Understanding “On-Water” Catalysis of Organic Reactions. Effects of H⁺ and Li⁺ Ions in the Aqueous Phase and Nonreacting Competitor H‑Bond Acceptors in the Organic Phase: On H₂O versus on D₂O for Huisgen Cycloadditions

For a typical Huisgen cycloaddition, carried out on water, the behavior of water molecules at the oil–water interface depended on the properties of the reactants. With weakly basic reactants, a small quantity of added H⁺ (HClO₄, 0.0001–0.01 M) present in the aqueous phase had negligible effects, but larger quantities of H⁺ (HClO₄, 0.1–3.0 M) increased the catalytic effect and caused protons to cross the water–organic interface and affect the products. Added Li⁺ ions (LiClO₄, 0.1–3.0 M) had no effect for on-water reactions but enhanced the rates and endo products for in-water reactions. For these cycloaddition reactions, the product endo:exo ratios, when compared to those in organic solvents, can be used to distinguish between the on-water and in-water modes. Comparisons of organic reactions on H₂O and on D₂O indicate that on-water catalysis ranges from weak to strong trans-phase H-bonding for reactants with basic p<i>K</i>_a < <i>ca</i>. −6 and to interfacial proton transfer for reactants with higher basic p<i>K</i>_a > ca. 2 (p<i>K</i>_a of conjugate acid). Water shows a chameleon-type response to organic molecules at hydrophobic surfaces