Water as a Reaction Solvent

Even though synthesis of organic chemicals in the body occurs very efficiently in water, chemists have traditionally been taught that water is not generally a good solvent for canying out synthetic reactions, because of either its poor solvent properties or the hydrolytic instability of reagents 

Non-toxic Opportunity for replacing VOCs Naturally occurring Inexpensive Non-flammable High specific heat capacity -exothermic reactions can be more safely controlled Distillation is energy intensive Contaminated waste streams may he difficult to treat High specific heat capacity - difficult to heat or cool rapidly 

The scope of possible reactions using water as a solvent is quite remarkable and water is much under-utilized as a solvent in many academic and industrial research institutions, largely through lack of knowledge and a culture of using organic solvents. Several other examples 

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Scheme 5.14 Some examples using water as a reaction solvent...

Potential for new synthetic methodologies. Compared to reactions in organic solvents, the use of water as a reaction solvent has... 

There are many other examples in the literature where sealed-vessel microwave conditions have been employed to heat water as a reaction solvent well above its boiling point. Examples include transition metal catalyzed transformations such as Suzuki [43], Heck [44], Sonogashira [45], and Stille [46] cross-coupling reactions, in addition to cyanation reactions [47], phenylations [48], heterocycle formation [49], and even solid-phase organic syntheses [50] (see Chapters 6 and 7 for details). In many of these studies, reaction temperatures lower than those normally considered near-critical (Table 4.2) have been employed (100-150 °C). This is due in part to the fact that with single-mode microwave reactors (see Section 3.5) 200-220 °C is the current limit to which water can be safely heated under pressure since these instruments generally have a 20 bar pressure limit. For generating truly near-critical conditions around 280 °C, special microwave reactors able to withstand pressures of up to 80 bar have to be utilized (see Section 3.4.4). 

In the aldolisation step, pure water as a reaction solvent significantly inproved the yield of aldol by suppressing the acetalization of butyraldehyde. The hydrogenation kinetics with pure water as a solvent were determined at pressures of 40-80 bar and temperatures of 60-90 °C. The triol selectivity was 100% in all experiments studied. 

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