Kinetics of the Birch reduction

Because of contradictions in the literature, we reinvestigated the kinetics of the Birch reduction, i.e. the hydrogenation of benzene and its derivatives in metal ammonia solutions (MAS: containing solvated electrons e⁻ and metal cations M⁺) with alcohols to yield the corresponding cyclohexa-1,4-dien compounds (e.g. 2 Li+2CH₃OH+C₆H₆⟹2CH₃OLi+C₆H₈). The kinetics of this reaction are obscured since the hydrogen reaction proceeds parallel to it (2Li+2CH₃OH⟹2CH₃OLi+H₂). The two reactions differ in their activation energies (6.5 and 22.5 kJ/Mol resp.); and in the series of the alkali metals Li, Na and K the rate of the Birch reduction decreases, whereas that of the hydrogen reaction increases. However, in the metal concentration range around 0.01 M. both reactions have within the experimental error the same reaction order with respect to the metal (≈︁0.8). Both are accelerated by addition of alkali cations common to the dissolved alkali metal, and both are decelerated by addition of alkali cation complexing cryptands. Thus we conclude that the cations are involved in the kinetics of both reactions, probably by forming intermediate ion pairs or shifting pre-equilibria in which solvated electrons are involved. The experimental data of both reactions can be described very well with the rate laws v(B) = k_Bf²[e⁻][Li⁺)[CH₃OH][C₆H₆]and v(H)=k_Hf²[e⁻][Li⁺](CH₃OH]resp. (f activity coefficients after Debye-Hückel) inserting the concentrations of e⁻ and Li⁺ as calculated from the known thermodynamics of LiAS. The experimental rate constants k_B and k_H are the products of the rate constants of the rate determining steps and the equilibrium constants of the pre-equilibria.