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,&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%Me,SO (v/v) mixture at 25 "C. The intrinsic reactivity (in the Marcus sense) of 4, as determined from the Brensted plot for phenoxide reactions @*" = 0.38) is low: log = 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 n-acceptor capability of a conjugated 2,4,6trinitrophenyl 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 itsp-nitro group at low pH (pH < 6). The pKi value associated with the ionization of the resulting nitronic acid (C-4H) is z 5, which compares well with similar data previously reported in the literature.

Nucleophilicities of Para-Substituted Phenoxide Ions in Water and Correlation Analysis

“…(7). For example, the N values of 9.69, 8.62, and 5.88 have been estimated for 4-methylphenoxide ion (pK a = 10.19) [31], 4-bromophenoxide ion (pK a = 9.34) [35], and 4-nitrophenoxide ion (pK a = 7.14) [31], respectively. As can be seen, these values agree well with those calculated through to the N versus σ p correlation (Eq.…”

Rapid Marcus Curvature Due to Extremely Strong Solvational Imbalances inthe Deprotonation of a Trinitrobenzylic Carbon Acid by Oximate Bases

“…In the case of 1, the deproviously found in nucleophilic processes, [14Ϫ16] this has not tonation by the series of oximates studied always occurs been the case in deprotonation of carbon acids. [17] In fact, under highly thermodynamically favorable conditions. (Due two linear and essentially superimposable Brønsted plots to the low pK a value of 1, no common oximates were availdescribe the behaviour of phenoxide and oximate bases in able for study of the ionization process in the reverse directhe ionization of 2,2Ј,4,4Ј-tetranitrodiphenylmethane 2, a tion, corresponding to ∆G°Ն 0 for reaction (5); this would compound with a much lower acidity (pK a ϭ 10.50) but an have provided an initially increasing rate plot prior to the intrinsic reactivity comparable with 1 [∆G 0 (1) ϭ observed levelling.)…”

“…62.8 kJ·mol Ϫ1 ; ∆G 0 (2) ϭ 70.2 kJ·mol Ϫ1 for a phenoxide Hence, the W r term, which will reflect mainly the enerreference] in 50:50 (v/v) H 2 O/Me 2 SO. [13] [17] In this instance getic cost of desolvation of the oximate reagent, will be the there is no evidence at all of levelling in the oximate reactivpredominant factor and the intrinsic barrier term will conity (Equation 7). [17] tribute only slightly, if at all, to the measured ∆G value, i.e.…”

“…Data for phenoxide bases are taken from ref. [17] Solvational imbalances have been shown to be the major consequently, the actual proton transfer step now appears to be the rate-limiting step. The corresponding reaction pro-factor determining the occurrence of rapid levelling in the reactivity of oximates in nucleophilic processes.…”

“…resulting in a marked decrease in the α-effect of oximate pK a BH values taken from ref. [17] Ϫ [d] k Ϫp OXH ϭ (k p Ox K a OXH )/K a 1 . Ϫ [e] ref.…”