An electron withdrawing group in organic chemistry is an individual atom or functional group that withdraws electron density from a conjugated system. Electron withdrawing groups attach to conjugated aromatic compounds and make the conjugated system more electrophilic so that the ring system can undergo nucleophilic attack.
Electron withdrawing groups are also known as deactivating groups because when they are attached to a ring system they deactivate the system by delocalizing the electron density. Scientists classify functional groups and atoms as three different types of electron withdrawing groups. Weakly deactivating groups cause electrophiles to attack the aromatic ring system at the ortho and the para positions. Examples of weakly deactivating groups include all halogens. Moderate and strong deactivating groups direct attacks to the meta position. Examples of moderate deactivating groups include cyano groups, carboxylic acids, esters, aldehydes and ketones. Some strong deactivating groups include nitro groups, trihalides and quaternary amines. Organic functional groups can also be activating and are referred to as electron donating groups because they donate lone pair electrons to the conjugated ring system and increase the ring’s electron density. With electron donating functional groups, the ortho and para positions are the most nucleophilic, and thus electrophiles tend to add to these positions.