Jacques Lelievre

Transition state imbalance in the ionization of nitroaromatic hydrocarbons: 2,2′,4,4′-tetranitrodiphenylmethane and 2,4,4′-trinitrodiphenylmethane in aqueous dimethyl sulphoxide solutions

Rates of reversible proton abstraction from 2,2′,4,4′-tetranitrodiphenylmethane and 2,4,4′-trinitrodiphenylmethane by a variety of bases in 50% water–50% dimethyl sulphoxide have been measured at 25 °C. Bronsted αCH and βB values have been determined and show a charge imbalance in the transition states for these reactions, in accord with findings for similar carbon acids involving extensive charge delocalization on ionization. Intrinsic rate constants (in the Marcus sense), ko′ have been obtained for these systems, and are shown to vary considerably with choice of catalyst type. As expected, high intrinsic energy barriers for these ionizations are predicted, arising from the extensive molecular and solvent reorganization required to produce planar, delocalized carbanions, Data for the former compound have also been obtained in water, which allow a direct estimation of its pKaH2O value to be made; pKaH2O= 13.16.

The crystal structure of the σ-complex of 4,6-dinitrobenzofuroxan and potassium 2-nitropropenide

A crystalline σ-complex (4d) of 4,6-dinitrobenzofuroxan and potassium 2-nitropropenide has been prepared and characterized by single-crystal X-ray analysis. There are two K+ and two independent anionic 2-nitropropane DNBF moieties per asymmetric unit. A cis-configuration of the hydrogen and NO2 substituents is observed. This structure determination suggests a syn-elimination process for the loss of nitrous acid from 4d which readily occurs in Me2SO solution to give the carbanion (6d) of 4,6-dinitro-7-methylbenzofuroxan.

Proton abstraction from bis-(2,4-dinitrophenyl)methane by methoxide ion

Rate constfnts for hydrogen or deuterium abstraction from bis-(2,4-dinitrophenyl)methane by methoxide ion have been determined in methanol, methanol–dimethyl sulphoxide, and deuteriated methanol solutions, together with the temperature variation of proton abstraction and carbanion protonation for the hydrogen-containing substrate in methanol. Isotope and solvent effects confirm the rate-limiting proton transfer.

Structure and conformation of polynitrodiphenylmethanes and related carbanions

1 H and 13C n.m.r. data are reported for a series of ortho and para nitro-substituted diphenylmethanes and related diphenylmethyl anions. In the case of the neutral molecules, a predominance of the skew conformation is suggested for mono- and di-o nitro-substituted derivatives, e.g. 2,4,4′-trinitro- and 2,2′4,4′-tetranitro-diphenylmeth anes, while a perpendicular conformation is preferred by the tri-ortho nitro-substituted derivative, e.g. 2,2′,4,4′,6-pentanitrodiphenylmethane. 13C shifts and 1JcαHα coupling constants indicate that, in all carbanions studied, the exocyclic α-carbon is sp2 hybridized and carries only a small fraction of the negative charge. For carbanions with different substituents on the aromatic rings, e.g. 2,4,4′-trinitro- and 2,2′,4,4′,6-pentanitrodiphenylmethyl carbanions, the negative charge appears to be preferentially delocalized in the more substituted aromatic ring.From arguments based upon the observed n.m.r. and u.v. spectroscopic data, the two rings in diphenylmethyl anions with up to two ortho nitro substituents are essentially coplanar, whereas steric interactions prevent such coplanarity in those with three or four such substituents.

Ionization of nitrotriphenylmethanes. Remarkable kinetic evidence for steric inhibition to resonance and F-strain

Rate and equilibrium data for the reversible deprotonation of 2,2′,2″,4,4′,4″-hexanitro-, 2,2′,4,4′,4″-pentanitro-, 2,4,4′,4″-tetranitro-, 4,4′,4″,-trinitro-, 4,4′-dinitro- and 4-nitro-triphenylmethanes (2a–f) by hydroxide ion have been measured in various H2O–dimethyl sulphoxide (DMSO) mixtures at 25 °C. The increase in acidity (pKa) brought about by the introduction of a first p-nitro group in triphenylmethane (2g) to give (2f) is very large and equal to about 12 pK units while the acid-strengthening influence of each of the second and third p-nitro groups is only of the order of 2 pK units. This suggests that only one p-nitrophenyl ring of the 4,4′-dinitro- and 4,4′,4″-trinitro-triphenylmethyl anions [(C-2e), (C-2d)] is in a favourable position for effective conjugation with the exocylic sp2 carbon atom of these carbanions at any given time. Accordingly, the effects exerted by the additional p-nitrophenyl ring(s) are mainly inductive in nature. Support for these ideas is the observation that the increases in acidity observed on going from (2f) to (2e) to (2d) are essentially the result of the corresponding increases in the kinetic acidity (k–pH2O). Addition of a first o-nitro group to (2d) to form (2c) further increases the kinetic acidity but it also results ina large decrease in k–pH2O, consistent with a preferential stabilization of the corresponding 2,4,4′,4″-tetranitrotriphenyi-methyl carbanion (C-2c) by the 2,4-dinitrophenyl ring. Significantly the introduction of the second and third o-nitro groups enhances the thermodynamic acidity while it decreases markedly the kpOH and k–pH2O values. It is suggested that these anomalous variations are the reflection of unfavourable steric interactions arising from the accumulation of o-nitro groups in the triphenylmethane system.

The relative acidities of 4-nitro-, 2,4-dinitro-, and 2,4,6-trinitro-toluene

The rates of proton abstraction and carbanion protonation have been measured for the title compounds in methanolic dimethyl sulphoxide and, where possible, in aqueous dimethyl sulphoxide solutions. These data allow estimates of their pKa values in water to be made, and provide a revised value for the pKa of 4-nitrotoluene in dimethyl sulphoxide solution.

Solvent dependence of the ionization of nitrophenylmethanes

The solvent dependence of proton abstraction from various nitrophenylmethanes has been examined for aqueous dimethyl sulphoxide and methanolic dimethyl sulphoxide solutions. Even though the compounds studied vary considerably in thermodynamic acidity (pKa), their proton-abstraction rates all show the same solvent dependence. It is suggested that the transition states for these reactions all occur at similar positions on the reaction pathway, and that transition state imbalances exist for these proton transfers.