Homologous series
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(intermolecular 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), 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).
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 remains constant in a homologous series, specific members of the series may have different structures or entirely different properties, though qualitatively, reactivity usually remains the same. 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 | |
| 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) | CH4O, n = 1 | - OH |
| Carboxylic acids | CnH2n+1COOH (n ≥ 0) | CH2O2, n = 0 | - COOH |
| Carbohydrates | Cn(H2O)n (n ≥ 1) | C6H12O6 |
Where n represents the number of carbon atoms present.
A Homologation reaction is any chemical process converting one member of a homologous series to the next.
Homologous Oxides
Homologous series are not unique to organic chemistry. Titanium, vanadium, and molybdenum oxides all form homologous series. E.g. VnO2n-1 for 2 < n < 10