CIE AS Chemistry

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First teaching 2020

Last exams 2024

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3.5.1 Production of Aldehydes & Ketones

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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.

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Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.