Environmental problems aqueous solvents

Carbonylation of Halides - Pd(tppts)3-catalysed carbonylation of bromo-benzene (Equation 7) in the presence of NEt3 in an aqueous/toluene (1/1) two phase system at 150°C and 15 bar CO afforded the triethylammonium salt of benzoic acid (100% yield).464,465 Rates were rather low (TOF s of 3.3-17 h ) but no decomposition of Pd(tppts)3 (tppts/Pd 12.5) was observed and the catalyst could be recovered quantitatively and recycled 464 However, in a second recycle extensive decomposition of the catalyst occurred with formation of palladium black. Generally in carbonylation reactions of halides the formation of stoichiometric amounts of either HX or halide salts still remains a problem of environmental concern despite the attractiveness due to the presence of the aqueous solvent. 

Another class of solvents which presents environmental problems comprises the polar aprotic solvents, such as dimethylformamide and dimethyl sulfoxide, that are the solvents of choice for, e.g. many nucleophilic substitutions. They are high boiling and not easily removed by distillation. They are also water-miscible which enables their separation by washing with water. Unfortunately, this leads inevitably to contaminated aqueous effluent. 

Sc(OTf)3 is more soluble in water than in organic solvents such as dichloromethane. The catalyst can be recovered almost quantitatively from the aqueous layer by simple extraction after the reaction was complete (Sch. 2), and it could be re-used. The recovered catalyst is also effective in the 2nd reaction, and the yield of the 2nd run is comparable with that of the 1st run (Eq. 1) [4], Because Sc(OTf)3 can be successfully recovered and re-used in many other reactions, it is expected to solve severe environmental problems caused by mineral acid- or Lewis acid-promoted reactions in the chemical industry. 

While continuous use of LnfOTOs is possible, it is also easy to recover Ln(OTf)3 compounds themselves. Lanthanide triflates are more soluble in water than in organic solvents such as dichloromethane. Almost 100% of Ln(OTl)3 was quite easily recovered from the aqueous layer after the reaction was completed and could be reused. The reactions are usually quenched with water and the products are extracted with an organic solvent (for example, dichloromethane). The lanthanide triflate is in the aqueous layer and removal of the water is all that is required to give the catalyst which can be used in the next reaction (Scheme 14-1). It is noteworthy that lanthanide triflates are expected to solve some severe environmental problems induced by Lewis acid-promoted reactions in industry chemistry [20]. 

Investigations using hydrotrope solutions in reaction kinetics have varied from the direct analysis of the influence on the kinetics by the solubilization per se [29] of aromatic esters to more elaborate reaction systems. Microwave heating was early shown as an efficient way to enhance organic reactions [30,31], However, the use of common organic solvents causes environmental problems in connection with microwave heating and aqueous solutions offer safe and convenient... 

As a cautionary note PTC should not be considered a panacea for all of the problems associated with green chemistry. Two-phase reactions involving water are often difficult to deal with industrially, particularly if the water is contaminated with trace amounts of hazardous organic substances. In some cases it may be more practical, cost effective and environmentally prudent to avoid production of aqueous waste in favour of a recyclable less benign solvent. 

In addition to all the good features of the Stille couplings, there are a few problems with the use of RSnMles or RSn(n-Bu)3 in aqueous solutions. These compounds are rather volatile and water-insoluble but this can be overcome with the aid of co-solvents. However, the products of the reaction still contain alkyltin species which are toxic and environmentally unacceptable. Furthermore, only one of the four Sn-C units take active part in the... 

The method for the manufacture of polypropylene by the Ziegler-Natta process, which has been in widespread use for several decades, involved some years ago a polymerization in a relatively volatile solvent, for example a light petroleum fraction. That was the drawback of this process, since in the separation and subsequent drying of the polymer formed the solvent could not be completely recovered. Problems are thus experienced in fulfilling environmental protection requirements. An additional obstacle was the large volume of aqueous waste that is generated during workup of the polymer suspension. 

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