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

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 °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 Ϫ4 s Ϫ1 ; 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 Ϫ6 – 10 Ϫ4 s Ϫ1. 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.