Alkanes
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Please help me to explain the term homologous series.
Jenkin writes
Most A-level textbooks will have something on this. Briefly, the main features of any homologous series are as follows.
All members of a particular homologous series can be represented by a general formula. For example, the alkanes make up a homologous series having the general formula CnH2n + 2, where n is the number of carbon atoms. If n =1, the formula is CH4, methane; if n = 2, the formula is C2H6, ethane; if n =5, C5H12, pentane, but the general formula will not distinguish between isomers.
The members of a homologous series show a gradual change in physical properties such as boiling point and melting point: you can check and confirm this by looking up values in the Nuffield Book of Data.
The members of a homologous series will undergo similar chemical reactions. In the case of alkanes, there are two good examples. They all burn in air or oxygen, forming carbon dioxide and water; they all react with chlorine or bromine in the presence of ultraviolet radiation, forming halogenoalkanes and a hydrogen halide. You will find details in textbooks.
You can think of each homologous series as a family of compounds: alkanes, alkenes, alcohols, aldehydes, ketones and so on. Because all members of a series behave in a similar way you, need not learn the reactions of each member of a series separately, thankfully, as there are so many of them!
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updated: 06 March 2005
