Waste-free large-scale syntheses without auxiliaries for sustainable production omitting purifying workup
Gas–solid and stoichiometric solid–solid reactions are usually quantitative and waste-free and thus particularly sustainable and environmentally benign. It is therefore shown how these can be scaled up with an emphasis for their use in chemical production. There is no necessity for purifying workup with solvent consumption (that means real solvent-freeness), waste disposal and other excessive energy input. These techniques avoid auxiliaries such as solvents, supports and catalysts, but the latter may be applied as gases with easy removal after reaction. Heat development is usually not a problem. Heat removal can be handled and is described also for extreme cases, as is the availability of equipment, in particular suitable milling techniques. The solid-state mechanism, which involves phase rebuilding (anisotropic molecular migrations within crystals), phase transformation and crystal disintegration, is the basics for the technical design. If (intermediate) melting cannot be avoided by cooling below eutectic temperatures, melt reactions at higher temperatures than required for solid-state reactions can be also rapid, uniform and quantitative, when direct crystallization of the product occurs. This version has an increased risk for side reactions as it does not profit from the crystal packing of the starting materials, but it profits from the crystal structure of the high melting product. For preparative purposes “large-scale” starts with the kg-range that can be easily reached in research institutes. It provides the basics for further scale up in industrial environments. Numerous typical examples are given for organic and some inorganic syntheses. These include solvent-free salt formations, complexations, condensations of amines, heterocyclic syntheses, Knoevenagel condensations, cascade reactions, halogen additions, stereo- and regio-specific protective reactions, and redox reactions. All of these may be of technical importance. Some of these involve now easily obtained products that cannot be achieved by solution reactions.