Introduction to Aldehydes & Ketones

This lesson covers: 

1. The carbonyl group in aldehydes and ketones
2. Synthesising aldehydes and carboxylic acids from primary alcohols
3. Synthesising ketones from secondary alcohols

Aldehydes and ketones contain a carbonyl group

Aldehydes and ketones are both carbonyl compounds, meaning they contain a carbon-oxygen double bond called the carbonyl group.

The key difference lies in the position of this functional group:

• Aldehydes have their carbonyl group at the end of a carbon chain. Their names end in -al.
• Some common examples of aldehydes are:

• Ketones have their carbonyl group within the carbon chain. Their names end in -one and often have a number indicating the carbon with the C=O group.
• Some common examples of ketones are:

Oxidising primary alcohols to aldehydes and carboxylic acids

Primary alcohols can be oxidised to aldehydes and further to carboxylic acids using acidified potassium dichromate(VI) (K₂Cr₂O₇) as an oxidising agent.

The oxidation reactions are:

Where:

• [O] represents the oxidising agent.
• R is an alkyl group.

The orange dichromate(VI) ion (Cr₂O₇^2-) is reduced to the green chromium(III) ion (Cr³⁺) during these oxidation reactions.

Controlling alcohol oxidation to get aldehydes or acids

You can control how far primary alcohols are oxidised by using different reaction conditions:

• Gently heating the alcohol with acidified K₂Cr₂O₇ in a distillation set-up stops at the aldehyde product.
• Refluxing the alcohol (or aldehyde) with acidified K₂Cr₂O₇ continues the oxidation to the carboxylic acid.

Oxidising secondary alcohols to ketones

Ketones are produced when secondary alcohols are oxidised with acidified K₂Cr₂O₇.

The oxidation reaction is:

• [O] represents the oxidising agent.
• R and R’ represent alkyl groups.

Ketones cannot be oxidised further by acidified K₂Cr₂O₇.