CH2CHCH2

CH2CHCH2 is a shorthand representation used in organic chemistry to denote the allyl fragment, a three-carbon moiety with the formula C3H5. It often refers to the allyl radical CH2=CH-CH2• or to the prop-2-en-1-yl substituent CH2=CH-CH2−. In both cases the allyl unit is characterized by conjugation between a terminal vinyl group and a central carbon, which stabilizes reactive intermediates.

Structure and resonance: The allyl radical is a resonance-stabilized species. The unpaired electron is delocalized over the three carbons, giving two major resonance forms: •CH2-CH=CH2 and CH2=CH-CH2•. This delocalization makes the allyl fragment relatively more persistent than many other simple radicals, though it remains highly reactive under typical conditions.

Occurrence and generation: Allyl fragments appear as intermediates in many radical processes, including the pyrolysis or photolysis of allyl halides (such as allyl chloride), hydrogen abstraction from propene, and various polymerization reactions. They also appear as functional groups in allylated products and as ligands or linking units in larger organic frameworks.

Reactivity and applications: Allyl radicals readily participate in radical addition to carbon–carbon multiple bonds, contributing to polymerization and crosslinking. The allyl group is a versatile synthetic handle in organic chemistry, enabling formations of homoallylic alcohols, ethers, and esters through allylation-type reactions. It also serves as a protecting or directing group in select contexts and as a building block for more complex molecules.

See also: allene (propadiene), allyl chloride, allylation, radical polymerization.