The Effect of Nitro Substitution upon Diphenylmethane Reactivity

Article abstract of DOI:10.1021/jo00296a014

Rate and equilibrium data for the reversible deprotonation of 2,2',4,4',6,6'-hexanitro- and 2,2',4,4',6-pentanitrodiphenylmethanes (1a and 1b) by hydroxide ion and various carboxylate and phenoxide ions have been measured in 50percent aqueous DMSO at 25 deg C.The equilibrium acidities of 1a and 1b (pK_a^CH) are much higher than anticipated from the acidifying effect measured for o-nitro substitution similar in comparing similar data for 2,2',4,4'-tetranitro-, 2,4,4'-trinitro- and 4,4'-dinitrodiphenylmethanes. (1c, 1d, and 1e, respectively).This result is attributed to steric factors.These are known to prevent a mutual coplanarity of the two phenyl rings in diphenylmethyl anions like those deriving from 1a and 1b (C-1a and C-1b) which possess more than two o-nitro groups. 1a and 1b would thus behave as α-(2,4,6-trinitrophenyl)- and α-(2,4-dinitrophenyl)-substituted 2,4,6-trinitrotoluenes rather than polynitrodiphenylmethanes, both compounds giving rise to a carbanion which must be viewed as having a sp²-hybridized exocyclic carbon conjugated with only one aromatic picryl ring.The intrinsic reactivities of 1a and 1b, as determinated from the Broensted plots for carboxylate and phenoxide ion reactions, are found to be low and the same order as those measured for 1c and 1d, despite the fact that those latter compounds give essentially planar carbanions with a highly delocalized negative charge.This suggests that the lesser contribution of the charge dispersion to the stabilization of C-1a and C-1b is approximately balanced by an enhanced solvational demand of the nitro groups in the conjugated picryl ring.