Molecular-level insights into chemical reactions in high-temperature water.
Akiya, Naoko
2001
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<sub>c</sub></italic>) for H<sub>2</sub>O<sub>2</sub> dissociation in HTW are sensitive to the solvation effects. Molecular dynamics simulations showed that the free energy of solvation for H<sub>2</sub>O<sub>2</sub> in water decreases along the reaction coordinate (<italic>r</italic><sub>OO</sub>) at <italic>T<sub>r</sub></italic> = 1.15 and rho<italic><sub>r</sub></italic> = 1.25. The density-dependent <italic>k</italic> and <italic>K<sub>c</sub></italic> at <italic>T<sub>r</sub></italic> = 1.15 were evaluated using the simulated activation and reaction volumes, respectively. At 0.25 < rho<italic><sub> r</sub></italic> < 1, <italic>k</italic> increases with density whereas <italic> K<sub>c</sub></italic> remains unchanged, because H<sub>2</sub>O<sub>2</sub>-water interactions are less favorable than TS-water interactions but are similar to OH-water interactions. At 1 < rho<italic><sub>r</sub></italic> < 2.75, both <italic>k</italic> and <italic>K<sub>c</sub></italic> decrease with increasing density because of the diminishing isothermal compressibility of water. At 250--380°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 (H<sub> 3</sub>O<super>+</super>). Water also drives the reaction mechanism toward E2 through the preferential solvation of the key intermediate.Subjects
Chemical Reactions Environmentally Benign Chemistry Environmentally-benign Chemistry Green Chemistries High Insights Level Molecular Supercritical Water Temperature
Types
Thesis
Metadata
Show full item recordCollections
Remediation of Harmful Language
The University of Michigan Library aims to describe library materials in a way that respects the people and communities who create, use, and are represented in our collections. Report harmful or offensive language in catalog records, finding aids, or elsewhere in our collections anonymously through our metadata feedback form. More information at Remediation of Harmful Language.
Accessibility
If you are unable to use this file in its current format, please select the Contact Us link and we can modify it to make it more accessible to you.