The sheer scale of organic chemistry means that it is important to categorise compounds with similarities in order to make sense of the vast number of organic compounds possible. Categorisation relies on similarities between and within structures, giving rise to the concept of homologous series.
Syllabus reference S3.2.4Structure 3.2.4 - Successive members of a homologous series show a trend in physical properties.
- Describe and explain the trend in melting and boiling points of members of a homologous series.
Guidance
Tools and links
- Structure 2.2 - What is the influence of the carbon chain length, branching and the nature of the functional groups on intermolecular forces?
Trends in properties
The chemical properties of the members of an homologous series are due to the presence (or lack of) functional groups. As all members of the same series have the same functional groups, then the chemical properties are similar. They are not identical because chemical properties are also modified by other factors, such as the position of the functional group, or branching in the carbon chain.
A case in point is the reactivity of the halogenoalkanes. This is known to depend on the position of the halogen atom on the carbon chain, as well as the shape of the carbon chain itself.
The physical properties of an homologous series show a trend as the series is ascended. This is due to the increased relative mass, as each member of the series differs from the previous member by one -CH2- unit.
Physical properties include:
- 1 density
- 2 melting point
- 3 boiling point
- 4 diffusion rate
- 5 solubility
- 6 refractive index
- 7 conductivity
Both density and boiling point depend on the intermolecular forces in the compound. These may be due to London dispersion forces only, or a combination of London dispersion force and dipole-dipole attractive forces. In either case, the London dispersion force increases as the relative molecular mass increases, causing a corresponding increase in density and boiling point.
Solubility in water depends on the ability of the water molecules to attract dipoles in the organic compound. Functional groups may contain dipoles that make the organic molecule soluble to a lesser, or greater extent. These dipoles are said to be hydrophilic (water loving). However, the carbon chain itself has no dipoles and is not attractive to water. It is said to be hydrophobic (water hating).
The difference between each successive member of an homologous series is one -CH2- unit. The CH2 group is non-polar (hydrophobic) and this increases the percentage of the molecule that is unattractive to water.
Organic compounds consequently tend to be less soluble as an homologous series is ascended.
C
bonds and the nitrile series has C