1. A substituent on a pi system either donates a lone pair in or pulls density out through resonance.
2. Pi-donors carry a lone pair adjacent to the pi system and push it in, raising the HOMO.
3. Pi-acceptors carry an electronegative multiple bond or empty orbital that withdraws density, lowering the LUMO.
4. The same donor/acceptor logic works on any pi system, as an enamine (donor) versus an enone (acceptor).
5. On a benzene ring this fixes the EAS outcome, donors activate and direct ortho/para while acceptors deactivate and direct meta.
6. Summary
Donor = lone pair adjacent · pushes density in · raises HOMO · activates · o/p director · stabilizes cations. Acceptor = electronegative multiple bond · pulls density out · lowers LUMO · deactivates · meta director · stabilizes anions/enolates.
Quiz yourself
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A pi-donor — oxygen has a lone pair adjacent to the ring that it donates into the pi system, raising the HOMO and activating the ring.
–NO₂ is a pi-acceptor that withdraws density from the ring, most strongly from the ortho/para positions, so the electrophile attacks the least-destabilized meta position.
A pi-donor — pushing a lone pair toward the empty orbital delivers electron density to the cation. Acceptors, which pull density away, stabilize adjacent anions/enolates instead.
The nitrogen lone pair donates into the C=C (pi-donor), building up electron density on the far carbon — the classic enamine nucleophile.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.