On-water reactions are a group of

organic reaction Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, Mechanistic Organ ...

s that take place as an

emulsion An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...

in water and have an unusual

reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...

acceleration compared with (i) the same reaction in an

organic solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...

, or (ii) the corresponding

dry media reaction A dry media reaction or solid-state reaction or solventless reaction is a chemical reaction system in the absence of a solvent. The drive for the development of dry media reactions in chemistry is * economics (save money on solvents) * ease of pur ...

. This effect has been known for many years but in 2005 researchers in the group of K. Barry Sharpless published a systematic study into this phenomenon. The rate acceleration is found in certain

Claisen rearrangement The Claisen rearrangement is a powerful carbon–carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of an allyl vinyl ether will initiate a ,3sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl, d ...

s. In one typical example of this reaction at

room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...

the

chemical yield In chemistry, yield, also referred to as reaction yield, is a measure of the quantity of moles of a product formed in relation to the reactant consumed, obtained in a chemical reaction, usually expressed as a percentage. Yield is one of the pri ...

was found to be 100% on water after 120 h compared with 16% for the same reaction in toluene and 73% in the neat reaction. : Enhanced reactivity is also found in

cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...

s. The reaction of

quadricyclane Quadricyclane is a strained, multi-cyclic hydrocarbon with the formula CH2(CH)6. A white volatile colorless liquid, it is highly strained molecule (78.7 kcal/mol). Isomerization of quadricyclane proceeds slowly at low temperatures.Petrov, V. A; ...

with

DEAD Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...

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 Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colorless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...

takes 36 hours and the neat reaction takes even longer (48 hours). : Other reactions with

apolar In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. Polar molecules must contain one or more polar ...

reactant In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

s such as

Ene reaction In organic chemistry, the ene reaction (also known as the Alder-ene reaction by its discoverer Kurt Alder in 1943) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a compound containing a multiple bond (the enophile ...

s and

Diels–Alder reaction In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a peric ...

s also exhibit rate accelerations. An explanation is not available but it involves

hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...

and the presence of a small amount of dissolved solute. This reaction type is of interest to

green chemistry Green chemistry, also called sustainable chemistry, is an area of chemistry and chemical engineering focused on the design of products and processes that minimize or eliminate the use and generation of hazardous substances. While environmental che ...

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 a coupling reaction between an

indole Indole is an aromatic heterocyclic organic compound with the formula C8 H7 N. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring. Indole is widely distributed in the natural environmen ...

and a

quinone The quinones are a class of organic compounds that are formally "derived from aromatic compounds

uch as benzene or naphthalene Uch ( pa, ; ur, ), frequently referred to as Uch Sharīf ( pa, ; ur, ; ''"Noble Uch"''), is a historic city in the southern part of Pakistan's Punjab province. Uch may have been founded as Alexandria on the Indus, a town founded by Alexand ...

by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double ...

takes place at

without catalyst in water in 82%

even though reactants and products are insoluble in this medium. The reaction is much less efficient in homogeneous systems such as

dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) at ...

and

acetonitrile Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...

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: : In this reaction the

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

''methyl 2-octynoate'' reacts with

triphenylphosphine Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists a ...

to an intermediate

zwitterion In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups. : With amino acids, for example, in solution a chemical equilibrium wil ...

ic allenolate, a dipolarophile for the

1,3-dipole In organic chemistry, a 1,3-dipolar compound or 1,3-dipole is a dipolar compound with delocalized electrons and a separation of charge over three atoms. They are reactants in 1,3-dipolar cycloadditions. The dipole has at least one resonance st ...

2-phenylnitrone. The primary

regioselective In chemistry, regioselectivity is the preference of chemical bonding or breaking in one direction over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a strong base ...

+2 dipolar cycloaddition product then rearranges to a dihydroisoxazole with regeneration of the phosphine. This reaction only takes place in water with

lithium chloride Lithium chloride is a chemical compound with the formula Li Cl. The salt is a typical ionic compound (with certain covalent characteristics), although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlor ...

added even though the reactants do not dissolve in this medium. In organic solvents such as

no reaction takes place.

Reaction in presence of water

An alternative classification with broader scope is suggested by Yujiro Hayashi as he describes certain organocatalytic

Aldol reaction The aldol reaction is a means of forming carbon–carbon bonds in organic chemistry. Discovered independently by the Russian chemist Alexander Borodin in 1869 and by the French chemist Charles-Adolphe Wurtz in 1872, the reaction combines two carb ...

s as taking place in the presence of water. The observed effect in these reactions is not rate acceleration (that would be ''on water''), but an increase in

enantioselectivity In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

. In the context of

organocatalysis In organic chemistry, organocatalysis is a form of catalysis in which the rate of a chemical reaction is increased by an organic catalyst. This "organocatalyst" consists of carbon, hydrogen, sulfur and other nonmetal elements found in organic com ...

, both concepts of on-water reactions and in-the-presence-of-water reactions were criticized in 2007 as not so environmentally friendly by

Donna Blackmond Donna Blackmond, Ph.D., (born April 19, 1958) is an American chemical engineer and the John C. Martin Endowed Chair in Chemistry at Scripps Research in La Jolla, CA. Her research focuses on prebiotic chemistry, the origin of biological homochirali ...

. 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 due to water suppressing reaction deactivation.

References

{{reflist, colwidth=30em , refs= ''Organic Synthesis “On Water”'' Arani Chanda and Valery V. Fokin Chemical Reviews 2009 109 (2), 725-748 {{doi, 10.1021/cr800448q ''Organic Reactions in Aqueous Media with a Focus on C-C Bond Formations'' Chao-Jun Li Chem. Rev. 1993, 93, 2023-2035 {{doi, 10.1021/cr00022a004 ''Organic synthesis reactions on-water at the organic–liquid water interface'' Richard N. Butler and Anthony G. Coyne Org. Biomol. Chem., 2016,14, 9945-9960 {{doi, 10.1039/C6OB01724J ''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. ''Angewandte Chemie'' (, meaning "Applied Chemistry") is a weekly peer-reviewed scientific journal that is published by Wiley-VCH on behalf of the German Chemical Society (Gesellschaft Deutscher Chemiker). Publishing formats include feature-lengt ...

21/2005 p 315

{dead link, date=February 2019, bot=medic{{cbignore, bot=medic ''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 - 87
Abstract
/ref> ''Organocatalysis on water. Regioselective + 2cycloaddition of nitrones and allenolates''David González-Cruz, David Tejedor, Pedro de Armas, Ezequiel Q. Morales and Fernando García-Tellado Chem. Commun., 2006, 2798 - 2800, {{doi, 10.1039/b606096j ''In Water or in the Presence of Water?'' Yujiro Hayashi

2006, 45, 8103 – 8104 {{doi, 10.1002/anie.200603378 ''Water in Organocatalytic Processes: Debunking the Myths'' Donna G. Blackmond, Alan Armstrong, Vyv Coombe, and Andrew Wells

2007, 46, 3798 – 3800 {{doi, 10.1002/anie.200604952 Organic chemistry