Kinetic study of the proton transfer reaction between 1-nitro-1-(4-nitrophenyl)alkanes and P1-t-Bu phosphazene base in THF solvent

The rates of proton transfer reactions between C-acids of the series of nitroalkanes with increasing bulk of R=H, Me, Et, i-Pr substituents as 4-nitro-phenylnitromethane (1), 1-(4-nitrophenyl)-1-nitroethane (2), 1-(4-nitrophenyl)-1-nitropropane (3), 2-methyl-1-(4-nitrophenyl)-1-nitropropane (4) and the P1-t-Bu phosphazene have been measured in THF at pseudo-first-order conditions. The product of the proton transfer reaction with P1-t-Bu is associated into ion pair. The equilibrium constants decrease along with growing bulk of alkyl substituent in the reacting C-acid: >100 000, 1170, 590, and 11.8 for 1, 2, 3, 4, respectively. The second order rate constants (k2H) for this very strong base and C-acids are rapidly declining: 9357, 2.31, 0.66, 0.09 dm3 mol−1 s−1 for 1, 2, 3, 4, respectively, and could not be accounted for the small values of the enthalpies of activation ΔHH≠=6.1,18.0,20.7and11.1kJmol−1. The appropriate values of the entropies of activation were all negative and relatively large ΔSH≠=−149.7,−176.5,−178.7,−227.8Jmol−1deg−1 indicating an importance of steric and ordering effects in forming the transition state. The primary deuterium kinetic isotope effects are large showing tendency of reverse relation towards steric hindrance of the reacting C-acids, kH/kD=15.8, 13.6, 13.2 for 1, 2, and 3, respectively. The explanation of the unusual slow proton transfer abstraction caused by this extremely strong base is undertaken.