Although aromatic substitution reactions usually occur by an electrophilic mechanism, aryl halides that have electron-withdrawing substituents can also undergo a nucleophilic substitution reaction, termed SNAr. The electron-withdrawing substituent(s) must be oriented ortho and/or para to the halogen. The reaction proceeds by nucleophilic attack of the nucleophile on the ring carbon bound to the halogen leaving group, an intermediate carbanion is formed which is termed a Meisenheimer complex. The Meisenheimer complex is stabilized via resonance by the electron-withdrawing groups. Expulsion of the halogen restores aromaticity to the ring and completes the reaction. Draw curved arrows to show the movement of electrons in this step of the mechanism.