AQA A Level Chemistry复习笔记8.3.3 Testing Aldehydes & Ketones

Distinguishing Between Aldehydes & Ketones

• Weak oxidising agents can be used to distinguish between an aldehyde and a ketone
  • The aldehyde will be oxidised to a carboxylic acid, but the ketone will not undergo oxidation

   

Using Tollens’ Reagent – The Silver Mirror Test

• Tollens’ reagent contains the silver(I) complex ion [Ag(NH3)2]+
• This is formed when aqueous ammonia is added to a solution of silver nitrate
  • Tollens’ reagent is also known as ammoniacal silver nitrate

   

• 2cm depth of 0.1 mol dm-3 silver nitrate solution is added to a test tube
• Drops of 2.0 mol dm-3 sodium hydroxide are added until a brown precipitate of silver(I) oxide appears
• Concentrated ammonia is then added drop by drop until the brown precipitate just disappears and the reagent is ready to use
• A few drops of the suspected aldehyde are added to the test tube
• The test tube is gently warmed with Tollens’ reagent
• The silver(I) complex ion solution, [Ag(NH3)2]+, is colourless
• As the aldehyde is oxidised, it causes the [Ag(NH3)2]+ ions to become reduced to solid metallic silver, Ag
• This is why a positive test result is called a “silver mirror”

The Ag+ ions in Tollens’ reagent are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms

Practical tip

• Always make sure you use very clean glassware for a Tollens' test to ensure you get the best possible result