes have a lower b.p. than the alcohol and can therefore be distilled off as soon as it forms
• aldehyde is condensed into a liquid and collected

Question 5

What to do if aldehyde is not distilled off

Answer 5

• further refluxing can be done with excess oxidising agent to oxidise the aldehyde to a carboxylic acid

Question 6

To make a ketone

Answer 6

• warm secondary alcohol is slowly added to the oxidising agent
• since the ketone cannot be further oxidised, it does not need to be distilled off straight away after it has been formed

Question 7

Reactions of aldehydes and ketones

Answer 7

1) Reduction (aldehydes to primary alcohols and ketones to secondary alcohols)
2) Nucleophilic addition of HCN
3) Reaction of HCN

*drawings in notes

Question 8

Tests

Answer 8

1) Fehling’s solution- aldehydes
2) Tollens reagent- aldehydes
3) 2,4-DNPH- aldehydes and ketones + undergo condensation

Question 9

Fehling’s solution ( alkaline solution containing copper(II) ions)
- used to detect presence of
-CHO group

Answer 9

• clear blue solution turns opaque red/orange precipitate
• Cu2+ is reduced to Cu+
• aldehyde is oxidised to carboxylic acid
• ketones cannot be oxidised therefore gives negative result

Question 10

Tollens reagent (aq alkaline solution of silver nitrate in excess ammonia solution)
- used to detect presence of
-CHO group

Answer 10

• Silver mirror of Ag atoms
• Ag+ is reduced to Ag
• aldehyde is oxidised to carboxylic acid
• ketones cannot be oxidised therefore gives negative result

Question 11

2,4-DNPH
- detects the presence of carbonyl compounds which have C=O group

Answer 11

• orange precipitate (can be purified by recrystallisation)

Question 12

Iodoform reaction

Answer 12

• Tri-iodomethane forms a yellow precipitate with methyl ketones
• ethanal also contains the above group and also forms yellow precipitate
• the reagent is heated with an alkaline solution of iodine and a sodium salt + tri-iodomethane is formed (yellow precipitate)

*drawing in notes