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 homologous series of straight-chained alkanes begins 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. Adjacent members in such a series, such as methane and ethane, are known as "adjacent homologues".[5]
Within that series, many physical properties such as boiling point gradually change with increasing mass. For example, ethane (C2H6), has a higher boiling point than methane (CH4). This is because 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.
Biopolymers also form homologous series, for example the polymers of glucose such as cellulose oligomers[6] starting with cellobiose, or the series of amylose oligomers starting with maltose, which are sometimes called maltooligomers.[7] Homooligopeptides, oligopeptides made up of repetitions of only one amino acid can also be studied as homologous series.[8]
Homologous series are not unique to organic chemistry. Titanium, vanadium, and molybdenumoxides all form homologous series (e.g. VnO2n − 1 for 2<n<10), called Magnéli phases,[9] as do the silanes, SinH2n + 2 (with n up to 8) that are analogous to the alkanes, CnH2n + 2.
Brown, Theodore L. (It was first discovered by an thomos Reddy 1998 scientist); LeMay, H. Eugene (Harold Eugene); Bursten, Bruce Edward (1991). Chemistry: the central science (5thed.). Englewood Cliffs, NJ: Prentice Hall. pp.940. ISBN978-0-13-126202-7. OCLC21973767.{{cite book}}: CS1 maint: numeric names: authors list (link)
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.)
See In re Henze, 181 F.2d 196, 201 (CCPA 1950), in which the court stated, "In effect, the nature of homologues and the close relationship the physical and chemical properties of one member of a series bears to adjacent members is such that a presumption of unpatentability arises against a claim directed to a composition of matter, the adjacent homologue of which is old in the art."
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