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Alcohols - Oxidation (DP IB Chemistry: SL)

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Oxidation of Primary Alcohols

  • The products of oxidation of alcohols depends on the class of alcohols
  • Here is a reminder of the three classes of alcohols:

Hydroxy Compounds Classifying Alcohols, downloadable AS & A Level Chemistry revision notes

The three classes of alcohols

Primary alcohols

  • Primary alcohols can be oxidised to form aldehydes which can undergo further oxidation to form carboxylic acids
  • The oxidising agents of alcohols include acidified K2Cr2O7 or acidified KMnO4
  • Acidified potassium dichromate(VI), K2Cr2O7, is an orange oxidising agent 
    • Acidified means that that the potassium dichromate(VI) is in a solution of dilute acid (such as dilute sulfuric acid) 
    • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced
      • When alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions

  • Acidified potassium manganate(VII), KMnO4, is a purple oxidising agent 
    • As with acidified KMnO4 the potassium manganate(VII) is in an acidic medium to allow reduction of potassium manganate(VII) to take place
      • When alcohols are oxidised, the purple manganate ions (MnO4-) are reduced to colourless Mn2+ ions 

Carboxylic Acids & Derivatives Oxidation of Primary Alcohols and Aldehydes, downloadable AS & A Level Chemistry revision notes

Further Oxidation

  • If the aldehyde is not distilled off, further oxidation with excess oxidising agent will oxidise it to a carboxylic acid
  • The reaction takes some time to complete and requires sustained heating

Test for alcohols

  • The oxidation using acidified dichromate provides the basis for the test for alcohols as the reaction gives a strong colour change from orange to green
  • Unfortunately, it does not work for tertiary alcohols, which cannot be oxidised

Hydroxy Compounds Tests Alcohols, downloadable AS & A Level Chemistry revision notes

The test for primary and secondary alcohols

Oxidation of Secondary Alcohols

  • Secondary alcohols can be oxidised to form ketones only
  • To get a good yield of the ketone the reaction requires some sustained heating

Hydroxy Compounds Oxidation of Secondary Alcohols, downloadable AS & A Level Chemistry revision notes

Oxidation of Secondary Alcohols

  • Tertiary alcohols do not undergo oxidation
    • This is because there must be a hydrogen on the functional group carbon, which breaks off to form water
    • There are only C-C bonds on the functional group carbon in a tertiary alcohol

Distillation & Reflux

The difference between using distillation and heating under reflux

  • To  produce an aldehyde from a primary alcohol the reaction mixture must be heated
  • The aldehyde product has a lower boiling point than the alcohol ( since it has lost the H-bonding) so it can be distilled off as soon as it forms 
  • Distillation can be carried out using a simple side arm arrangement which acts as an air condenser or the vapours can be made to pass through a condenser

Hydroxy Compounds Oxidation of Primary Alcohols, downloadable AS & A Level Chemistry revision notes

Oxidation of ethanol by acidified K2Cr2O7 to form an aldehyde by distillation

Heating under reflux

  • For reactions that require sustained heating the apparatus has to be modified
  • To prevent loss of volatile reactants the apparatus includes a condenser in the vertical position which returns components back into the reaction flask
  • This is known as heating under reflux (reflux means re-boiling)

Heating under Reflux Apparatus

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Stewart

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Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.