Key features of a homologous series

Homologous compounds belong to a series of organic molecules that have the same general formula and similar chemical properties.

A homologous series has three distinctive characteristics:

• All members contain the same functional group. This specific group of atoms within a molecule is responsible for the characteristic chemical reactions of that molecule.
• All members have the same general formula. This formula is a blueprint that can predict the molecular formula of any member in the series.
• Each subsequent compound in the series differs by a CH₂ unit. This incremental addition of a CH₂ group is what forms the next homologue in the series.

Determining molecular formulas

To determine the molecular formula of any member of a homologous series, you can apply its general formula. Here are two examples:

Worked example 1 - Calculating the molecular formula of an alkene

Calculate the molecular formula of an alkene with 7 carbon atoms.

Step 1: Identify the general formula for alkenes

The general formula for alkenes is C_n_H_2n_.

Step 2: Substitute the value for n (number of carbon atoms)

For an alkene with 7 carbon atoms, n = 7.

Step 3: Solve the formula to get the molecular formula

The molecular formula of the alkene = C_7_H_2×7_ = C_7_H_14_.

Worked example 2 - Calculating the molecular formula of an alcohol

Calculate the molecular formula of an alcohol with 4 carbon atoms.

Step 1: Identify the general formula for alcohols

The general formula for alcohols is C_n_H_2n+1_OH.

Step 2: Substitute the value for n (number of carbon atoms)

For an alcohol with 4 carbon atoms, n = 4.

Step 3: Solve the formula to get the molecular formula

The molecular formula of the alcohol = C_4_H_2×4+1_OH = C_4_H_9_OH.

Common homologous series

Here's a table outlining various homologous series, with examples for each:

Carbon skeletons in organic compounds

Organic compounds are built around a carbon skeleton, which is the backbone of the molecule. There are two primary types:

1. Aromatic skeletons contain benzene rings, which are hexagonal structures of carbon atoms with a ring of delocalised electrons.

2. Non-aromatic skeletons can be either:

• Straight-chained e.g. butane.
• Branched e.g. 2-methylpropane.
• Cyclic e.g. cyclobutane.

Compounds are also classified based on the types of bonds between carbon atoms:

• Saturated compounds have only single bonds between carbon atoms, like in alkanes.
• Unsaturated compounds contain double or triple bonds (C=C, C≡C), or are aromatic.

Finally, an alkyl group is a fragment of a molecule, a carbon chain with the general formula C_nH_2n+1.

For example, the methyl group (-CH₃) is the smallest alkyl group, circled in the molecule below.