Esterification, acid catalysis equilibrium

An alcoholysis reaction, in which an ester reacts with an alcohol to form a new ester and a new alcohol, is called a transesterification reaction. An example of this alcoholysis reaction is given in Figure 19.5. Transesteiification reactions are slow reactions because alcohols are poor nucleophiles and esters have very basic leaving groups hence, they occur under support of acid catalysis (Bmice, 2004). Similar to esterifications, transesteiification reactions are equihbiium reactions (Hoydonckx, Vos, Chavan, Jacobs, 2004) hence, an excess of alcohol and a direct extraction of products is needed to shift the equilibrium. 

The most common and most thoroughly studied type of homogeneous catalysis is acid-base catalysis. It includes hydrolysis, alcoholysis, esterification, and condensation reactions among many others. It is characterized by the fact that the equilibrium between base and conjugate acid, or between acid and conjugate base, is coupled with the actual catalytic cycle. 

Ester formation is a standard organic reaction between an alcohol and a carboxylic acid, which is an equilibrium reaction that has been shown to occur under catalysis by either acid or base. In polyesterification involving an organic acid, the substrate is itself the catalyst. It has been noted (Pilati, 1989) that, among the many reaction mechanisms, Scheme 1.1 is the most likely for acid-catalysed esterification, with the second reaction being the rate-determining step. 

Several authors have reported kinetic studies on the esterification of lactic acid with ethanol, carried out with different catalysts (mainly heterogeneous catalysis), at different temperatures, lactic acid/ethanol ratio and different eoneentrations of aqueous lactic acid. " In most studies, the presence of oligomers has not been considered, since their amount is irrelevant at equilibrium. ... 

Pervaporation-assisted catalysis is a typical example of an operation eflide-ntly carried out in extractor-type catalytic membrane reactors. Esterification is by far the most studied reaction combined with pervaporation. " Esters are a class of compounds with wide industrial appUcation, from polymers to fragrance and flavour industries. Esterification, a reaction between a carboxylic acid and an alcohol with water as a by-product, is an equilibrium-limited reaction. So, this is a typical reaction that can be carried out advantageously in a extractor-type membrane reactor. By selectively removing the reaction product water, it is possible to achieve a conversion enhancement over the thermodynamic equilibrium value based on the feed conditions. 

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