Solution equilibria 109 Esterification

Esterification is a homogeneous equilibrium reaction when all the species are liquids. The equilibrium constant value is commonly determined by following the concentration of acid with time by titration of solution aliquots with NaOH. As a homogeneous equilibrium, the concentrations of the liquids are not set to 1, but can be determined as M= moles/L of solution, assuming the liquids are ideal. 

Lactonization, like esterification, is an equilibrium process. y-Lactones and 8-lactones are so readily formed that the carboxylic acid itself can provide the required acidic catalyst, and substantial amounts of the lactone are typically present in solutions of 4- or 5-hydroxy acids respectively (Table 7.3). Interestingly, the proportion of lactone is usually higher for five-membered rings than for six-membered rings. 

In Helfferich s relation it is implicitly assumed that the diffusion of the reactants through the pore liquid is fast enough so that equilibrium between the pore liquid and the supernatant solutions exists. Several papers, however, report a particle size effect on the esterification rate [428—... 

In the membrane reactor shown in Figure 13.16(c), the chemical reaction and the separation step use the same membrane. However, in some processes it is desirable to separate reaction and separation into two distinct operations. If the net result of the process is to change the products of the chemical reaction, the process is still classified under the broad heading of membrane reactor. Two examples in which chemical reaction and separation are physically separated are shown in Figure 13.17. Figure 13.17(a) shows the use of a pervaporation membrane to shift the equilibrium of the de-esterification reaction [39,40], A portion of the organic solution in the esterification reactor is continuously circulated past the... 

Enzymatic reactions in organic media have been a major issue in the field of biocatalysis over the last two decades. Carboxylesterases (mostly lipases) have been used in monophasic organic solution under controlled values of water activity (ajj for catalyzing ester formation the reaction equilibrium can be shifted towards ester formation by interesterification or transesterification [1]. Direct esterification is often hampered by water formation, which may increase o , thus negatively influencing the equihbrium. 

In acid solution the first reaction is similar except that the intermediate is neutral but the carboxylate is stUl the better leaving group. The second esterification is now all right because f.rxhanol can attack the protonated carboxylic acid and water can be driven out after a second -- onation. This second step is an equilibrium with water and methanol about equal in leaving c cp ability but MeOH is in large excess as solvent. We have omitted proton transfer steps. 

The first example is an esterification (or lactonization as the product is a cyclic ester or lactone cr the forwards direction and an ester hydrolysis in the backwards direction. Ester hydrolysis a catalysed by acid or base but esterification by acid only. In addition, even a weak base will be enough to turn the starting material into the carboxylate anion, which will not cyclize. The equilibrium is to the right in acid solution and to the left in basic solution. 

In an ongoing project, we have set out to investigate the nature and extent of interactions exhibited by the ionic liquid constituents on various solutes. In particular, the Fischer-type esterification was chosen as a model reaction as its kinetics and thermodynamic equilibrium were assumed to be sensitive towards small changes in... 

The solvent also influences the reaction through its effect on the relative soln-bility of substrate and product. Ideally, the substrate would be solubilized, while the insoluble product precipitates from solution. Thus, the recovery of product would be improved, thermodynamic equilibrium in the solvent phase would be driven toward ester synthesis, and product inhibition would be prevented. This scenario can occur for the lipase-catalyzed esterification of saccharides and nonpolar, saturated fatty acids. When employing polar solvents such as acetone, saccharides remain solubilized, while the product, saccharide-fatty acid monoester, precipitated. ... 

The same enzyme catalyses the esterification of racemic 129 in non-aqueous solution with vinyl acetate. The released alcohol is CH2=CHOH, the enol of acetaldehyde it immediately forms acetaldehyde which self condenses and is removed from the equilibrium. The enzyme is filtered off, the enantiomerically pure alcohol (S) -129 and acetate (R)-130 separated by flash chromatography, and the ester hydrolysed to the alcohol without racemisation. Either method (esterification or hydrolysis) gives both enantiomers of a range of secondary alcohols.31... 

Figure 17.11 Equilibrium adsorption isotherms of different organic solutes onto (a), (b) modified cellulose fibres via esterification with octyl anhydride, and (c) virgin fibres, as a function of the equilibrium solute concentration at pH 6.5-7 and room temperature, (reproduced from reference [51]). (Continued)...

