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Homologous series
From Wikipedia, the free encyclopedia
In chemistry, a homologous series is a series of organic compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group, and shows a gradation in physical properties as a result of increase in molecular size and mass (see relative molecular mass). For example, ethane has a higher boiling point than methane since it has more Van der Waals forces with neighbouring molecules. This is due to the increase in the number of atoms making up the molecule. Organic compounds in the same homologous series vary by a CH2.
Alkanes (paraffins), alkenes (olefins), Methoxyethane (ethers), and alkynes (acetylenes) form such series in which members differ in mass by 14, 12, and 10 atomic mass units, respectively. For example, the alkane homologous series begins with methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), and pentane (C5H12), each member differing from the previous one by a CH2 group (or 14 atomic mass units).
Similarly, there is the alcohol homologous series that starts with methanol (CH4O), ethanol (C2H6O), as primary alcohols, isopropanol (C3H8O) as a simple secondary alcohol, and a simple tertiary alcohol is tert-butanol (C4H10O).
Even while the general formula are the same, they have different structures that can lead the exact same compound to different properties, although they will always present the same chemical properties while as a homologous compound.
Compounds in each set have the same little group of atoms called the functional group. Most chemical properties of organic compounds are due to the presence of the functional group.
| Homologous series | General formula | Example | Functional group |
| Alkanes | CnH2n + 2 (n more than or equal to 1) | CH4, n = 1 | |
| Alkenes | CnH2n (n more than or equal to 2) | C2H4, n = 2 | C = C |
| Alkynes | CnH2n − 2 (n more than or equal to 2) | C2H2, n = 2 | C ≡ C |
| Alcohols | CnH2n + 2O (n more than or equal to 1) | CH4O, n = 1 | - OH |
| Carboxylic acids | CnH2nO2 (n more than or equal to 1) | CH2O2, n = 1 | - COOH |
Where n represents the number of carbon atoms present.
Homologization is any chemical process converting one member of a homologues series to the next. For example the reaction of aldehydes and ketones with diazomethane or methoxymethylenetriphenylphosphine effectively inserts a methylene unit in the hydrocarbon chain and the reaction product is the next homologue. Chain homologization in sugars occurs in the Kiliani-Fischer synthesis.