Reaction, greenness waste, quantities

The most widely used homogeneous catalysts are simple acids and bases which catalyse well-known reactions such as ester and amide hydrolysis, and esterification. Such catalysts are inexpensive enough that they can be neutralized, easily separated fi om organic materials, and disposed of. This, of course, is not a good example of green chemistry and contributes to the huge quantity of aqueous salt waste generated by industry. 

Clay minerals are effective catalysts of many organic reactions, frequently showing product, regio, or shape selectivity. The other important advantage of clay minerals as catalysts is that the reaction conditions are frequently milder than in traditional procedures. The catalyst can be eliminated by filtration, and the use of hazardous solvents can be avoided, which reduces the quantity of waste. Consequently, clay minerals are often employed as catalysts in the so-called green chemistry. 

The reactions and mechanisms covered in this chapter represent a growing modern trend in organic chemistry. Organometallic catalysts facilitate reactions that are otherwise impossible, are very difficult, or would require many synthetic steps in order to accomplish. The use of these catalysts causes a net decrease in the number and quantity of reagents, solvents, and purifications necessary in an overall synthetic sequence. This decrease means that the chemical waste from an industrial process can be dramatically reduced. Intentionally designing a chemical procedure or process to decrease waste and toxic by-products is now a whole chemical field in and of itself that is called green chemistry. 

Green chemistry consists of chemicals and chemical processes designed to reduce or eliminate impacts on the environment. The use and production of these chemicals may involve reduced waste products, non-toxic chemicals, and improved efficiency. Industrial chemists evaluate chemical pathways and their economic and environmental costs by calculating the relative efficiency of the chemical reactions involved. Percentage yield provides a means of comparison of the theoretical and actual quantity of product, and used to be the main way of evaluating reaction efficiency. However, calculation of atom economy has become a more important means... 

One of the main efforts towards the development of sustainable chemistiy is the reduction in the generation of waste. The largest amount of waste produced by a reaction is typically associated with the solvent employed as it is the component added in the largest quantity and, usually not incorporated into the final product but rather removed and disposed of at the end of the process. For these reasons, many efforts are being devoted to the development of catalytic systems that can operate under solvent-free conditions. When the use of a solvent is mandatory, the efforts are directed towards the use of sustainable solvents. In this context, water, which is the only natural solvent, is the preferred choice. Other green alternatives are ionic liquids or supercritical carbon dioxide. 

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