allylicity

Allylicity refers to the tendency of a chemical reaction to occur at or involve the allylic position of a molecule. The allylic position is the carbon atom adjacent to a carbon-carbon double bond. This position is particularly reactive due to the resonance stabilization of intermediates formed at this site.

Reactions exhibiting allylicity often involve the formation of allylic carbocations, carbanions, or radicals. For example, in SN1 reactions, a leaving group at the allylic position can depart, forming a carbocation that is stabilized by resonance with the adjacent pi system of the double bond. This resonance delocalization lowers the activation energy for the reaction, making it proceed more readily than at a saturated carbon. Similarly, allylic carbanions and radicals are also resonance-stabilized, influencing the outcome of reactions like SN2 substitutions or free radical chain reactions.

Common reactions where allylicity plays a significant role include allylic bromination using N-bromosuccinimide (NBS), allylic oxidation, and nucleophilic substitutions at the allylic position. Understanding allylicity is crucial for predicting the regioselectivity and stereoselectivity of many organic transformations. The presence of the double bond influences the reactivity of the allylic carbons, making them distinct from other saturated carbon centers within the same molecule.