CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.5.1 Production of Aldehydes & Ketones

Production of Aldehydes & Ketones

  • Aldehydes and ketones are carbonyl compounds containing a C=O group
  • They can be prepared from the oxidation of primary and secondary alcohols respectively

Oxidising agents

  • The oxidising agents used to prepare aldehydes and ketones from alcohols include acidified potassium dichromate (K2Cr2O7) and acidified potassium manganate (KMnO4)
  • Acidified with dilute sulfuric acid, potassium dichromate(VI), K2Cr2O7, is an orange oxidising agent
    • When the alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions
  • Acidified with dilute sulfuric acid, potassium manganate(VII), KMnO4 is a purple oxidising agent
    • When the alcohols are oxidised the purple manganate ions (MnO4) are reduced to colourless Mn2+ ions

 

Carbonyl Compounds Oxidising Agents, downloadable AS & A Level Chemistry revision notes

The oxidising agents change colour when they oxidise an alcohol and get reduced themselves

Synthesis of aldehydes

  • To make an aldehyde, warm primary alcohol is slowly added to the oxidising agent
  • The formed aldehyde has a lower boiling point than the alcohol and can therefore be distilled off as soon as it forms
  • The aldehyde is then condensed into a liquid and collected

Carbonyl Compounds Synthesis of Aldehydes, downloadable AS & A Level Chemistry revision notes

Aldehydes are formed from the oxidation of primary alcohols

Synthesis of ketones

  • To make a ketone, warm secondary alcohol is slowly added to the oxidising agent
  • Since the formed ketone cannot be further oxidised it does not need to be distilled off straightaway after it has been formed

 

Carbonyl Compounds Synthesis of Ketones, downloadable AS & A Level Chemistry revision notes

Ketones are formed from the oxidation of secondary alcohols

Exam Tip

If the aldehyde formed is not distilled off, further refluxing with excess oxidising agent will oxidise the aldehyde to a carboxylic acid.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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