Testing for Aldehydes and Ketones

Question 1

What chemically is tollen’s reagent?

Answer 1

• Contains silver ion complex Ag(NH3)2 +
• (silver nitrated + aqueous ammonia)
• Colourless solution

Question 2

How does tollen’s reagent work?

Answer 2

• Tests for Aldehydes
• Gently warmed with aldehyde
• Aldehyde is oxidised
• Ag(NH3)2 + ions reduced to solid metallic silver Ag
• Silver mirror is formed if aldehyde present
• Ketone, no reaction, cannot be oxidised by tollen’s reagent

Question 3

What is Fehling’s solution

Answer 3

• Alkaline solution containing Copper (II) ions
• Blue from Copper (II) ions

Question 4

How does Fehling’s solution work?

Answer 4

• Tests for aldehydes
• Aldehyde oxidised
• Becomes carboxylic acid
• Cu 2+ ions (blue) reduced to Cu + ions and copper oxide forms (brick red)
• If ketone reacts, nothing happens, not oxidised, so remains blue

Question 5

How can acidified potassium dichromate be used to distinguish between aldehydes and ketones?

Answer 5

• Aldehyde oxidised, orange to green
• Ketone not oxidised, no colour change