6.3.8 Oxidation of alcohols

What is the oxidising agent in the oxidation of alcohols?

Alcohols can be oxidised using acidified potassium dichromate(VI).

acidified using dilute sulfuric acid.

What can the oxidising agent be represented as in equations?

The oxidising agent can be represented as [O] in equations.

What is the first stage in oxidising primary alcohols?

Stage 1: A primary alcohol is oxidised to an aldehyde.

What is the second stage in oxidising primary alcohols?

Stage 2: The aldehyde is oxidised further to a carboxylic acid.

What are the Conditions for oxidation of primary alcohols

To obtain an aldehyde:

• Use distillation.

• The aldehyde is volatile and has a relatively low boiling point.

• It distils off as soon as it forms.

• This prevents further oxidation into a carboxylic acid.

To obtain a carboxylic acid:

• Heat under reflux.

• Use excess acidified potassium dichromate(VI).

• The reflux condenser cools vapours and returns them to the reaction flask.

• This allows the aldehyde to remain in contact with the oxidising agent and undergo further oxidation into a carboxylic acid.

What are secondary alcohols oxidised to?

Secondary alcohols are oxidised to ketones.

What is the test for aldehydes?

• Fehling's solution and Benedict's solution can be used to test for aldehydes.

• Both contain copper(II) ions, which give the solutions their blue colour.

• When warmed with an aldehyde, the aldehyde is oxidised and copper(II) ions are reduced to copper(I) ions.

• An orange-brown precipitate of copper(I) oxide, Cu2O forms.

What is observed if there is a positive result for the test of aldehydes?

- Blue solution → Orange-brown precipitate.

What is observed if there is a positive result for the test of Ketones?

• Do not react with Fehling's solution under these conditions.

• The solution remains blue.