Journal of the Chemical Society. Perkin transactions II p. 1899 - 1903 (1990)
Update date:2022-08-03
Topics:
Terrier, Francois
Xie, Hai-Qi
Lelievre, Jacques
Boubaker, Taoufik
Farrell, Patrick G.
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 deg C.The increase in acidity (pKa) brought about by the indroduction 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 isonly of the order of 2 pK units.This suggests that only one p-nitrophenyl ring of the 4,4'-dinitro- and 4,4',4''-trinitrotriphenylmethyl anions <(C-2e), (C-2d)> is in a favourable position for effective conjugation with the exocyclic 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 in going from (2f) to (2e) to (2d) are essentially the result of the corresponding increases in the kinetic acidity (kOHp).Addition of a first o-nitro group to (2d) to form (2c) further increases the kinetic acidity but it also results in a large decrease in kH2O-p, consistent with the preferential stabilization of the corresponding 2,4,4',4''-tetranitrotriphenylmethyl 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 kOHp and kH2O-p values.It is suggested that these anomalous variations are the reflection ofunfavourable steric interactions arising from the accumulation of o-nitro groups in the triphenylmethane system.
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