CH2CHX

CH2CHX is a generic shorthand used in organic chemistry to denote a two-carbon fragment in which the second carbon bears a substituent X. The notation does not specify bond order or precise connectivity by itself; X is frequently a halogen (fluorine, chlorine, bromine, or iodine), but other leaving groups or substituents may be represented as X depending on the context. In many schemes CH2=CHX is used to denote a vinyl halide, whereas CH2CHX (without an equals sign) can represent a two-carbon fragment where X is attached to the second carbon of an ethyl-type skeleton. The actual structure and reactivity therefore depend on the drawing conventions used in a given source.

Vinyl halide interpretation. When CH2=CHX is implied, the fragment is a vinyl halide in which the halogen is attached to a vinylic carbon. Vinyl halides are common substrates in palladium-catalyzed cross-coupling reactions such as the Heck, Suzuki-Miyaura, and Sonogashira couplings, enabling formation of new carbon–carbon bonds at the vinyl position. They also participate in hydrohalogenation and various rearrangements, with reactivity influenced by the identity of X and by substitution on the double bond. SN2 pathways are typically not applicable at vinylic centers.

Alkyl halide interpretation. If X is on a saturated carbon (an alkyl-halide interpretation of CH2CHX), the fragment behaves as a standard alkyl halide. Such substrates undergo SN2 or E2 reactions depending on substitution and base, and can participate in cross-coupling and radical processes under appropriate catalytic conditions. Halomethyl or halovinyl motifs are common intermediates in pharmaceutical synthesis and in polymerizable monomers.

Synthesis and applications. CH2CHX fragments are prepared by halogenation of alkenes or alkanes, substitution of alcohol-derived precursors, or by selective functionalization of existing two-carbon motifs. They serve as versatile building blocks in organic synthesis, enabling diversification of carbon frameworks and incorporation of halogen functionality for further transformations.