On Water Reaction

On Water Reaction

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. This effect has been known for many years but in 2005 researchers in the group of K. Barry Sharpless presented a systematic study into this phenomenon.

The rate acceleration is found in certain Claisen rearrangements. In one typical example of this reaction at room temperature the chemical yield was found to be 100% on water after 120 hours compared to 16% with the same reaction in toluene or 73% in the neat reaction.

Enhanced reactivity is also found in cycloadditions. The reaction of quadricyclane with DEAD is a 2σ + 2σ + 2π cycloaddition that on water takes place within 10 minutes at room temperature with 82% yield. The same reaction in toluene takes 24 hours at 80°C with 70% yield. An emulsion reaction in fluorinated cyclohexane takes 36 hours and the neat reaction takes even longer (48 hours).

Other reactions with apolar reactants such as Ene reactions and Diels-Alder reactions also exhibit rate accelerations. An explanation is not available but it involves hydrogen bonding and the presence of a small amount of dissolved solute. This reaction type is of interest to green chemistry because it greatly reduces the usage of organic solvents, reaction product isolation is relatively easy, and it increases the yields and chemical purity with little extra expenditure, if not less.