benzyl group
Summary
A benzyl group is C₆H₅CH₂−, a benzene ring attached through a CH₂ carbon (abbreviated Bn). That CH₂ is the benzylic position, where cations and radicals are resonance stabilized by the ring, so benzylic sites are unusually reactive.
Detail
Meaning: A benzyl group is C₆H₅CH₂−, a phenyl ring joined to the rest of a molecule through a CH₂ carbon. The carbon next to the ring is the benzylic position. A benzylic carbocation or radical is stabilized by resonance because the positive charge or unpaired electron delocalizes into the ring. As a result, benzylic C–H bonds are relatively weak and benzylic halides react readily by SN1 and SN2.
On the MCAT: Expect side-chain reactions on alkylbenzenes. Hot KMnO₄ or another strong oxidant converts an alkyl side chain with at least one benzylic hydrogen into a benzoic acid (the carbon chain is cut back to the carboxyl carbon). tert-Butylbenzene has no benzylic H and resists this. NBS with light or heat brominates the benzylic carbon by a radical mechanism. Benzyl ethers and esters serve as protecting groups for alcohols and carboxylic acids, and H₂ with Pd/C removes them. You may be asked to rank carbocation stability or SN1 rate: benzylic is comparable to allylic and well above a simple primary carbon.
Common confusion: Benzyl is not phenyl. Phenyl is C₆H₅− bonded directly to another atom, while benzyl has the extra CH₂. Benzyl alcohol (C₆H₅CH₂OH) behaves like an ordinary primary alcohol, whereas phenol (C₆H₅OH) is far more acidic. The special stabilization belongs to the benzylic carbon, not to the ring carbons.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.