Testing Aldehydes & Ketones (AQA A Level Chemistry)

Tollens' Reagent Testing

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”

Positive Test Result:

• When Tollens’ reagent is gently warmed with an aldehyde, the silver mirror is formed

  • This is the positive test result

• When Tollens’ reagent is gently warmed with a ketone, no silver mirror will be seen, as the ketone cannot be oxidised by Tollens’ reagent, so no reaction takes place

  • This is a negative test result

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