Homologous Series of Organic Molecules

This lesson covers: 

1. Key features that define a homologous series
2. How to use general formulas to determine molecular formulas
3. Common examples of important homologous series
4. The structure of the carbon skeleton in organic molecules

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 is repsonsible for the characteristic chemical reactions of that molecule.
• All memebers 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_nH_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 = C₇H_2×7 = C₇H₁₄

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_nH_2n+1OH.

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 = C₄H_2×4+1OH = C₄H₉OH

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 hexagaonal 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.