Homologous series

Which substances belong to the homologous series?

There are many classes of chemical compounds, mainly organic, which form homologous series. The best-known homologous series are the sets of alkanes, alkenes and alkynes, where the number in the name refers to the number of carbon atoms present in the molecule.

Homologues

This is what we call the individual compounds belonging to a given homologous series. More often than not, the first few compounds in a given series have their common names, while in the next few, the core of the name is derived from Greek or Latin numerals denoting the number of specific atoms in the molecule.

Examples of homologous series

1. Alkanes with the general formula C_nH_2n+2

Table 1 Initial elements of the homologous series of alkanes

2. Alkenes with the general formula C_nH_2n

Table 2 Initial elements of the homologous series of alkenes

3. Alkynes with the general formula C_nH_2n-2

Table 3 Initial elements of the homologous series of alkynes

4. Cycloalkanes with the general formula C_nH_2n

Table 4 Initial elements of the homologous series of cycloalkanes

5. Monohydroxy alcohols with the general formula C_nH_2n+1OH

Table 5 Initial elements of the homologous series of  monohydroxy alcohols

6. Monocarboxylic acids with the general formula C_nH_2n+1COOH

Table 6 Initial elements of the homologous series of monocarboxylic acids

Homologues in inorganic chemistry

Homologous series are observed not only in organic chemistry. There are many inorganic compounds that can tend to form homologues. For example, silicon forms chemical compounds with hydrogen analogous to hydrocarbons, resulting in the formation of a homologous series. These are so-called silanes and the repeating structure in them is SiH₂:

SiH₄

Si₂H₆

Si₃H₈

Si₄H₁₀

We can expect similar behaviour from hydrogen compounds with selenium, as it is in the same group as sulphur. As a result, we can expect some of its similar properties.