allylic

Allylic is an adjective used in organic chemistry to describe a position or substituent that is adjacent to a carbon–carbon double bond. The allylic carbon is the sp3 carbon bonded to the double bond without itself being part of the double bond; the allyl group refers to a three-carbon fragment, typically CH2–CH=CH2 or its isomer CH2=CH–CH2-, connected to a molecule through the non-vinylic carbon.

The allylic position is important because species formed at this site—radicals, cations, or anions—are stabilized by resonance with the neighboring π-system. This delocalization influences reactivity, selectivity, and the outcome of many transformations. In practice, allylic chemistry encompasses substitutions, oxidations, rearrangements, and radical processes that involve the allylic framework.

Common contexts include: allylic substitution, where nucleophiles or electrophiles replace at the allylic carbon with possible migration of the double bond (SN2 or SN2' pathways); allylic oxidation, which targets the allylic C–H bonds to give allylic alcohols or enones (for example, selenium dioxide in allylic oxidation); and radical processes such as allylic bromination using N-bromosuccinimide. The allyl group is also a versatile partner in transition-metal-catalyzed reactions, notably the palladium-catalyzed Tsuji–Trost allylation, which forms carbon–carbon bonds at the allylic position.

Allylic chemistry thus centers on the reactivity of the carbon atoms adjacent to alkenes, governed by resonance stabilization and characteristic reaction pathways that upper-level synthetic strategies frequently exploit.