Illustrated Glossary of Organic Chemistry

Electron withdrawing group (EWG): An atom or group that draws electron density from neighboring atoms towards itself, usually by resonance or inductive effects.

Trifluoroacetate ion is a weaker base than acetate ion because the trifluoromethyl group is attracting electron density away from the carboxylate. The trifluoromethyl group is an electron withdrawing group:


Acetate ion


Trifluoroacetate ion

Aniline is a weaker base than ammonia because the benzene ring is attracting electron density away from the nitrogen lone pair by resonance. The benzene ring is an electron withdrawing group:


In ammonia the lone pair is localized on the nitrogen atom.


In aniline, nitrogen's lone pair is delocalized by resonance as demonstrated by this resonance hybrid.