Homologous Series and IUPAC Naming

Organic chemistry handles millions of compounds by organising them into families and naming them systematically. A student who understands the homologous series concept and can apply IUPAC rules can name any compound on the syllabus and predict its properties without memorising each one individually.

Homologous Series¶

A homologous series is a family of organic compounds that:

1. Share the same general formula
2. Have the same functional group (which gives the series its characteristic reactions)
3. Show a gradual, regular change in physical properties (boiling point, density, viscosity) as chain length increases
4. Have similar chemical properties — all members react in the same way
5. Differ from adjacent members by one CH₂ unit (molecular mass increases by 14 each step)

The four homologous series required for CSEC:

IUPAC Naming¶

Knowing which series a compound belongs to tells you its reactions. The IUPAC system then gives it a name from its chain length and functional group.

Organic compounds are named systematically. The name is built from:

• A prefix indicating the number of carbons in the longest continuous chain
• A suffix indicating the functional group (and therefore the series)

Prefixes:

Suffixes:

• Alkanes: –ane
• Alkenes: –ene
• Alcohols: –ol
• Carboxylic acids: –anoic acid

Examples: ethanol = eth (2 carbons) + ol (alcohol); propane = prop (3 carbons) + ane.

For alkenes and alcohols, the position of the double bond or –OH group is given by a number. Propan-1-ol has the –OH on carbon 1; propan-2-ol has it on carbon 2.

For branched alkanes, identify the longest continuous chain as the parent name. Name branches as alkyl groups (methyl, ethyl, etc.) and give each branch a position number, counting from the end of the chain that gives branches the lowest numbers.

Drawn Structural Formulae¶

A name tells you what a compound is. A drawn structure shows how the atoms are connected. CSEC questions use both, so you need to be comfortable converting between them.

Organic structures can be written in two ways:

Condensed structural formula: shows the bonding arrangement without drawing every bond, e.g. CH₃CH₂OH for ethanol.

Fully displayed (structural) formula: shows every bond and atom explicitly.

Key Alkanes¶

The Functional Groups¶

Structural Isomerism¶

The same molecular formula can correspond to more than one structural arrangement. These different structures are called isomers, and they come up often in exam questions.

Structural isomers are compounds with the same molecular formula but different structural arrangements of atoms. Because atoms are connected differently, isomers have different physical properties (such as boiling point).

Chain Isomerism¶

The carbon skeleton is arranged differently (straight vs. branched).

Positional Isomerism¶

The functional group or double bond is at a different position on the same carbon skeleton.

More branched isomers generally have lower boiling points because their compact shape reduces surface contact area and weakens intermolecular forces.