benzylic

Benzylic refers to the benzylic carbon, the carbon atom directly attached to a benzene ring in benzyl derivatives. The term is used to describe both the position and the group involved, as in benzylic hydrogens, benzylic carbons, or a benzylic substituent such as benzyl (Ph-CH2–).

The benzylic position exhibits increased reactivity due to resonance stabilization of intermediates. Hydrogen atoms at the benzylic carbon are relatively acidic compared to ordinary alkanes, because the benzyl carbanion is stabilized by delocalization into the aromatic ring. Similarly, benzyl radicals and benzyl carbocations are stabilized by resonance with the ring, which accelerates reactions at this position.

Typical transformations include radical halogenation at the benzylic position (often with NBS), benzylic oxidation (oxidation of a benzyl group to benzaldehyde, then to benzoic acid under stronger oxidation conditions), and benzylic substitution that proceeds via stabilized benzylic carbocations (SN1) or via SN2 for primary benzylic halides. The archetypal example is toluene, which can be converted to benzyl bromide by bromination at the benzylic position or oxidized to benzoic acid.

In nomenclature, benzylic describes the position relative to a benzene ring; benzyl describes the substituent group, Ph-CH2–. Distinctions from allylic are noted: both benefit from resonance stabilization, but benzylic refers specifically to attachment to a benzene ring.