Nucleophilic Aromatic Substitution (NAS) is a chemical reaction where a nucleophile replaces a leaving group on an aromatic ring. This process typically occurs in aromatic compounds that have electron-withdrawing groups, which stabilize the negative charge that forms during the reaction. Common nucleophiles include amines, hydroxides, and cyanides. In NAS, the reaction often proceeds through a two-step mechanism. First, the nucleophile attacks the aromatic ring, forming a negatively charged intermediate called a Meisenheimer complex. Then, the leaving group departs, restoring the aromaticity of the ring and resulting in the substitution product. This reaction is important in organic synthesis and pharmaceuticals.