Equilibrium condensation reaction

PA-6 synthesis from, 177 Equilibrium condensation reaction, 164 Equilibrium constants, for polyamidations, 150-152... 

The field of generating functions was actually opened to polymer science with the work of Burchard [1], DuSek [2], and Gordon [4]. Gordon introduced the cascade theory [5] to derive finally the degree of polymerization in various systems. In contrast to radical polymerization the concept of directionality [6] is not of importance for equilibrium condensation reactions. Extensive work on the subject has been done by Tang [26, 27]. 

Nylon 11 is produced hy the condensation reaction of 11- aminounde-canoic acid. This is an example of the self condensation of an amino acid where only one monomer is used. The monomer is first suspended in water, then heated to melt the monomer and to start the reaction. Water is continuously removed to drive the equilibrium to the right. The polymer is finally withdrawn for storage ... 

Theoretically, a fall in concentration of oxaloacetate, particularly within the mitochondria, could impair the ability of the citric acid cycle to metabolize acetyl-CoA and divert fatty acid oxidation toward ketogenesis. Such a fall may occur because of an increase in the [NADH]/[NAD+] ratio caused by increased P-oxida-tion affecting the equilibrium between oxaloacetate and malate and decreasing the concentration of oxaloacetate. However, pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate, is activated by acetyl-CoA. Consequently, when there are significant amounts of acetyl-CoA, there should be sufficient oxaloacetate to initiate the condensing reaction of the citric acid cycle. 

A unique characteristic of polyesters is their ability to undergo additional condensation reactions during processing or when in the solid state. These reactions redistribute the molecular weight of the polymer until a dynamic equilibrium is established. Water, when present at high temperatures in polyester melts, can depolymerize polyesters via a hydrolysis reaction. For this reason, manufacturers must carefully dry the polymer before processing. 

Failure to remove the alcohols generated in either of the equilibrium condensation steps will reduce the efficiency of the polymerization process. This effect can be explained by Le Chatelier s principle, which was discussed in Chapter 3. The volatile alcohols produced during polymerization act as a chemical stress on the product side of the reaction, which inhibits polymerization. Another implication of the equilibrium nature of this polymerization process is seen in the molecular weight distribution of the final polymer. All polyesters contain a few percent of low molecular weight oligomers, regardless of the polymerization process. 

Next to reactions catalyzed by transaminases, hydrolase-catalyzed reactions also lead to limitations regarding the equilibrium. This problem occurs during ester synthesis, because this condensation reaction produces water. The equilibrium is shifted by high amounts of water towards the reactants therefore, an efficient removal is necessary to reach high conversions. Here, two process setups of Unichema Chemie B V will be discussed illustrating in situ product removal [41]. The first setup is based on azeotropic distillation of the water produced... 

Base catalyzed condensation reactions of esters and ketones have an additional factor of importance in determining the product, and this is the fact that the overall reaction, as well as the intermediate steps, is highly reversible. The final product may be rate or equilibrium determined, and in the latter case the result may depend on the relative acidity of the various possible products. In a highly basic medium the product will be partly in the form of a salt and the stability of the salt is then a product-determining factor. Failure of a condensation to take place may be due either to an insufficiently high concentration of carbanions or to the instability of the product. The reactions of ethyl isobutyrate will illustrate both points.419... 

Condensation reactions are conveniently written as carbanion reactions, and yet it is clear that the metallic cation is important too. For example, sodium and lithium give quite different results in the condensation of acetophenone and tert-butyl acetate.422 The various rate and equilibrium constants depend on the nature of the associated metal. Lithium, zinc, and magnesium, which give the aldol condens-... 

Reverse hydrolysis a thermodynamically controlled equilibrium process, in which a free monosaccharide reacts with a nucleophile under exclusion of a water molecule and hence chemically, can be considered a condensation reaction. 

This method is called the Fischer esterification. It s a condensation reaction where the loss of a water molecule accompanies the joining of the alcohol portion to the acid portion. The acid gives up the OH and the alcohol gives up the H to make the water molecule. All steps in the mechanism are reversible (that is, it establishes an equilibrium), so removing the ester as soon as it forms is helpful. Removal of the ester is normally easy since esters typically have lower boiling points than alcohols and carboxylic acids. Figure 12-20 illustrates the mechanism for the acid-catalyzed formation of an ester by the reaction of an alcohol with a Ccirboxylic acid. 

The product yield of a thermodynamically controlled reaction depends on pH when acids and bases participate in the reaction. This pH-dependence can be analyzed using known values of p AT -values of the acidic and basic groups of the reactants and the products. For thermodynamically controlled processes the apparent eqnilibrium constant for the product yield in condensation reactions, K, mnst be determined. This equilibrium constant is defined by the following equation ... 

We have seen that elements can be separated based on their volatility, either through gas-solid or gas-liquid reactions. There are many types of reactions that form a continuum between equilibrium condensation (or its inverse, evaporation) on the one hand and purely kinetically controlled reactions, such as Rayleigh distillation, on the other. In some cases, isotopic fractionation can assist in identifying the processes involved. 

This paper provides an example of how accurate continuum models can open the door to the modeling of condensed-phase processes where solvation free energies have a very large influence on reaction energetics. It additionally offers a case study of how to first choose a model on the basis of experimental/tlieoretical comparisons over a relevant data set, and then apply tliat model with a greater expectation for its utility. The generality of this approach to other (equilibrium) electrochemical reactions seems promising. 

---