What is a Homologous Series?
A family of compounds with the same functional group and general formula, where consecutive members differ by CH₂. They show a gradual trend in physical properties and similar chemical properties.
Key Homologous Series
| Series | General Formula | Functional Group | Suffix |
|---|---|---|---|
| Alkanes | CnH2n+2 | C−C (single bonds) | -ane |
| Alkenes | CnH2n | C=C (double bond) | -ene |
| Alcohols | CnH2n+1OH | −OH (hydroxyl) | -ol |
| Carboxylic acids | CnH2n+1COOH | −COOH | -oic acid |
| Aldehydes | CnH2nO | −CHO | -al |
| Ketones | CnH2nO | C=O (internal) | -one |
| Esters | — | −COO− | -oate |
| Amines | CnH2n+1NH₂ | −NH₂ | -amine |
IUPAC Naming Steps
- Find the longest carbon chain → root name (meth-, eth-, prop-, but-)
- Number from the end nearest to the functional group
- Name any branches (methyl, ethyl) with position numbers
- Add the suffix for the functional group
Think About It
Why do boiling points increase down a homologous series?
As chain length increases, the number of electrons increases, which strengthens London dispersion forces. Stronger intermolecular forces require more energy to overcome → higher boiling point.