IB Chemistry home > Syllabus 2016 > Organic Chemistry > Homologous series

Syllabus ref: 10.1

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.

Definition

An homologous series is a group of compounds with similar structural features that can be represented by a general formula whose members differ by one -CH2- unit.

You can think of an homologous series as a 'family' or group of compounds, whose chemical behaviour is very similar and whose physical properties show trends when ascending the members of the group (in terms of number of carbon atoms).

Each homologous series has a class name that must be distinguished from the name of the functional group that it contains.

For example: The homologous series of alcohols (class) contains the hydroxyl functional group.


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General formula

A general formula is one in which the numbers of carbons are represented by the letter 'n' and the numbers of other elements as a function of 'n'

So if there are two carbons and four hydrogens, n is equal to 2 (the number of carbons) and the value for the hydrogens = 2n. The general formula becomes CnH2n.

General Formula data

Series
General formula
Alkanes
CnH(2n+2)
Alkenes
CnH2n
Alkynes
CnH(2n-2)
Alcohols
CnH(2n+1)OH
Halogenoalkanes
CnH(2n+1)X
Aldehydes
CnH(2n+1)CHO
Ketones
CnH(2n+2)CO
Carboxylic acids
CnH(2n+1)COOH
Amines
CnH(2n+1)NH2
Amides
CnH(2n+1)CONH2
Esters
CnH(2n+1)COOCnH(2n+1)
Ethers
CnH(2n+1)OCnH(2n+1)
Nitriles
CnH(2n+1)CN

Learning general formulae is not necessary, just be aware that they exist and that all homologous series can be represented in this way.


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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 molecular mass, as each member of the series differs from the previous member by one -CH2- unit.

Physical properties include:

  1. 1 density
  2. 2 melting point
  3. 3 boiling point
  4. 4 diffusion rate
  5. 5 solubility
  6. 6 refractive index
  7. 7 conductivity

Both density and boiling point depend on the intermolecular forces in the compound. These may be due to Van der Waal's forces only, or a combination of Van der Waal's and dipole-dipole attractive forces. In either case, the Van der Waal's force increases as the relative molecular mass increases, causing a corresponding increase in density and boiling point.

Boiling points of the alkanes

The 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 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.


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