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In chemistry, a homologous series is a series of compounds with the same general formula, usually varying by a single parameter—such as the length of a carbon chain. Examples of such series are the straight-chained alkanes (paraffins), and some of their derivatives (such as the primary alcohols, aldehydes, and (mono)carboxylic acids). The single-ring unbranched cycloalkanes form another such series.
Compounds within a homologous series typically have a fixed set of functional groups that gives them similar chemical and physical properties. (For example, the series of primary straight-chained alcohols has an hydroxyl at the end of the carbon chain.) These properties typically change gradually along the series, and the changes can often be explained by mere differences in molecular size and mass.
The name "homologous series" is also often used for any collection of compounds that have similar structures or include the same functional group, such as the general alkanes (straight and branched), the alkenes (olefins), the carbohydrates, etc. However, if the members cannot be arranged in a linear order by a single parameter, the collection may be better called a "chemical family" or "class of homologous compounds" than a "series".
Example: straight-chained alkanes
The homologous series of straight-chained alkanes begins with methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), and pentane (C5H12). In that series, successive members differ in mass by an extra methylene bridge (-CH2- unit) inserted in the chain. Thus the molecular mass of each member differs by 14 atomic mass units.
Within that series, many physical properties such as boiling point gradually increase with increasing 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 distort 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.
|Homologous series||General formula||Repeating unit||Functional group(s)|
|Straight-chain alkanes||CnH2n + 2 (n ≥ 1)||−CH2−||H3C− ... −CH3|
|Straight-chain perfluoroalkanes||CnF2n + 2 (n ≥ 1)||−CF2−||F3C− ... −CF3|
|Straight-chain alkyl||CnH2n + 1 (n ≥ 1)||−CH2−||H3C− ... −CH2−|
|Straight-chain 1-alkenes||CnH2n (n ≥ 2)||−CH2−||H2C=C− ... −CH3|
|Cyclic alkanes||CnH2n (n ≥ 2)||−CH2−||Singly-bonded ring|
|Straight-chain 1-alkynes||CnH2n − 2 (n ≥ 2)||−CH2−||HC≡C− ... −CH3|
|polyacetylenes||C2nH2n + 2 (n ≥ 2)||−CH=CH−||H3C− ... −CH3|
|Straight-chain primary alcohols||CnH2n + 1OH (n ≥ 1)||−CH2−||H3C− ... −OH|
|Straight-chain primary monocarboxylic acids||CnH2n + 1COOH (n ≥ 0)||−CH2−||H3C− ... −COOH|
|Straight-chain azanes||NnHn + 2 (n ≥ 1)||−NH−||H2N− ... −NH2|
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. VnO2n − 1 for 2 < n < 10), as do the silanes, SinH2n + 2 (with n up to 8) that are analogous to the alkanes, CnH2n + 2.
- Charles Gerhardt (1843) "Sur la classification chimique des substances organiques" (On the chemical classification of organic substances), Revue scientifique et industrielle, 14 : 580-609. From page 588: "17. Nous appelons substances homologues celles qui jouissent des même propriétés chimiques et dont la composition offre certaines analogies dans les proportions relatives des éléments." (17. We call homologous substances those that have the same chemical properties and whose composition offers certain analogies in the relative proportions of elements.)