Dissociation equilibrium of nitroform in polar solvents studied by pulse radiolysis

Abstract

The dissociation equilibrium C(NO₂)₃H ⇌ C(NO₂)^–₃+ H⁺_sol and the related kinetics have been studied for various solvents (alcohols, acetone, and water) by means of pulse radiolysis. Nitroform anions were produced by radiation-chemical reduction of tetranitromethane which was dissolved in these liquids. The dissociation constant K of nitroform decreases with decreasing dielectric constant; its value ranges from 6.3 × 10^–1 M for water to 2.5 × 10^–7 M for t-butanol. K is essentially controlled by the rate constant k_n for the neutralization of C(NO₂)^–₃ by solvated protons (3.4 × 10⁴ M^–1 s^–1 in water and 5.0 × 10⁹ M^–1 s^–1 in t-butanol), while the rate of dissociation of C(NO₂)₃H depends relatively little on the nature of the solvent. A linear relationship has been found between log k_n and log K. Nitroform anions are also neutralized by proton transfer from solvent molecules. The rate constant for this process does not vary much with the properties of the solvent. Equivalent conductivities for C(NO₂)^–₃+ H⁺_sol ion pairs have also been determined.