Association of Butyllithium in Diethyl Ether and in Tetrahydrofuran.

Abstract

BuLi exists in Et2O and THF as an unreactive tetramer in a dynamic equil. with the reactive monomer. The equil. const. for the monomer-tetramer equil. is ∼10-4 mol3/L3 in THF and ∼10-16 mol3/L3 in Et2O. Dissocn. into monomer is substantial below 10-2 M in THF, but insignificant at 10-4 M in Et2O. The endothermicity of the dissocn. in Et2O is ∼20 kJ mol-1 of monomer, and ΔG° for the dissocn. is of the same magnitude, meaning that the TΔS° contribution is low. The high reactivity of BuLi when dissolved in THF or other strongly coordinating solvents as compared with the ethereal soln. is due to the much higher concn. of monomer. Kinetic data for the reaction of BuLi with PhCN and CF3CO2Me were also obtained. [on SciFinder(R)]