There exists a close relationship between different organic compounds. This similarity in behavior has made the study of millions of organic compounds easier. They can be classified into few families. A series of related compounds in which any two adjacent molecules differ by -CH₂- group is called homologous series. For example, consider alkanes;
CH₄
CH₃-CH₃
CH₃-CH₂-CH₃
CH₃-CH₂-CH₂-CH₃
Methane
Ethane
Propane
Butane
Note the difference between adjacent alkanes, they differ by the same unit -CH₂-. This means you can represent next member by simply adding -CH₂- unit. A series of related compounds in which adjacent member differ by -CH₂- is called a homologous series. Similarly alcohols also form homologous series.
CH₃-OH
CH₃-CH₂-OH
CH₃-CH₂-CH₂-OH
Methanol
Ethanol
Propanol
These compounds also differ by the same unit -CH₂-. All the classes of organic compounds including alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, carboxylic acids etc. form homologous series.
Table 13.1 shows the general formulae of some homologous series
| Homologous series | General formula |
|---|---|
| Alkanes | CₙH₂ₙ₊₂ |
| Alkenes | CₙH₂ₙ |
| Alkynes | CₙH₂ₙ₋₂ |
| Alcohols | CₙH₂ₙ₊₁OH |
| Carboxylic acids | CₙH₂ₙ₊₁COOH |
From the general formula you can easily determine the molecular formula of any member of the series. For examples,
Alkanes have general formula CₙH₂ₙ₊₂ which can be used to determine the molecular formula for any member of alkane series by putting number of carbon atoms in the general formula.
Examples:
Methane
C₁H₄
Ethane
C₂H₆
Propane
C₃H₈
Note that n=1 for methane, n=2 for ethane and n=3 for propane
Alkenes have general formula CₙH₂ₙ which can be used to determine the molecular formula for any member of alkene series.
Examples:
Ethene
C₂H₄
Note that n=2
Alcohols have general formula CₙH₂ₙ₊₁OH which can be used to determine the molecular formula for any member of alcohol series.
Examples:
Methanol
C₁H₃OH
Note that n=1
Write the molecular formulae of the following compounds using general formulae