Radical transformations of organic solvents under sub- and supercritical conditions

Abstract

A comparative study of chemical transformations of ethanol, acetone and dimethyl carbonate was performed in a temperature range from 200 to 300 °C. These compounds are widely used as solvents for processing vegetable raw materials. Therefore, knowing their behaviour in a reaction medium is necessary to understand the mechanism of sub- and supercritical extraction of the vegetable raw materials and the role of solvents in this processуы. It was established that the investigated compounds under certain conditions function as a source of free radicals; thus, they can contribute to the radical decomposition of the major components in plant biomass. The Schwarz toluene method was used to detect free radicals in reaction media. It is shown that radical breakdown of bonds in the acetone and dimethyl carbonate molecules occurs already at a temperature of 200 °C, and in ethanol - only under supercritical conditions at temperatures over 250 °C. Alkylation is the main reaction occurring during the thermal treatment of toluene and solvents mixtures, which leads to a high yield of various alkylbenzenes for all investigated xylene solvents. In a temperature range from 200 to 250 °С, acetone has the highest alkylating capability and ethanol - the lowest one. At temperatures above 250 °С, the alkylating capability of solvents is similar, with a slight prevalence of that of dimethyl carbonate. Unlike other solvents, acetone participates in nucleophilic addition reactions along with radical transformations under process conditions. The products obtained as a result of these reactions at temperatures above 250 °C are predominant. The main ones are the products of the aldol and crotonic condensation of acetone. Based on the results of a study, the pathways of the radical decomposition of solvents molecules and the formation of products of thermal treatment were proposed.