Water reactions are a group that take place as an emulsion in water and happen at an unique, accelerated reaction rate as compared to identical reaction in an organic solvent. An emulsion is a mixture of two or more unblendable liquids, such as oil and water emulsions. Although this group of organic reactions has been fact for many years, it was only 2005 when K. Barry Sharpless displayed a systematic study into this anomaly. This rate of acceleration is seen in certain types of Claisen rearrangements. A Claisen rearrangement is a carbon-carbon bond creating chemical reaction that was uncovered by Rainer Ludwig Claisen. When the allyl vinyl ether is heated, it will start a sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl.

Cycloadditions also enhance the reaction with water. A cycloaddition is where two or more unsaturated molecules join when the formation of a cyclic adduct is present, and there is a net reduction of the bond multiplicity. Some other water reactions with apolar reactants like Ene reactions and Diels-Alder reactions also shows an increases rate of reaction. In theory it is due to the involvement of hydrogen bonding and also the small traces of dissolved solute. It is this particular type of reaction that has gained the interest in ‘green chemistry’ because it drastically reduces the use of organic solvents. It also increases the chemical purity, and its yields.

One documented study of a coupling reaction involving a quinine and an indole that takes place at room temperature without catalyst in water produces an eighty two percent yield. This is done even though the reactants and products are insoluble in this particular medium. Other types of reactions with water can be less efficient, such as, dichloromethane, acetonitrile and tolens, as well as the solvent absent reaction. An indole is an aromic heterocyclic organic compound that has bicyclic makeup. Its makeup consists of a six-member benzene ring that is fused to a five-member nitrogen containing pyrrole ring. Indole is frequently used in the production of fragrances and also are precursors to many pharmaceuticals. Quinones contain a completely conjugated cyclic dione structure.

Yujiro Hayashi introduced an alternative classification that contained a wider scope. It stated that certain organocatalytic Aldol reactions are taking plce in the presense of water. The affect that is shown is not rate acceleration, but rather an increase in enantioselectivity. Enantioselectivity is one of two steroisomers that are non-superposable complete mirror images of one another. Opposite Enantiopure compounds are just that, only one of the sides has the molecules that can be detected.

In both concepts within the context of organacatalysts of on-water-reactions and in-the-presence-of-water reactions have came under some scrutiny in 2007. It was said that they were not as environmently friendly as first thought. This argument was brought forward by Donna Blackmond. She sited that the separation of reaction product will need an organic solvent anyway. In an aqueous system the water phase can be less then ten percent of the total, while another component makes up the real solvent.

 
According to Blackmond, separation of reaction product from the water phase usually requires organic solvent anyway and in reported aqueous systems the water phase can in reality be less than 10% of the total reaction mixture with another component forming the actual solvent. Blackmond also notes that in reported instances, the observed rate-acceleration in presence of water is in fact due to water suppressing reaction deactivation.