An Inquiry-Based Approach to Electrophilic Aromatic Substitution: Quantifying and Identifying Nitration Products by1H NMR

Nitration of benzene is one of the prototypical examples of electrophilic aromatic substitution. The regiochemistry of nitration is dictated by the substituent(s) on the aromatic ring. Electron-donating groups direct electrophiles ortho/para to the substituent and dominate over the influence of electron-withdrawing substituents. The reactions of 2′- and 3′-hydroxyacetophenone with nitric acid in acetic anhydride result in nitration product mixtures that can be easily distinguished by proton nuclear magnetic resonance ( 1 H NMR) spectroscopy. The relative amounts of each product can be determined by integrating the areas of the signals produced by the methyl protons of their respective acetyl groups. The aromatic hydrogens of each of the nitration products formed in either the reaction of 2′- or 3′-hydroxyacetophenone are all unique; each proton corresponds to a single signal. The signals produced by the aromatic protons, then, can be matched to the acetyl group on the ring by comparing the integrated areas of the signals produced by the three methyl protons of the acetyl group with the integrated areas of each aromatic protons since the areas appear in 3:1 ratios. Analysis of the coupling patterns (multiplicities) of the signals produced by the aromatic protons enables students to identify the nitration products formed in the reactions.