Nitrations of acetanilides by reagents of NO2X type

The distribution of products in the nitration of a series of eight substituted acetanilides has been studied using differing nitrating agents.With mixed nitric and sulfuric acids, attack para to the acetamido group is favored, while with acetyl nitrate or nitronium tetrafluoroborate, predominant ortho substitution occurs. It is suggested that ortho substitution results from SN2 displacement by a pair of substituent electrons (on nitrogen or carbonyl oxygen) on the species NO2X (X = BF4 or OAc), leading to formation of the most readily accessible σ complex, while the para substitution favored in mixed acids results from substitution in the conjugate acid of acetanilide.Nitroacetanilides follow an unusual path with acetyl nitrate, yielding the corresponding dinitrobenzene in addition to the expected dinitroacetanilides.Nuclear magnetic resonance spectroscopy efficiently demonstrates the structures of the reaction products, and reveals the preferred intramolecularly hydrogen-bonded conformation of ortho-nitroacetanilides.