STERIC EFFECTS ON THE INTRINSIC REACTIVITY OF NITROTRIPHENYLMETHANES

Rate constants (k(p)B, k(-p)BH) for the reversible deprotonation of (2,2',2'',4,4',4''-hexanitrotriphenyl), (2,2',4,4',4''-pentanitrotriphenyl), (2,4,4',4''-tetranitrotriphenyl), and (4,4',4''-trinitrotriphenyl)methanes (1a-d) by primary aliphatic amines and piperidine and morpholine as well as by phenoxide anions and hydroxide anion have been measured in 50% H2O-50% Me2SO and 20% H2O-80% Me2SO at 25-degrees-C. Addition of a first ortho-nitro group to 1d to form 1c increases markedly the thermodynamic acidity (DELTA-pK = 3.88 in 80% Me2SO); concomitantly, there is a relatively modest increase in the kinetic acidity (k(p)OH) but a large decrease in the rate of carbanion reprotonation by the solvent (k(-p)H2O)). This result is consistent with a preferential stabilization of the (2,4,4',4''-tetranitrophenyl)methyl carbanion (C-2c) by the 2,4-dinitrophenyl ring. Contrasting with this behavior, the introduction of the second and third o-nitro groups has the effect to further enhance the thermodynamic acidity, while decreasing both the k(p)B, and k(-p)BH values to an unexpected extent. It is suggested that the anomalous variations of k(p)B and k(-p)BH on going from 1c to 1b to 1a are the reflection of the unfavorable steric interactions which arise from the accumulation of o-nitro groups in the triphenylmethane system. There are two other remarkable manifestations of these steric effects: the first is the finding of extremely low intrinsic reactivities for 1b and 1a; the log k0 value for 1a is among the lowest reported values for the ionization of a carbon acid; the second is the observation of a reversal in the order of catalytic efficiency of the amine catalysts. While primary amines are normally less reactive than secondary amines in the ionization of 1d and 1c, they become markedly more reactive in the ionization of 1b and 1a. Steric effects are also suggested to be responsible for the finding of an abnormal dependence of the beta(B) values upon the solvent composition.