Homologous series

In <a href="/facts/Organic_chemistry/djxhxaNg">organic chemistry</a>, a homologous series is a <a href="/facts/Sequence/gV2S4uqp">sequence</a> of <a href="/facts/Chemical_compound/37FXqqtv">compounds</a> with the same <a href="/facts/Functional_group/GXkSPsjM">functional group</a> and similar <a href="/facts/Chemical_property/uMVxKspS">chemical properties</a> in which the members of the series differ by the number of repeating units they contain. This can be the length of a <a href="/facts/Carbon_chain/vm7k4zsv">carbon chain</a>, for example in the straight-chained <a href="/facts/Alkane/Qam7Hvmf">alkanes</a> (paraffins), or it could be the number of <a href="/facts/Monomer/Gw3CltMb">monomers</a> in a <a href="/facts/Homopolymer/eF32E50K">homopolymer</a> such as amylose. A homologue (also spelled as homolog) is a <a href="/facts/Chemical_compound/37FXqqtv">compound</a> belonging to a homologous series.
Compounds within a homologous series typically have a fixed set of functional groups that gives them similar chemical and <a href="/facts/Physical_properties/ujcMUceW">physical properties</a>. (For example, the series of primary straight-chained <a href="/facts/Alcohols/Y0SCTDkh">alcohols</a> has a <a href="/facts/Hydroxyl/1Hs2QAEv">hydroxyl</a> at the end of the <a href="/facts/Carbon_chain/vm7k4zsv">carbon chain</a>.) 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 <a href="/facts/Alkane/Qam7Hvmf">alkanes</a> (straight and branched), the <a href="/facts/Alkene/RtYcS6kU">alkenes</a> (olefins), the <a href="/facts/Carbohydrate/Geuuzxsf">carbohydrates</a>, 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 <a href="/facts/Charles_Fr%25C3%25A9d%25C3%25A9ric_Gerhardt/BawMTtCF">Charles Gerhardt</a>. A <a href="/facts/Homologation_reaction/r7X7yYDt">homologation reaction</a> is a <a href="/facts/Chemical_process/EBSGtK3y">chemical process</a> that converts one member of a homologous series to the next member.

Homologous series open-in-new

Homologous series