Theoretical Models for Solvation and Catalysis in Carbonyl Addition

Alternative modes of catalysis of formaldehyde hydration by a single ancillary water molecule are investigated by ab initio calculations at the STO-3G level. A cyclic transition state (FW2†) involving formaldehyde with two molecules of water is characterized and is only 0.8 kcal mol^-1 higher in energy than the isolated reactants. The results indicate that gas-phase formaldehyde hydration probably proceeds via FW2† in a concerted mechanism with a Gibbs free energy of activation of 27 kcal mol^-1. Addition of a water dimer to formaldehyde via FW₂* is predicted to occur in water with a Gibbs free energy of activation of 16 kcal mol^-1, in agreement with experiment. Empirical extrapolation to the liquid phase of entropies of activation calculated for reaction of one or two waters in the gas phase suggests that a mechanism involving three water molecules would be consistent with experiments for dioxan solution. Specific solvation by four water molecules is predicted to stablize the zwitterionic adduct H₂O+CH₂O^- (an unbound state in the gas phase) by 111 kcal mol^-1 relative to H₂O⁺CH₂O^- and 4H₂O or by 37 kcal mol^-1 relative to (H₂O)₃ and CH₂O·₂H₂O. Thus a stepwise mechanism for formaldehyde hydration also may be feasible, although the energy barrier to formation of a solvated zwitterionic intermediate has not yet been calculated.