Alkylidene

An alkylidene is a divalent group in organic chemistry consisting of a carbon atom that is doubly bonded to the rest of the molecule and singly bonded to two substituents. The general representation is R2C=, with the double bond connecting the alkylidene carbon to the adjacent atom. The carbon is typically sp2-hybridized. The term is used for both purely organic substituents and organometallic ligands, where the alkylidene carbon forms a double bond to a metal center (M=CR2).

Common examples include methylidene, often written as =CH2, which is the simplest alkylidene, and isopropylidene, written as (CH3)2C=. The two substituents attached to the alkylidene carbon define the specific group. In naming, alkylidene often appears as a substituent name or as part of a ligand description, as in M=CR2 complexes. The concept also serves in protective-group chemistry, where isopropylidene groups are used to mask diols in carbohydrate and related chemistry.

Applications and context: Alkylidene ligands are central to organometallic chemistry and catalysis. Metal alkylidenes participate in reactions such as olefin metathesis, where M=CR2 species shuttle alkene fragments to form new carbon–carbon double bonds. In organic synthesis, alkylidene fragments can appear as reactive intermediates or as structural motifs in molecules featuring exocyclic or internal double bonds. The term also appears in specialized naming of compounds that contain alkylidene linkages or protective groups.