In chemistry, a homologous series is a series of compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group. All the compounds within a homologous series have the same general molecular formula and the same functional group (carbon=carbon, hydroxyl, carboxyl, ester, etc.), and can be prepared using similar methods. Compounds within a homologous series show gradual change in physical properties due to increased molecular size and mass, caused by the longer carbon chains (see relative molecular mass). For example, ethane (C2H6), has a higher boiling point than methane (CH4). This is because an ethane molecule experiences greater dipole moments, as in a large molecule, the electron cloud tends to be distorted at random to a greater extent. Thus, the London Dispersion Forces between ethane molecules are higher than that between methane molecules, resulting in stronger forces of intermolecular attraction, raising the boiling point.
Alkanes (paraffins), alkenes (olefins), ethers, and alkynes (acetylenes) form such series in which members differ in mass by 14 atomic mass units. 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). The CH2 group is called Methylene Group.
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 though the general formula structures or entirely different properties, though qualitatively, reactivity usually remains the same.
v Compounds in each series typically have a similar group of atoms called a functional group. Most chemical properties of organic compounds are due to the presence of functional groups.
| Homologous series | General formula | Example | Functional group |
| Straight Chain Alkanes | CnH2n + 2 (n ≥ 1) | CH4, n = 1 | |
| Straight Chain Perfluoroalkanes | CnF2n + 2 (n ≥ 1) | CF4, n = 1 | |
| Alkyl | CnH2n + 1 (n ≥ 1) | CH3, n = 1 | |
| Alkenes and Cyclic Alkanes | CnH2n (n ≥ 2) | C2H4, n = 2 | C = C |
| Alkynes | CnH2n − 2 (n ≥ 2) | C2H2, n = 2 | C ≡ C |
| Alcohols | CnH(2n + 1)OH (n ≥ 1) | CH3OH, n = 1 | - OH |
| Carboxylic acids | CnH2n+1COOH (n ≥ 0) | CH2O2, n = 0 | - COOH |
| Carbohydrates | Cx(H2O)y (n ≥ 1) | C6H12O6 |
Where n represents the number of carbon atoms present.
A Homologation reaction is any chemical process which converts one member of a homologous series to the next member.
Read more about Homologous Series: Inorganic Homologous Series
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“In the order of literature, as in others, there is no act that is not the coronation of an infinite series of causes and the source of an infinite series of effects.”
—Jorge Luis Borges (1899–1986)