Esterification Chapter water removal

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. 

Inhibition of the enzymes can be caused by water, as already shown in Chapter 9.2.2.2. Therefore esterification reactions are very difficult to handle, because the produced water has to be removed from the reactor to avoid enzyme inhibition. Increasing the CO2 flow-rate would be one possibility for removing the produced water from the reactor, but this results in higher investment and operation costs. 

Although the previous two sections of this chapter emphasized hydrolytic processes, two mechanism that led to O or N-acylation were considered. In the discussion of acid-catalyzed ester hydrolysis, it was pointed out that this reaction is reversible (p. 654). Thus it is possible to acylate alcohols by acid-catalyzed reaction with a carboxylic acid. This is called the Fischer esterification method. To drive the reaction forward, the alcohol is usually used in large excess, and it may also be necessary to remove water as it is formed. This can be done by azeotropic distillation in some cases. 

The mechanism for acid-catalyzed esterification of carboxylic acids is completely reversible, but is driven to the right (toward the ester product) but application of Le Chatelier s principle and removal of water (also see Chapter 18, Section 18.6.3). Another application of Le Chatelier s principle uses a large excess of the alcohol (butanol for the formation of 70) to shift the equilibrium toward the ester by increasing the probability that it will react with alcohol rather than with water. Note that if ester 70 is treated with an acid catalyst and water rather than butanol, the mechanism shown will convert the ester back to the acid. In other words, if water replaces butanol, the mechanism from 70 to 21 is that for acid-catalyzed hydrolysis of an ester to a carboxylic acid (Section 20.2). 

In acid-catalyzed esterification reactions (an esterification is a reaction that produces an ester), the alcohol part of the ester is commonly used as the solvent for alcohols with low boiling points. In the conversion of 21 70, butanol is used as the solvent to generate the butyl ester. Formation of an ethyl ester would use ethanol as a solvent and formation of a methyl ester would use methanol. If an ester is formed from an alcohol with a high boiling point, a solvent such as benzene or toluene is often used and an excess of the alcohol is then added to that solvent. Azeotropic distillation (Chapter 18, Section 18.6.3) using these solvents allows removal of the water product and facilitates formation of the ester. 

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