Emulsifiers from solid and liquid polyols: different strategies for obtaining optimum conversions and selectivities

The present paper provides a general overview of the factors involved in bo th the kinetics and the selectivity of partial acylation reactions of polyo ls (sugars). Different kinetic strategies for maximum production of interme diate esters of various polyols and monosaccharides are reported and discus sed. Physicochemical requirements for obtaining maximum selectivities and c omplementary strategies for reducing reaction times are discussed. The reac tions studied include glycerol, glucose, fructose, mannose, sorbitol and an alkyl glucoside as precursors. The high selectivity towards the monoglycer ide in the presence and absence of a solvent has been attributed to a combi nation of the precipitation of the desired ester, use of glycerol in excess and the relatively low solubility of the fatty acid in the system. Unlike the reaction in the presence of a solvent, the reaction in a solvent-free m edium produces the diester first. The monoester only accumulates in the med ium as a consequence of disproportionation and glycerolysis reactions of th e diester formed earlier. Selective esterification of solid sugars (polyols ) which have an intermediate solubility in acetone is favored at low temper atures at which a sufficient amount of polyol dissolves and concurrent prec ipitation of the desired product can be achieved. By contrast, use of eleva ted temperatures is more appropriate for selective partial esterification o f polyols, which are the most soluble in the solvent employed. Polyols (sug ars) which are the less soluble into the liquid reaction phase cannot be ea sily esterified. Diffusional limitations on the rate of dissolution of the solid precursor can be minimized by increasing the surface area of the soli d polyol. (C) 2001 Elsevier Science B.V. All rights reserved.