aryl

Meaning: An aryl group (Ar) is an aromatic ring that has lost one hydrogen and is bonded to the rest of a molecule through a ring carbon. Phenyl, C₆H₅−, is the simplest example, and Ar plays the role for aromatics that R plays for alkyl groups. The carbon that carries the attachment is sp² hybridized and is part of the delocalized π system.

On the MCAT: The usual question is reactivity of an aryl halide, Ar–X. It generally does not undergo SN2, because the ring blocks backside attack and the carbon is sp². It generally does not undergo SN1, because the aryl cation is very unstable. The C–X bond is also stronger than in an alkyl halide, partly from sp² character and resonance with the ring. Substitution can occur under special conditions, such as a strongly electron-withdrawing group (for example NO₂) ortho or para to the leaving group, which allows nucleophilic aromatic substitution. You may also be asked to name or draw aryl substituents, for example in aryl ketones or aryl ethers.

Common confusion: Aryl is not benzyl. Benzyl is C₆H₅CH₂−, attached through an sp³ carbon next to the ring. Benzylic halides react readily by SN1 and SN2 because the ring stabilizes the carbocation or transition state by resonance. If the halogen is on the ring itself, it is aryl and is much less reactive.