Oxidation of alcohols

The oxidising agents

Only two appear in this standard, and each has its own colour change.

• The colour change is the observation — quote it whenever you describe a test.
• If no colour change occurs, the alcohol was tertiary.

What each class gives

• Primary alcohol → aldehyde → carboxylic acid
• Secondary alcohol → ketone, and no further
• Tertiary alcohol → no reaction

Why the class matters

• Oxidation replaces a hydrogen on the carbon bearing the –OH.

• A primary alcohol has two such hydrogens, so it can be oxidised twice — first to an aldehyde, then to a carboxylic acid.

• A secondary alcohol has one, so it is oxidised once, to a ketone. The ketone has none left, so it stops there.

• A tertiary alcohol has none, so it cannot be oxidised at all without breaking a carbon–carbon bond.

• Count the hydrogens on the C–OH carbon and the answer follows immediately.

Stopping at the aldehyde

For a primary alcohol, the conditions decide which product you get.

• Distil as the reaction proceeds → stop at the aldehyde.

  • The aldehyde has a lower boiling point than the alcohol, because it has no O–H and so cannot hydrogen bond to itself.
  • It therefore evaporates first and can be removed from the mixture before it is oxidised further.

• Reflux with excess oxidising agent → go all the way to the carboxylic acid.

  • The condenser returns everything to the flask, so the aldehyde stays in contact with the oxidant and is oxidised again.

• These two words — distil and reflux — are the whole difference. State the one you mean.

Writing the equations

• The oxidising agent is usually shown as [O] in an organic equation, which is accepted at Level 3:

$$\text{CH}_3\text{CH}_2\text{OH} + [\text{O}] \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O}$$

$$\text{CH}_3\text{CHO} + [\text{O}] \rightarrow \text{CH}_3\text{COOH}$$

• Note the first step produces water and the second does not.