A readily observed base-catalyzed isotopic exchange in a 2,4-dinitroalkyl benzene

The behaviour of solutions of the sodium salt of 4-(2,4-dinitrophenyl)pentanoic acid in D2O is readily monitored by 1H NMR spectroscopy. In the presence of excess NaOD, these show exchange of hydrogen at the 3-position of the benzene ring with kOD = 1.23(0.03) × 10−4 M−1 s−1 at 25 °C. Rates of exchange using solutions with triethylamine buffers show no dependence on buffer concentration and an estimate of the acid dissociation constant, pKa ≈ 29.3, of the dinitrobenzene is made. Under the conditions, there is no detectable exchange at the benzylic position.