Aldehydes and Ketones - Carbonyl Compounds (A-Level Chemistry)

Carbonyl Compounds

Carbonyl Functional Group

The carbonyl functional group is:

Properties of carbonyl compounds

Carbonyl molecules do not contain hydrogen bonds, however, they do have permanent dipole-dipole forces.

The C=O bond is polarised as the O atom is more electronegative than the carbon atom.

The smaller carbonyl compounds can form hydrogen bonds with water molecules, as the lone pair of electrons on the polarised oxygen atom attracts the hydrogen atoms in the water molecules. They are soluble in water.

Aldehydes

Structure of aldehydes

Aldehydes have the carbonyl functional group at the end of the molecule.

The name of an aldehyde ends with the suffix “al”.

The general structural diagram for an aldehyde is:

The images below show structural and skeletal diagrams of common aldehydes with their IUPAC names.

Oxidation of aldehydes

Primary alcohols can be oxidised to form aldehydes. Aldehydes, in turn, react with oxidizing agents to make carboxylic acids. Some examples include:

• Potassium manganate (VII) – Heat dilute acidified potassium manganate (VII) gently with an aldehyde; the solution turns from purple to colourless.
• Potassium dichromate (VI) – Heat dilute acidified potassium dichromate (VI) gently with an aldehyde; the solution turns from orange to green.

The oxidizing agent is represented by the symbol [O].

For example, in the oxidation of ethanal to form ethanoic acid, the balanced symbol equation is:

CH₃CHO + [O] → CH₃COOH

Ketones

Structure of ketones

Ketones are molecules which also contain the carbonyl functional group.

Ketones have the carbonyl functional group in the middle of the carbon chain.

The name of a ketone ends with the suffix “one”.

A number is used to denote the carbon atom on which the carbonyl group is bonded.

The general structural diagram for a ketone is:

The first ketone in the homologous series is therefore propanone.