The acidity of weak carbon acids. Part 4. The kinetic acidities of 4-nitro-, 4,4′-dinitro and 4,4′,4″-trinitrotriphenylmethanes

Abstract

Rate coefficients for the base-catalysed detritiation of 4,4′,4″-trinitro-,4,4′-dinitro- and 4-nitrotriphenyl-methanes in dimethyl sulphoxide have been measured at 25.0 °C. The bases are a series of secondary aliphatic amines and tetramethylguanidine and of benzoate anions. For 4,4′-dinitrophenylmethane and piperidine, the rate coefficients for detritiation at 55.0 °C and, for α-[²H₁]-4,4′-dinitrotriphenylmethane and piperidine, dedeuteriation at 25.0 °C were also measured. The kinetic isotope effect and activation parameters indicate the rate-determining step to be the ionisation process. Brønsted coefficients, β, have been calculated using the known or previously measured pK_a values of the conjugate acids of the bases in dimethyl sulphoxide. The β values for both carbon acids are significantly greater for the benzoate anion than for the nitrogen base catalysis, and those for the dinitro-carbon acid are less than those for the trinitro-acid. No reactivity-selectivity relationship exists. This behaviour is discussed in relation to Marcus and related theories.