Carboxylic acids continued esterification

Because amorphous carbon as graphite heats up strongly under MW irradiation [4], its use as a sensitizer has been widely reported [5-10] (Sect. 7.1). Recently, MW-as-sisted esterification of carboxylic acids with alcohols was performed on activated carbon in good yields (71-96%) [98]. For our part, when charcoal powder was used as a support, we had difficulty in desorbing the reaction products [15]. Even with a continuous extractor, the desorption was never quantitative. The desorption of reaction products from graphite powder is much easier than from amorphous carbon powder. 

Various combinations of reactant(s) and process conditions are potentially available to synthesize polyesters [Fakirov, 2002 Goodman, 1988], Polyesters can be produced by direct esterification of a diacid with a diol (Eq. 2-120) or self-condensation of a hydroxy carboxylic acid (Eq. 2-119). Since polyesterification, like many step polymerizations, is an equilibrium reaction, water must be continuously removed to achieve high conversions and high molecular weights. Control of the reaction temperature is important to minimize side reactions such as dehydration of the diol to form diethylene glycol... 

Shieh et al." reported the use of a continuous flow reactor for the esterification of a 50 to 100 g scale of carboxylic acids to methyl esters with dimethyl carbonate and 1,8-Diazabicyclo[5.4.0]undec-... 

Problem 5.5 A nonstoichiometric mixture of a dicarboxylic acid and a glycol having 20 mol % excess of the latter and a carboxyl group concentration of 5.64 mol/kg was polymerized under nonequilibrium conditions with continuous removal of the by-product water, to about 80% esterification of the carboxylic acid groups. The mixture was then further polymerized in the presence of a strong acid catalyst which yielded the following data of the concentration of carboxylic acid groups in the mixture versus reaction time. 

Other recent relevant developments have been the replacement of conventional acid catalysts with greener analogues. Direct esterification of carbo Q lic acids and alcohols continues to be a focus of attention. As examples, diphenylammonium triflate 8 and bulky diatylammonium sulfonates were shown to catalyse ester condensation of carboxylic acids and alcohols efficiently, and without the need for azeotropic water removal in the former case. Pentafluorophenylammonium triflate 9 was shown to be an efficient and cost-effective catalyst not only for esterification, but also for thioesterification, transesterification and macrolactone formation without requiring a dehydrating system. The superior catal)4ic efficiency of 9 relative to 8 was ascribed to the lower basicity of the pentafluoroaniline counter amine compared to diphenylamine. In related work, polyaniline... 

Synthesis.— The perennial interest in the esterification of carboxylic acids, particularly hindered ones, continues. Isopropenyl esters, prepared by the zinc(ii)-catalysed addition of the hindered acids to propyne, undergo ready acid-catalysed exchange with alcohols, as shown in Scheme 24,... 

The assiunption that functional group reactivity is independent of chain length can be verified kinetically by following a polyesterification. The simple esterification is an add-catalyzed process in which protonation of the acid is followed by interaction with the alcohol to produce an ester and water. If significant polymer formation is to be achieved, the water must be removed continuously from the reaction to displace the equilibriiun, and the water eUminated can be used to estimate the extent of the reaction. Alternatively, the rate of disappearance of carboxylic groups can be mea-smed by titrating aliquots of the mixture. 

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