Oxidation of Alcohols (DP IB Chemistry)

Oxidation of Alcohols

• 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

Classification of primary, secondary and tertiary 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 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 the reduction of potassium manganate(VII) to take place

    • When alcohols are oxidised, the purple manganate ions (MnO₄^-) are reduced to colourless Mn²⁺ ions 

Diagram to show the oxidation of of primary alcohols

A primary alcohol can be oxidised to an aldehyde and then carboxylic acid

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 potassium dichromate(VI) 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

When primary, secondary and tertiary alcohols are reacted with acidified potassium dichromate(VI), primary and secondary alcohols cause a colour change from orange to green. There is no reaction with tertiary alcohols so the colour remains orange

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

Oxidation of secondary alcohols

Secondary alcohols oxidise to form ketones

• 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

Distillation

• 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

Diagram of distillation apparatus

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)

Diagram of reflux apparatus

Heating under reflux requires the condenser to be set up vertically