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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.〔''Unique Reactivity of Organic Compounds in Aqueous Suspension '' Sridhar Narayan, John Muldoon, M. G. Finn, Valery V. Fokin, Hartmuth C. Kolb, K. Barry Sharpless Angew. Chem. Int. Ed. 21/2005 p 3157 ()〕 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. ==Examples== In one study 〔''On Water-Promoted Direct Coupling of Indoles with 1,4-Benzoquinones without Catalyst'' Hai-Bo Zhang, Li Liu, Yong-Jun Chen, Dong Wang, Chao-Jun Li European Journal of Organic Chemistry Volume 2006, Issue 4 , Pages 869 - 873 (Abstract )〕 a coupling reaction between an indole and a quinone takes place at room temperature without catalyst in water in 82% chemical yield even though reactants and products are insoluble in this medium. The reaction is much less efficient in homogeneous systems such as dichloromethane, toluene and acetonitrile or even the solvent free reaction or even the water reaction but now at 50°C. : The ''On water'' effect is also studied in cycloadditions of the type:〔''Organocatalysis on water. Regioselective (+ 2 )-cycloaddition of nitrones and allenolates''David González-Cruz, David Tejedor, Pedro de Armas, Ezequiel Q. Morales and Fernando García-Tellado Chem. Commun., 2006, 2798 - 2800, 〕 : In this reaction the alkyne ''methyl 2-octynoate'' reacts with triphenylphosphine to an intermediate zwitterionic allenolate, a dipolarophile for the 1,3-dipole 2-phenylnitrone. The primary regioselective ()dipolar cycloaddition product then rearranges to a -dihydroisoxazole with regeneration of the phosphine. This reaction only takes place in water with lithium chloride added even though the reactants do not dissolve in this medium. In organic solvents such as toluene or dichloromethane no reaction takes place. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「On water reaction」の詳細全文を読む スポンサード リンク
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