Homologous series

The idea

A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is a homologous series; its members have very similar chemical properties.

A family of alcohols

Methanol, ethanol, propanol and butanol have formulae CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH. Each has an -OH group in place of one hydrogen of a carbon chain, but the chains have different lengths.

How do you expect the chemical properties of these four compounds to compare?

• They are very similar
• They are completely different
• They cannot be compared

Four alcohols

Open every tab. The -OH group is circled in orange.

A family with one group

The carbon chains get longer, but the same functional group replaces a hydrogen in each. The functional group decides the chemical properties.

A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is a homologous series. Its members have very similar chemical properties.

Notes

A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is a homologous series. Its members have very similar chemical properties.

Check yourself

What do the members of a homologous series have in common?

Which of these has chemical properties very similar to ethanol, C₂H₅OH?

In the series CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH, what stays the same?

What decides the chemical properties in a homologous series?

• The same number of carbon atoms. Members differ in the length of the carbon chain. CH₃OH has one carbon atom and C₄H₉OH has four.
• The same functional group replacing hydrogen in a carbon chain — correct. Yes!
• The same molecular formula. Members have different formulae. CH₃OH and C₂H₅OH are different compounds in the same series.
• Ethane, C₂H₆. Ethane has no -OH group, so it is not in the same series as ethanol.
• Ethanoic acid, CH₃COOH. Ethanoic acid has the -COOH group, a different functional group from -OH.
• Propanol, C₃H₇OH — correct. Yes!
• The functional group -OH — correct. Yes!
• The length of the carbon chain. The chains have 1, 2, 3 and 4 carbon atoms, so the length changes.
• The number of hydrogen atoms. The formulae have different numbers of hydrogen atoms: 4, 6, 8 and 10, counting the one in -OH.
• The length of the carbon chain. The functional group decides the properties of the compound, regardless of the length of the carbon chain.
• The functional group — correct. Yes!
• The colour of the compound. The book says it is the presence of the functional group that decides the properties.