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We all know what it means to put something "on ice", but what is a chemical reaction "on water"? This new expression has been coined by a team headed by K. Barry Sharpless, winner of the 2001 Nobel Prize for chemistry, to describe reactions of organic substances that are not water-soluble, yet react well or even considerably faster in the presence of water than in organic solvents. If water could also replace organic solvents more often on the industrial scale, it would save money, increase the safety of chemical facilities, and reduce stress on the environment. Another advantage is that after the reaction, the organic and aqueous phases separate, eliminating the need for complex isolation steps to obtain the product.

Until now, a central aspect in the area of aqueous organic chemistry has been the effort to improve the water- solubility of the substances involved. Has this been the wrong approach? Is the axiom that has been passed on from the days of alchemy, corpora non agunt nisi soluta (substances do not interact with each other if they are not dissolved), no longer valid? Do reactants not need to be water-soluble at all in order to react in an aqueous environment? It seems that the situation bears some rethinking. Says Sharpless, "In contrast to prior assumptions, it seems that in many cases the immiscibility of the organic and aqueous phases is a considerable advantage."

So what exactly does "on water" mean? The expression simply refers to the fact that the essentially insoluble reactants and the water are vigorously stirred together. This forms a suspension, meaning that the immiscible liquids are finely divided into tiny drops. The contact surface between the aqueous and organic phases is thus especially large.

Why certain important categories of reactions, such as the Claisen rearrangement, work so well in aqueous suspension is not yet clear. Particularly astonishing is the fact that the reactions occasionally go faster "on water" than in a mixture of the pure reactants (without any solvent). "Molecules at the interface between two different phases often behave differently than molecules within the phase." Sharpless speculates: "It is possible that the unique properties of molecules at the interface between the water and the hydrophobic, oily organic phase play an important role in speeding up the reactions."

This is not the first report of reactions “on water” Henry Shaw, Howard D. Perimutter, and Chen Gu with Susan D. Arco and Titos O. Quibuyen reported in J. Org. Chem. 1997, 62, 236, that free radical brominations can be advantageously performed by photolysis in a heterogeneous nature with a water phase. Water was advantageous in this situation because it is an excellent medium for free radical reactions and because the oxygen-hydrogen bond is so strong that it is inert. They also report running the brominations in the neat starting material liquid. Yields of brominated products were the same in the absence as in the presence of water. The advantages of the water noted by the authors were:

· the reaction on water can replace the reaction in carbon tetrachloride which is disfavored because of the environmental toxicity of carbon tetrachloride

· the hydrogen bromide was removed from the organic layers into the water or lost to the atmosphere

· by providing a diluting medium the free radicals are dispersed over the reaction volume reducing the recombinations near the window where the light enters the reactor

· by providing a heat sink the water maintains a more uniform temperature making the product distribution more dependable

· partitioning of the heavier than water products from the lighter than water starting materials creating a three phase mixture

An advantages that was not mentioned was the increase in reactor volume that may be required to reach the stirrer when working on scale.

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