Homologous series

In organic chemistry, a homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the members of the series differ by the number of repeating units they contain.[1][2] This can be the length of a carbon chain,[2] for example in the straight-chained alkanes (paraffins), or it could be the number of monomers in a homopolymer such as amylose.[3] A homologue (also spelled as homolog) is a compound belonging to a homologous series.[1]

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 a 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".

The concept of homologous series was proposed in 1843 by the French chemist Charles Gerhardt.[4] A homologation reaction is a chemical process that converts one member of a homologous series to the next member.

  1. ^ a b "Glossary of Terms Used in Medicinal Chemistry (IUPAC Recommendations 1998)". Archived from the original on 2017-08-25. Retrieved 2007-12-22.
  2. ^ a b Brown, Theodore L. (Theodore Lawrence); LeMay, H. Eugene (Harold Eugene); Bursten, Bruce Edward (1991). Chemistry: the central science (5th ed.). Englewood Cliffs, NJ: Prentice Hall. pp. 940. ISBN 978-0-13-126202-7. OCLC 21973767.
  3. ^ Saarela, K. (2013-10-22). Macromolecular Chemistry—8: Plenary and Main Lectures Presented at the International Symposium on Macromolecules Held in Helsinki, Finland, 2–7 July 1972. Elsevier. p. 88. ISBN 978-1-4832-8025-7.
  4. ^ 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 proportion of elements.)