Esterification by azeotropic distillation with toluene

Enanthaldehyde, -METHYL-/3-OXO-, DIMETHYL ACETAL, 32, 79 Enanthic acid, e-oxo, 31, 3 Epichlorohydrin, 31, 1 Esterification, by azeotropic distillation with toluene, 30, 30, 31 of ethanol with phosphorus trichloride, 31, 111... 

This reaction, known as Fischer esterification, requires the presence of an acid catalyst. Because the carboxylic acid and the ester have similar reactivities, the reaction is useful only if a method can be found to drive the equilibrium in the direction of the desired product—the ester. In accord with Le Chatelier s principle, this is accomplished by using an excess of one of the reactants or by removing one of the products. An excess of the alcohol is used if it is readily available, as is the case for methanol or ethanol. Or water can be removed by azeotropic distillation with a solvent such as toluene. 

An example of a typical batch preparation of a polyester is one where 1.2 moles of propylene glycol, 0.67 mole of maleic anhydride, and 0.33 mole of phthalic anhydride are combined. Propylene glycol is used in excess to compensate for loss during the reaction. The condensation at 150-200 °C lasts for 6-16 hours, with constant removal of water, the byproduct. An aromatic solvent, like toluene or xylene, is often added to the reaction mixtures to facilitate water removal by azeotropic distillation. Esterification catalysts, like toluene sulfonic acid, reduce the reaction time. In addition. 

Esters of low volatility are accesible via several types of esterification. In the case of esters of butyl and amyl alcohols, water is removed as a binary azeotropic mixture with the alcohol. To produce esters of the lower alcohols (methyl, ethyl, propyl), it may be necessary to add a hydrocarbon such as benzene or toluene to increase the amount of distilled water. With high boiling alcohols, ie, benzyl, furfuryl, and P-phenylethyl, an accessory azeotroping Hquid is useful to eliminate the water by distillation. 

The best approach to improving separations is to work toward reactions that achieve 100% yields at 100% conversions. Frequently, this will require more selective catalysts. The previous chapter contained an example moving in this direction. Toluene was disproportionated to benzene and xylenes using a silica-modified zeolite catalyst.23 After removal of benzene and unchanged toluene by distillation, the xylene remaining was a 99% para-isomer. It was clean enough to put directly into the process of oxidation to terephthalic acid. This avoided the usual separation of xylenes by crystallization or by a molecular sieve. There are times when an equilibrium can be shifted by removal of a product or by-product continuously to give 100% conversion. The familiar esterification with azeotropic removal of water or removal of water with a molecular sieve is an example. 

When either or both the acid and the alcohol are volatile, the reaction may be carried to completion by distilling out the water produced in the reaction, usually as an azeotrope. The azeotrope can be water with one of the components of the reaction mixture or with an inert solvent which is insoluble in water. Generally, an azeotrope is selected which has a toiling point below 100 C and which condenses into two phases. The butyl alcohols and their higher homologues form azeotropes with water which behave in this manner, as do the inert solvents btozene, toluene, chloroform, ethylene dichloride, and carton tetrachloride. When methyl, ethyl, or propyl alcohol is used in an azeotropic esterification, one of the inert solvents can be used to produce a two-phase distillate. 

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