groups of related (having the same chemical functions and the same type of structure) organic compounds that differ from one another only by one or more methylene (CH₂) groups. (The CH₂ group is called the homologous difference.)

Homologous series are known for saturated hydrocarbons (alkanes) of the general formula C_nH_2n+2 (methane CH₄, ethane C₂H₆, propane C₃H₈, and so on), unsaturated hydrocarbons (alkenes) of the general formula C_nCH_2n (ethylene C₂H₄, propylene C₃H₆, butylene C₄H₈, and so on), and saturated monohydric alcohols of the general formula C_nH_2n+1OH (methyl alcohol CH₃OH, ethyl alcohol C₂H₅OH, propyl alcohol C₃H₇OH, and so on). Homologous series are often named for their first member (for example, methane series, ethylene series, and so on). The compounds of each homologous series (especially the middle members) are characterized by relatively regular changes in certain physical properties. Thus, the boiling points of neighboring members in the middle of a homologous series (for compounds with an unbranched chain) differ by approximately 20°-25°C. (In higher members of a series this magnitude gradually decreases.) For example, alcohols with a linear chain —n-propyl [CH₃CH₂CH₂OH], n-butyl [CH₃CH₂—CH₂— CH₂OH], n-amyl [CH₃CH₂CH₂CH₂CH₂OH], and n-hexyl [CH₃(CH₂)₄CH₂OH]—boil, respectively, at 97.4°, 117.7°, 137.8°, and 157.2°C. Also corresponding to changes in composition by the homologous difference are changes including molecular size (by a constant magnitude), heat of combustion (by 628.02–669.89 kilojoules [150–160 kilocalo-ries]), and molecular refraction (by 4.6).

Closely related to the concept of homologous series are the concepts of isologous and genetic series. Isologous series are groups of organic compounds with the same carbon “skeleton” and the same functional groups, but differing in their degree of saturation. For example, the isologous ethane series is ethane (CH₃—CH₃), ethylene (CH₂ ‗ CH₂), and acetylene (CH≡CH). Genetic series are groups of organic compounds with the same carbon skeleton but different functional groups. For example, the genetic ethane series is ethane (CH₃—CH₃), ethyl chloride (CH₃—CH₂C1), ethyl alcohol (CH₃—CH₂OH), and so on.

The concepts of homologous, isologous, and genetic series play an important role in the systematization and classification of compounds in organic chemistry (see Figure 1); they were first clearly formulated by C. Gerhardt in 1844–45.

Figure 1