On water reactions are a group of organic reactions that take place as an emulsion in water and that exhibit an unusual reaction rate acceleration compared to the same reaction in an organic solvent, or compared to the corresponding dry media reaction.
“The notion of the special nature of water as a solvent for organic reactions began with examples of Diels–Alder reactions more than fifty years ago. The first quantitative data were reported by Rideout and Breslow, who showed that Diels–Alder reactions between nonpolar compounds proceeded at much higher rates in water (dilute homogeneous solution) than in organic solvents. Rate accelerations as high as 200-fold were noted in certain cases.”
“On Water”: Unique Reactivity of Organic Compounds in Aqueous Suspension Angewandte Chemie, Volume 44, Issue 21 May 20, 2005
There is a growing list of organic chemical reactions which have been shown to have considerable acceleration 'on water'. It's speculated that the unique properties of molecules at the macroscopic phase boundary between water and insoluble hydrophobic oils might play a role - but these effects are yet to have a formally accepted explanation.