Alkylen

Alkylen is a term sometimes encountered in organic chemistry to denote a divalent hydrocarbon fragment derived from alkanes by the removal of two hydrogen atoms. In modern usage, the term alkylene is preferred, and alkylen is mainly found in older literature or as a transliteration from other languages. As a bridging moiety, an alkylen fragment provides two points of attachment to other atoms or groups, and its presence can influence the geometry and reactivity of a molecule.

The simplest examples are the methylene group (-CH2-), connecting two substituents, and the ethylene group (-CH2-CH2-), which links two moieties through two consecutive carbon atoms. Longer alkylene chains, such as -CH2-CH2-CH2- and beyond, function as extended linking units in many compounds, including polymers and ligands.

In nomenclature, alkylene is the systematic term for these divalent hydrocarbon fragments. They are commonly encountered in polymer science as bridging units in polyolefins, in organometallic chemistry as ligands or spacers, and in synthetic organic chemistry as building blocks for complex architectures. Synthesis of alkylene fragments can proceed via reductive dehalogenation of dihalides, dehydration of diols, or methylenation-type transformations that insert a CH2 unit between two fragments.

Safety and handling of compounds bearing alkylene linkages follow general hydrocarbon guidelines: they are typically flammable and may pose standard health hazards for volatile organic compounds. See also alkylene and methylene for related concepts.