• 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