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 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.

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.

  Inorganic Homologous Series

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Homologous series are not unique to organic chemistry. Titanium, vanadium, and molybdenum oxides all form homologous series (e.g. V_nO_2n-1 for 2 < n < 10), as do the silanes, Si_nH_2n+2 (with n up to 8) that are analogous to the alkanes, C_nH_2n+2.

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