Green solvents
Organic solvents are toxic and volatile which are harmful to human health and creates pollution in environment Therefore it is necessary to replace these solvents by safer solvents like ionic liquids, supercritical CO2 or supercritical water and also solvent free system that utilizes the surface or interiors of clay zeolites, silica and alumina.Supercritical CO2 fluid is now becoming important commercial and industrial solvent for Chemical separations because of low toxicity and non inflamability. Ionic liquids (Welton 199 Holbery and seddon, 1999 Seddon 1997)are substances composed of cation and anions. Ionic liquids have emerged as green solvents because they have no measurable vapour pressure and that wide range of chemical reactions can be performed in them. Commonly used ionic liquids are those with alkyl ammonium, alkyl phophonium, N-alkyl pyridium cautions and Cl, Br-,I-,BF4- anions .Ionic liquids find their useful and important application from hydrogenation to biocatalytic reactions
Near critical water offers outstanding performance as benign solvents for both organic and ionic compounds .Friedal - Craft's acylation requires Lewis acids but in near critical water, phenol could be converted with acetic acid to produce 2-hydroxy acetophenone, 4 -hydroy acetophenone and phenyl acetate without addition of an acid catalyst.
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