We examined the hydrolysis of dibenzyl ether, benzyl t-butyl ether, methyl t-butyl ether, methylbenzoate, and diphenylcarbonate in high-temperature liquid water, both with and without added acid or base. The apparent reaction order for H+ did not exceed 0.2 for any of the compounds investigated. This result indicates that hydrolysis of these compounds in high-temperature water (HTW) does not follow the kinetics expected for specific acid catalysis (H+ reaction order = 1.0), as does the hydrolysis at ambient temperatures. Rather, the greater thermal energy in the HTW system allows protonation by water molecules to become faster than protonation by H+ at near-neutral conditions. Because the water-catalyzed path is faster, the occurrence of these acid-catalyzed reactions in HTW with no added acid is not due to the elevated value of K w, the ion product. This finding contradicts the conventional wisdom in this field.

We examined two crossed aldol condensations in pure liquid water at temperatures of 250, 300, and 350 °C. We synthesized benzalacetone from benzaldehyde and acetone, and chalcone from benzaldehyde and acetophenone. We provide evidence that these reactions are acid and base catalyzed in high-temperature water. Benzalacetone was always the major reaction product in its experiments and chalcone was almost always the major product in its synthesis experiments. At 350 °C and long reaction times (5 and 8 hours) the chalcone yields were surpassed by those of 3-phenylpropiophenone. The maximum molar yield from the benzalacetone synthesis was 24% at 250 °C and 5 hours, and the maximum yield of chalcone was 21% at 250 °C and 15 hours. At higher temperatures, the yield of the unsaturated ketone products was lower due to formation of degradation products such as benzoic acid, benzyl alcohol, benzylacetone, acetophenone, E-stilbene, propiophenone and diphenylethane. A reaction network with reversible formation of the unsaturated ketone, its degradation, and a path for benzaldehyde disproportion provided the basis for a quantitative reaction model. These results provide another illustration of the ability of HTW to facilitate acid-and base-catalyzed reactions without any added acid or base.

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The rearrangement of benzil is base (and not acid) catalyzed under conventional conditions (water-dioxane mixture around 100 uC). We examined this reaction in high-temperature water (HTW) between 300-380 uC with the intent of studying a reaction that proceeds solely by base catalysis in this more environmentally benign medium. The rearrangement proceeds in neutral HTW without addition of base, but the yield of rearrangement products is nearly insensitive to pH at near-neutral conditions. Adding larger amounts of base, however, leads to much higher yields and 100% selectivity to rearrangement products. Likewise, adding larger amounts of acid leads to comparable yields, but less than 100% selectivity. The selectivity to rearrangement products generally increased with pH at near-neutral and basic conditions whereas the selectivity to benzil decomposition products (a competing thermal pathway) exhibited a maximum at nearneutral conditions. We conclude that the benzil rearrangement is catalyzed by acid, base, and water in HTW. The dominant mechanism shifts as the pH changes. This system shows that mechanisms that are unimportant at conventional reaction conditions can become dominant in HTW. It also demonstrates the ability to use pH to direct the selectivity of a reaction in HTW.

We have elucidated the kinetics for both the rearrangement of benzil (1,2-diphenylethanedione) to benzilic acid and for the subsequent reactions of benzilic acid in pure high-temperature liquid water (HTW). The rearrangement is rapid, and the benzilic acid formed can react via two parallel pathways. One is decarboxylation to form benzhydrol and the other is a self-reaction to form diphenylketene plus benzophenone. Diphenylketene is hydrated in HTW to form diphenylacetic acid, which can decarboxylate to form diphenylmethane. Benzhydrol reacts slowly in HTW, but it forms diphenylmethane and benzophenone in equal amounts. This set of reaction pathways is shown to be consistent with the experimental data obtained from the reactions of benzil, diphenylacetic acid, and benzhydrol, individually, in HTW.