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In organic chemistry, a methylidene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon atom, which is connected to the remainder of the molecule by a double bond. The group may be written =CH2, where the '=' denotes the double bond. According to IUPAC, this structural element is named methylidene (name methylidene should be used for −CH2− group, also called “methylidene bridge“).
Many organic compounds are named and classified as if they were the result of substituting a methylidene group for two adjacent hydrogen atoms of some parent molecule (even if they are not actually obtained that way). Thus, for example, methylidenecyclopropene is named after cyclopropene.
The methylidene group should be distinguished from the compound methylidene, also called carbene, whose molecule is a methylidene group all by itself. This compound is usually denoted CH
2. The singlet excited isomer of the methylidene compound is systematically called methylidene.
The names "methylidene group" or "bridging methylidene" are also used for the substituent -CH
2-, that has the same composition as the methylidene group but is connected by single bonds to two distinct atoms in the rest of the molecule. This unit may be better called "methylidene bridge" or "methanediyl" to distinguish it from the doubly bonded methylidene group. The distinction is often important, because the double bond is chemically different from two single bonds.
The central carbon in 1,3-dicarbonyl compound is known as an activated methylidene group. This is because, owing to the structure, the carbon is especially acidic and can easily be deprotonated to form a methylidene group.