Functional Groups (AQA A-Level Chemistry): Revision Notes

3.1.1 Functional Groups

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. A homologous series is a family of compounds that share the same functional group but differ by a repeating unit, typically a -CH_2 group.

Functional Groups

• Alkene ($-C=C-$): Characterised by a carbon-carbon double bond.
• Alcohol ($-OH$): Characterised by a hydroxyl group.
• Carboxylic Acid ($-COOH$): Contains both a carbonyl ($C=O$) and hydroxyl ($-OH$) group.
• Amine ($-NH_2$): Contains an amino group. Functional groups define the type of reactions an organic compound can undergo. For example, alcohols can undergo dehydration reactions, while alkenes can undergo addition reactions.

Characteristics of a Homologous Series

A homologous series is a sequence of compounds that share the same functional group but vary in the length of their carbon chain. Each successive member of a homologous series differs by a $CH_2$ unit. Some key characteristics include:

• Same Functional Group: All members have the same functional group, which gives them similar chemical properties.
• Gradation in Physical Properties: As the carbon chain length increases, the physical properties (such as boiling point, melting point, and solubility) show a gradual change. For example, longer chains typically have higher boiling points.
• Similar Chemical Properties: Due to having the same functional group, compounds within a homologous series exhibit similar chemical behaviour.
• General Formula: Each series has a general formula. For example, the alkanes (a homologous series) have the general formula $C_nH_{2n+2}$