10.2.10 Alcohols - Oxidation

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

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 (Cr₂O_7^2-) 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 

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

The test for primary and 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

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

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

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