methylidene

Methylidene refers to a chemical substituent represented by the group =CH2, i.e., a methylene carbon that is double-bonded to the remainder of the molecule. In this context, the carbon of the methylidene group carries two hydrogens and forms a double bond with its partner in the structure.

The methylidene group is used to describe exocyclic methylene arrangements, where the =CH2 unit is attached to a larger framework at a carbon that participates in a double bond. This usage is distinct from the saturated methylene group (−CH2−), which is linked to adjacent atoms by single bonds, and from the methyl group (−CH3). In systematic naming, methylidene can appear as a substituent name in structures with an exocyclic double bond, or as part of the parenthetical descriptor in certain ring systems.

Examples of compounds featuring a methylidene group include methylenecyclopentane, in which a =CH2 unit is exocyclic to a cyclopentane ring. The term also appears in older or more specialized nomenclature to denote exocyclic methylene attachments in various rings and fused systems.

In organometallic chemistry, methylidene ligands are also described as M=CH2, where the methylene carbon is double-bonded to a metal center. Such alkylidene or methylidene ligands are involved in catalytic processes and can participate in migratory insertions and other organometallic transformations.

The term “methylidene” thus denotes a specific double-bonded methylene fragment used in structural naming and in certain ligand contexts, complementing broader discussions of methylene and methyl substituents.