Molecular-level insights into chemical reactions in high-temperature water.

Abstract

High-temperature water (HTW) is a suitable reaction medium for different types of organic chemistry. We investigated three model reaction systems to elucidate the roles of water for chemical reactions in HTW, using complementary experimental, computational, and theoretical tools. Formic acid decomposes by dehydration in the gas phase but by decarboxylation in HTW. Quantum chemical calculations revealed that water influences the relative stability of the transition state (TS) species, which determines the dominant decomposition pathway. The dehydration TS is more stable than the decarboxylation TS in the absence of water, so dehydration dominates in the gas phase. Water stabilizes the decarboxylation TS more than the dehydration TS, so decarboxylation dominates in HTW. Water also reduces the activation barriers for both decomposition pathways by acting as a proton relay in the TS structure. The rate constant (<italic>k</italic>) and equilibrium constant (<italic> K_c</italic>) for H₂O₂ dissociation in HTW are sensitive to the solvation effects. Molecular dynamics simulations showed that the free energy of solvation for H₂O₂ in water decreases along the reaction coordinate (<italic>r</italic>_OO) at <italic>T_r</italic> = 1.15 and rho<italic>_r</italic> = 1.25. The density-dependent <italic>k</italic> and <italic>K_c</italic> at <italic>T_r</italic> = 1.15 were evaluated using the simulated activation and reaction volumes, respectively. At 0.25 < rho<italic>_r</italic> < 1, <italic>k</italic> increases with density whereas <italic> K_c</italic> remains unchanged, because H₂O₂-water interactions are less favorable than TS-water interactions but are similar to OH-water interactions. At 1 < rho<italic>_r</italic> < 2.75, both <italic>k</italic> and <italic>K_c</italic> decrease with increasing density because of the diminishing isothermal compressibility of water. At 250--380&deg;C and 0.08--0.81 g/cm<super>3</super>, cyclohexanol dehydrates readily in HTW in the absence of added catalysts to form cyclohexene and 1- and 3-methyl cyclopentenes. Increasing temperature and water density increase the rate of cyclohexanol disappearance and methyl cyclopentenes formation. The analysis of experimental data and the results of kinetics modeling suggest that cyclohexanol dehydrates predominantly by the E2 mechanism. Water participates in the reaction as a reactant, product, and a source of acid catalyst (H₃O<super>+</super>). Water also drives the reaction mechanism toward E2 through the preferential solvation of the key intermediate.