The low intrinsic reactivity of picrylacetone: an index for the π-acceptor capability of a 2,4,6-trinitrophenyl structure

Abstract

Rates of deprotonation of picrylacetone, i.e., 2,4,6-trinitrophenylacetone 4, by a variety of bases B (phenoxide and carboxylate ions, OH^–) and of protonation of the resulting carbanion (C-4) by the conjugated acids BH⁺ have been measured in a 50%H₂O–50%Me₂SO (v/v) mixture at 25 °C. The intrinsic reactivity (in the Marcus sense) of 4, as determined from the Brønsted plot for phenoxide reactions (β^ArO= 0.38) is low: log k₀^ArO= 2.37. This value is more typical for the formation of a strongly resonancestabilized benzyl-type carbanion than for that of an enolate-type carbanion and it may be regarded as a good measure of the high π-acceptor capability of a conjugated 2,4,6-trinitrophenyl structure. Definitive evidence that the negative charge of C-4 is essentially delocalized through the picryl moiety comes from the observation that this carbanion undergoes instantaneous protonation at its p-nitro group at low pH (pH < 6). The pK_a′ value associated with the ionization of the resulting nitronic acid (C-4H) is ≈5, which compares well with similar data previously reported in the literature.