Intramolecular base catalysed hydrolysis of ortho-hydroxyaryl esters: the anomalous position of methyl 3,5-dinitrosalicylate on the Linear Free Energy Relationship plot
The pKa of the ortho-hydroxy group in methyl 3,5-dinitrosalicylate, HMDNS, is 2.45. Rate constants for the reaction of its conjugate base, MDNS− with hydroxide anion and water are 5.3 × 10−2 mol dm−3 s−1 and 6.6 × 10−6 s−1, respectively at 25 °C. The rate constant for the uncatalysed reaction of HMDNS and water is 6.5 × 10−6 s−1 and so there is no evidence for intramolecular general base catalysis of the water reaction with MDNS− by the weakly basic ortho-O−. By means of Brønsted plots the water reaction of MDNS− is compared with that of a group of other salicylate esters (β = 0) and also a structurally different group of esters (β = 0.4), both of which undergo intramolecular base catalysed hydrolysis. Although the title ester structurally belongs to the first set of compounds, its anomalous position on the plot clearly corresponds to the trend of the second set. This is explained in terms of differences in resonance stabilisation and hydrogen bonding in the transition state.