When temperature equilibrium is obtained, 2 ml of amine-H20-C02 was injected into 85% of phosphoric acid solution present in the reactor flask. The esterification reaction occurred and CO2 was released with N2 flow through the condenser and the condenser would condense evaporated water. Then, CO2 goes into an infrared analyzer and the software connected with the infrared show the amount of CO2 in % (peak) over the time. Experiment was repeated three time for verification of the amount of CO2 loading. For each peak, the determination of CO2 volumetric flow, based from CO2 vol% determinate by software, was calculated as below ... 

The solution was found by using an approach which is closely related to the dynamic resolution of cyanohydrins, i.e., a lipase-catalyzed enantioselective esterification employing vinyl acetate as acyl donor [236]. Thus, dynamic resolution was attempted by making use of the inherent instability of the racemic substrate, which - being a hemithioacetal - is in equilibrium with the... 

Despite these many studies, reactive distillation is not the best solution to couple reaction and separation, mainly for three reasons (1) in most of the cases, the esterification medium (reagents plus products) is a nonideal system (not really fit for vapor-equilibrium-based technology such as reactive distillation) (2) pure water cannot be selectively removed from the top or the bottom of the column and (3) reactive distillation is a high-energy consumption technology (because separation is based on conventional distillation) (Lim, Park, Hung, Sahimi, Tsotsis, 2002 Drioli Giomo, 2010). 

Even in the presence of large amounts of water, the miniemulsion process permits the synthesis of hydrophobic polyesters in a very simple manner and at very low temperatures in order to obtain stable polyester dispersions. The influence of several parameters on the esterification yield has been studied. On the one hand, any modification of the dispersed phase such as the hydrophobicity of the components, viscosity, and the reactant nature results in different yields. With increasing hydrophobicity of the monomers or decreasing viscosity, the yield increases. On the other hand, any modification of the surrounding environment of the droplets such as the interface nature, the ionic strength and the interface area, has no influence on the equilibrium. From a thermodynamic point of view, this polymerization presents the characteristics of a bulk or solution polymerization. Independently of the dispersion state in the range studied, the equilibrium is the same as in bulk or in solution polymerization with an organic phase saturated with water. It is however very unlikely that the reactions occur exclusively in the core of the particle, but in order to provide an answer to this question it would be necessary to conduct a kinetic study related to the interface area. 

We have now discussed Fischer esterification (formation of an ester in an acidic solution of an alcohol) and fhe hydrolysis of an ester in acidic water. When we discussed Fischer esterification, we pointed out that it is an equilibrium reaction. Ester hydrolysis in aqueous acid is also an equilibrium reaction. The two reactions proceed via the same nucleophilic addition/elimination mechanism, except that they are the reverse of each other. As first introduced in Section 10.6, the prmdple of microscopic reversibility states that for any reversible reaction, the sequence of intermediates and transition states must be the same but in reverse order for the backward versus forward reaction. In general, the reverse of protonation (Add a proton) is deprotonation (Take a proton away). The reverse of nucleophilic affack (Make a bond between a nucleophile and an electrophile) is leaving group departure (Break a bond to give stable molecules or ions). 

In addition to lactone formation, it is probable that extramolecular esterification may take place with the formation of aldonic esters of aldonic acids (e.g., gluconic acid gluconate) and chain polymerization also may occur. In such systems, the concentration of water present would be expected to exert a profound influence on the composition of the equilibrium solution. Lactic acid forms external esters (lactides), but this type of condensation through carboxyls and a-hydroxyls has not been observed for hexonic and pentonic acids. 

Analytical Approach and Interconversion Under mildly acidic conditions, GHB undergoes an internal condensation via Fischer esterification to produce a stable lactone. This involves two steps cyclization, followed by the elimination of water. The reverse reaction, the hydrolysis of GBL to GHB, can be driven in the presence of a strong base, which is the method used clandestinely to make GHB (Figure 7.29). The reverse reaction can occur in solution when GHB is spiked into a drink, many of which are slightly addic. However, once equilibrium between the add and the lactone form has been reached, no significant losses of GHB appear in typical exhibits over the time frame of days or weeks. ... 

Phenylboronic acid is primarily present in the anionic form in dilute aqueous solution, and on reacting with a diol, it undergoes esterification while maintaining its tetrahedral geometry (Figure 13.1a). It has been demonstrated that with esterification, a shift in acid-base equilibrium toward lower pH occurs. We should note that... 

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