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The nature of the rate enhancements caused by gradually increasing the mole traction of water in the solvent (from 0 to 1) for the cycloaddition reactions of pyridazinium-dicyanomethanide 1,3-dipole, 2, with the dipolarophiles ethyl vinyl ketone (a water-super dipolarophile) and methyl acrylate (a water-normal dipolarophile) in the organic solvents acetonitrile, acetone, methanol, ethanol, and tert-butyl alcohol at 37 °C are explored. In each case as the mole fraction of water surpassed ca. 0.9, exponential rate enhancements were triggered. When methanol replaced water, the rate enhancements were smaller, and no triggering effect was observed. The dramatic rate enhancement triggered for the water-super dipolarophile was significantly reduced as the temperature was raised in the range 29-64 °C. The results suggest that a dominant hydrogen-bonding effect operates in water-induced rate enhancements of 1 ,3-dipolar cycloaddition reactions with water-super dipolarophiles as well as the hydrophobic effect. The hydrogen- bonding effect involves secondary bridging hydrogen bonding from the primary water-solvation shell of the transition state and the growth of structured water clusters. Theoretical calculations strongly support these conclusions. [ABSTRACT FROM AUTHOR]

The article focuses on the significance of mechanistic differences between the transfers of hydrophilic and hydrophobic ions to low-polarity solvents. It differentiates hydrophobic ions and hydrophilic ions capability of transferring water to organic solvents. Furthermore, it emphasizes the significance of the amount of hydrophilic ions in its ability to increase water concentration in organic phase.