KINETICS, ISOTOPE EFFECTS OF THE REACTION OF 1-(4-NITROPHENYL)-1-NITROALKANES WITH DBU IN TETRAHYDROFURAN AND CHLOROBENZENE SOLVENTS

(R = Me, Et, i-PR; NPNE, NPNP, MNPNP respectively; L is H or D) with 1, 8-diazabicyclo [5.4.0]undec-7-ene (DBU) base in tetrahydrofuran (THF) and chlorobenzene (CB) solvent are reported. The products of these proton transfer reactions are ion pair absorbing at lambda-(max) = 460-480 nm. The equilibrium constants in THF were K(r)25-degrees-C = 260, 560, 3.7 and in CB K(r)25-degrees-C = 500, 220, 4.7 mol-1 dm-3 for NPNE, NPNP, MNPNP respectively. The thermodynamic parameters of the reactions are also quoted. The substrate reacts with DBU in both THF and CB solvents in a normal second-order proton transfer reaction. In the case of deuteron transfer, isotopic D/H exchange is much faster than internal return. The reactions show low values of enthalpy of activation DELTA-H+ =14.3, 18.1, 24.2 and 13.0, 15.1, 18.6kj mol-1 for NPNE, NPNP, and MNPNP in THF and CB respectively, and large negative entropies of activation - DELTA-S+ = 141, 139, 146; 140, 146, 160, J mol-1 deg-1 for the same sequence of substrates and solvents. The kinetic isotope effects are large, (k(H)/k(D)20-degrees-C = 12.2, 13.0, 10.1; 12.9, 12.0, 10.2 for the above sequence of substrates and solvents, show no difference with changes in either steric hindrance of the C-acids or polarity of the solvents.