electron withdrawing
Summary
An electron-withdrawing group pulls electron density toward itself through sigma bonds (inductive effect) or through pi bonds (resonance). It stabilizes nearby negative charge, so it usually makes acids stronger and bases weaker, and it makes carbonyl carbons more electrophilic.
Detail
Meaning: An electron-withdrawing group (EWG) is an atom or group that draws electron density away from the rest of the molecule. It acts in two ways. The inductive effect works through sigma bonds because the group is electronegative (F, Cl, O, N⁺, CF₃), and it weakens with distance. The resonance effect works through pi systems (C=O, C≡N, NO₂). An EWG stabilizes anions by spreading out negative charge, and it destabilizes carbocations. It also makes a carbonyl carbon more electrophilic.
On the MCAT: Expect acidity rankings. Trichloroacetic acid is stronger than dichloroacetic, which is stronger than chloroacetic, which is stronger than acetic acid. More EWGs, and EWGs closer to the acidic group, give a more stable conjugate base and a lower pKa. A para-nitro group makes phenol more acidic. An EWG near an amine lowers its basicity. On a benzene ring, most EWGs (NO₂, C=O, CN) deactivate the ring toward electrophilic aromatic substitution and direct meta.
Common confusion: Halogens are electron-withdrawing by induction, so they deactivate the ring. Their lone pairs donate by resonance, so they direct ortho/para. Do not assume that every deactivating group is meta-directing. Also, "electron-withdrawing" describes a substituent's effect, while "electrophile" describes a reacting species.
Sources
- Organic Chemistry by Klein
- Organic Chemistry by Wade
- Organic Chemistry by McMurry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.