Structure 3. Classification of matter - Functional groups: Classification of organic compounds

A homologous series is a series of compounds that can be described by the same general formula, such as $C_nH_{2n+2}$ for alkanes. Members have similar chemical properties and show a gradation in physical properties like boiling point.

Functional groups are specific atoms or groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

Saturated compounds contain only single bonds between carbon atoms (e.g., alkanes), while unsaturated compounds contain double (alkenes, $C=C$) or triple (alkynes, $C\equiv C$) carbon-carbon bonds.

Alcohols are characterized by the hydroxyl group ($-OH$). They are classified as primary ($1^{\circ}$), secondary ($2^{\circ}$), or tertiary ($3^{\circ}$) based on the number of carbon atoms attached to the carbon bearing the $-OH$ group.

Carbonyl groups ($C=O$) are found in both aldehydes and ketones. In aldehydes, the carbonyl group is at the end of the carbon chain ($-CHO$), while in ketones, it is within the chain ($-CO-$).

Carboxylic acids contain the carboxyl group ($-COOH$), which consists of a carbonyl group and a hydroxyl group attached to the same carbon atom.

Esters are derived from carboxylic acids where the hydrogen of the $-OH$ group is replaced by an alkyl group ($-COOR$). They are often responsible for fruity smells.

Amines contain the amino group ($-NH_2, -NHR, \text{ or } -NR_2$), and amides contain a carbonyl group linked to a nitrogen atom ($-CONH_2$).

Arenes (aromatic hydrocarbons) contain a benzene ring ($C_6H_6$), which consists of a hexagonal ring of carbon atoms with delocalized $\pi$ electrons.