AQA A Level Chemistry

Revision Notes

7.2.1 Aldehydes & Ketones

Oxidation of Aldehydes

 

 

  • The carbonyl group in an aldehyde is always situated at the end of the chain
    • When naming aldehydes, you do not include the ‘1’ in the name, the carbonyl carbon is always number 1 on the chain
  • The simplest aldehyde is methanal, with the only carbon being that of the carbonyl group
  • The carbonyl group in a ketone is always situated in the middle of the chain
  • The simplest ketone is propan-2-one, as you need an alkyl group either side of the carbonyl carbon in a ketone

Preparation of Aldehydes & Ketones

  • Aldehydes and ketones can be prepared by oxidising primary and secondary alcohols as shown below

 

 

Further Oxidation

  • During the oxidation of a primary alcohol to an aldehyde, the apparatus must be set up to distill off the aldehyde as it is produced
  • Further oxidation of primary alcohols can then take place
    • Aldehydes can be easily oxidised to form carboxylic acids
  • To oxidise a primary alcohol straight to a carboxylic acid, you would heat the reaction mixture under reflux
    • The aldehyde would still be produced, but as it evaporates it would condense and drop back into the reaction mixture, to be further oxidised to the carboxylic acid
  • The oxidising agent used for all of the oxidation reactions be acidified potassium dichromate
    • K2Cr2O7 with sulfuric acid, H2SO4
  • Ketones are very resistant to being oxidised, so no further oxidation reaction will take place with secondary alcohols
    • This is because ketones do not have a readily available hydrogen atom, in the same way that aldehydes (or alcohols) do
    • An extremely strong oxidising agent would be needed for oxidation of a ketone to take place
    • The oxidation will likely oxidise a ketone in a destructive way, breaking a C-C bond

Exam Tip

In the exam you can simply say that ketones cannot be oxidised!

Distinguishing Between 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
  • There are a number of tests that can be used to distinguish between aldehydes and ketones
  • You specifically need to know the following methods to distinguish between an aldehyde and a ketone:
    • Tollens’ reagent (this is the most commonly used method)
    • Fehling’s solution

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
  • If gently warmed with Tollens’ reagent, an aldehyde will become oxidised
  • 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

Carbonyl Compounds Tollens Reagent, downloadable AS & A Level Chemistry revision notes

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

Using Fehling’s Solution 

  • Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent
  • If an aldehyde is warmed with Fehling’s solution, the aldehyde will be oxidised and a colour change will take place
  • Fehling’s solution is blue, because of the copper(II) complex ions present
  • During the reaction, as the aldehyde is oxidised to a carboxylic acid, the blue Cu2+ ions are reduced to Cu+ ions and a brick red precipitate is formed
  • The brick red precipitate is copper(I) oxide
  • If a ketone is warmed with Fehling’s solution, no reaction takes place as the ketone will not be oxidised, so the solution will remain blue

Carbonyl Compounds Fehlings Solution, downloadable AS & A Level Chemistry revision notes

The copper(II) ions in Fehling’s solution are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to copper(I) ions in the Cu2O precipitate

  • Heating with acidified potassium dichromate could also be used to distinguish between an aldehyde and a ketone
    • The aldehyde would be oxidised, and you would see an orange to green colour change
    • The ketone would not be oxidised, so you would see no colour change

Summary of the Oxidation Reactions Table

7.2.1 Summary of oxidation reactions of alcohols and carbonyls, downloadable AS & A Level Chemistry revision notes

Exam Tip

You are expected to know all of the above methods which can be used to distinguish between an aldehyde and a ketone! However, Tollen’s reagent is the most commonly used method, if trying to identify an unknown sample for example.

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