Noncatalytic kinetic study on site-selective H/D exchange reaction of phenol in sub- and supercritical water

Abstract

The site-selective H/D exchange reaction of phenol in sub- and supercritical water is studied without added catalysts. In subcritical water in equilibrium with steam at 210-240 degrees C, the H/D exchange proceeds both at the ortho and para sites in the phenyl ring, with no exchange observed at the meta site. The pseudo-first-order rate constants are of the order of 10(-4) s(-1); 50% larger for the ortho than for the para site. In supercritical water, the exchange is observed also at the meta site with the rate constant in the range of 10(-6)-10(-4) s(-1). As the bulk density decreases, the exchange slows down and the site selectivity toward the ortho is enhanced. The enhancement is due to the phenol-water interaction preference at the atomic resolution. The site selectivity toward the ortho is further enhanced when the reaction is carried out in benzene/water solution. Using such selectivity control and the reversible nature of the hydrothermal deuteration/protonation process, it is feasible to synthesize phenyl compounds that are deuterated at any topological combination of ortho, meta, and para sites.

Mechanisms and Kinetics of Noncatalytic Ether Reaction in Supercritical Water. 1. Proton-Transferred Fragmentation of Diethyl Ether to Acetaldehyde in Competition with Hydrolysis

“…As seen in Figure 6, the rate constants for the competing fragmentation and hydrolysis cross at a medium density between 0.5 and 0.6 g/cm 3 ; no singularity is observed in the region of the critical density of water (0.32 g/cm 3 ), as previously noted for other supercritical water reactions. 7,14,30 In consequence, the weight of the hydrolysis becomes larger than that of the proton-transferred fragmentation at 0.6 g/cm 3 , whereas the latter is much more important than the former below 0.5 g/cm 3 . Subcritical Water Reaction.…”

“…Unlike ambient water, hot water mixes well with nonpolar organic compounds. Due to the strong and anisotropic fluctuations in the local electric field at high temperatures, hot water can induce chemical reactions that are impossible without acidic or basic catalysts under ambient conditions. − To establish the hydrothermal chemistry for the next generation, systematic studies in a manner friendly to the earth on the reaction of each functional group are necessary. As one of the most important functional groups, ether bonds (C−O−C) are abundantly accumulated in naturally produced organic compounds, in particular, coal, such polysaccharides as cellulose and starch, wood components, etc., that attract much attention for food and energy concerns.…”

Effect of Water Content on Conversion of d-Cellobiose into 5-Hydroxymethyl-2-furaldehyde in a Dimethyl Sulfoxide–Water Mixture

“…This suggests the high reactivity of the “solitary water”. The water molecules dissolved very slightly in organic solvent are more naked with the surface charges less canceled (in the short range) and less screened (in the long range); they are isolated from the other water molecules and have been named as the “solitary water”. − , The water–water close contact should be minor at the lowest water content of x w = 0.007. The water required for the continuous glycosidic-bond cleavage is automatically supplied by the subsequent simultaneously occurring dehydration of monosaccharides.…”

Structure assignment and H/D-exchange behavior of several glycosylated polyphenols

“…The unusual experimental observation prompted us to investigate the structures by molecular modeling to find a possible explanation. It is known from the literature that phenolic structures in which hydroxyl groups are present in meta-position to each other can undergo H-/D-exchange but require elaborate transition metal catalysis or other catalysis, 40 unusual reaction conditions such as sub-/super-critical solvents 41 or rather harsh reaction conditions. 42 Scheme 1 shows a few of such reactions.…”