In esterification reactions

Stannous oxalate is used as an esterification and transesterification catalyst for the preparation of alkyds, esters, and polyesters (172,173). In esterification reactions, it limits the undeskable side reactions responsible for the degradation of esters at preparation temperatures. The U.S. Bureau of Mines conducted research on the use of stannous oxalate as a catalyst in the hydrogenation of coal (174) (see Coal). 

Schemes are available, however, that start from the free carboxylic acid, plus an activator . Dicyclohexylcarbodiimide, DCC, has been extensively employed as a promoter in esterification reactions, and in protein chemistry for peptide bond formation [187]. Although the reagent is toxic, and a stoichiometric concentration or more is necessary, this procedure is very useful, especially when a new derivative is targeted. The reaction usually proceeds at room temperature, is not subject to steric hindrance, and the conditions are mild, so that several types of functional groups can be employed, including acid-sensitive unsaturated acyl groups. In combination with 4-pyrrolidinonepyridine, this reagent has been employed for the preparation of long-chain fatty esters of cellulose from carboxylic acids, as depicted in Fig. 5 [166,185,188] ...

Nevertheless, in some cases, this criterion is not sufficient for the choice of the solvent. For instance, Kuo and Parkin [78], demonstrated that hydrophobicity of solvent in the presence of lipase also affect selectivity and partition of reactants in esterification reactions. On the other hand, in the presence of certain solvents, even in low concentration, enzyme can be activated [13]. 

Another mode of semibatch operation involves the use of a purge stream to remove continuously one or more of the products of a reversible reaction. For example, water may be removed in esterification reactions by the use of a purge stream or by distillation of the reacting mixture. Continuous removal of product(s) increases the net reaction rate by slowing down the reverse reaction. 

Catalytic sol-gel lipase immobilizates were rapidly commercialized (by Fluka) after their invention in 1995 because of their remarkably stable activity in esterification reactions (and also in the kinetic resolution of chiral alcohols and amines) along with unique stability (residual activity of 70% even after 20 reaction cycles is common). The original procedure for the encapsulation produced by the fluoride-catalysed hydrolysis of mixtures of RSi(OCH3)3 and Si(OCH3)4 has been improved... 

We have already seen the latter compounds involved in esterification reactions (see Section 7.9.1), and seen the value of pyridine in removing acidic byproducts, e.g. HCl. Of course, N-acylpyridinium salts will easily be hydrolysed under aqueous conditions. 

At low temperatures, the activity of acid catalysts in transesterification is normally fairly low and to obtain a sufficient reaction rate it is necessary to increase the reaction temperature to >170 °C. Therefore, sulfonic acid resins can be used in esterification reactions where they perform well at temperatures <120 °C and particularly in the pretreatment of acidic oils. Under these reaction conditions, acidic resins are stable. Poly(styrenesulfonic add), for example, has been used in the esterification of a by-product of a vegetable oil refinery with a 38.1 wt% acidity at 90-120 ° C and 3-6 atm. It was not deactivated after the first batch and maintained a steady catalytic performance in the next seven batches [22]. 

Lithium hydroxide is used as an electrolyte in certain alkahne storage batteries and in the production of lithium soaps. Other uses of this compound include its catalytic applications in esterification reactions in the production of alkyd resins in photographic developer solutions and as a starting material to prepare other lithium salts. 

Despite the fact that glycolic acid has been successfully used as an acyl donor in esterification reactions with fatty alcohols, there are few reports dealing with the enzymatic ROP of glycolide [139], On the other hand, cyclic diesters based on ethylene glycol have been polymerized successfully by lipase catalysis and afford AA-BB-typepolyesters [140, 141],... 

Many publications advocate the use of solid acid resins in esterification reactions. A comprehensive review of organic reactions catalyzed by resins is that of Harmer and Sun. However, thermal-stability above 140°C and lack of structural integrity at high pressures severely limit the applicability of organic resins as catalysts for esterification and transesterification. 

In addition to its direct influence via the water activity in the system, the amount of water also influences the activity coefficients of the other components in the mixture, and therefore equiUbrium constants like K0 can vary with the water activity in the system (Table 1.5) [29, 63, 64]. This can be seen as a solvent effect on the equilibrium constant The tendency in esterification reactions is that increases with decreasing water activity, which means that it is favorable to use low water activity because of both the direct effect of water activity on the equilibrium and the influence of water on K0. 

Separation of products from the reaction mixture In situ product removal from enzymatic reactor via a nanofiltration or ultrafiltration membrane Removal of selected enantiomer via a liquid membrane Removal of water in esterification reactions via a pervaporation membrane... 

Enzyme activity was determined as the initial rates in esterification reactions between lauric acid and propanol at a molar ratio of 1 3, a temperature of 60°C, and an enzyme concentration of 5 wt% in relation to the substrates. At the beginning of the reaction, samples containing the mixture of lauric acid and propanol were collected and the lauric acid content was determined by titration with 0.04 N NaOH. After the addition of the enzyme to the substrates, the mixture was kept at 60°C for 15 min. Then, lauric acid consumption was determined. 

The history of reactive distillation dates back to the early 1920s. Pioneering work in this field was done by Backhaus [1] who filed 11 patents covering its use in esterification reactions. A detailed review was published some years later [2],... 

With the addition of a catalytic centre, an imprinted recognition site can be transformed into an enzyme mimetic material offering substrate selective catalysis. Over the past decade a few research groups have studied catalytic effects of imprinted metal oxides in esterification reactions. In this section a brief overview will be given on their efforts to understand the catalytic selectivity and structure of the active site. For additional reading an excellent review can be found by Davis et al. [46]. 

Acid Chlorides. Acid chlorides can also be used in esterification reactions (70). The product is the ester of the reacted acid chloride, with hydrochloric acid as a by-product ... 

In esterification reactions, 1 mol of carboxylic acid reacts with 1 mol of monohydric alcohol to produce 1 mol of ester and 1 mol water. 

Only few reports are known describing zeolites as catalysts in esterification reactions. Corma et al. [1] published a relationship between the characteristics of HY zeolite and the catiilytic activity in the esterification of carboxylic acids. Yadov Ma et al. [2] have demonstrated that zeolites were able to do esterification reaction. The present investigation describes the use of zeolite HP in vapour phase esterification of acetic acid with C2-C4 alcohols and the activity of HP is compared with that of HY, DHY, HZSM-5 and Y-AI2O3,... 

Zeolitic materials with Si to A1 ratios - with accent on zeolite Beta - were tested in esterification reactions. Hydrophilic materials proved to be inactive in the liquid phase esterification of apolar substrates. This may be ascribed to strong adsorption of water formed. More polar substrates are able to compete with water for the adsorption sites thus, the influence of the hydrophobicity of the catalyst on its activity becomes less pronounced. Such influence is negligible on the activity of the catalysts in gas phase reactions. Here, the activity is mainly dependent on the amount and the strength of acid sites. 

Fig. 5.2 -6 Bransted acidic ionic liquids used in esterification reactions [195,196].

Water activity of the reaction medium plays a central role in lipase catalyzed reactions (Berglund 2001). Different authors have described and demonstrated the usefulness of controlling water activity on lipase performance. In esterification reactions on cyclohexane media, the reaction rate increased with water activity in the low activity range however, it reached a maximum at a value of 0.84 and a subsequent increase in water activity led to a decrease in the reaction rate (Mat-sumoto et al. 2001). Results on the effect of water activity on enantioselectivity of lipases are rather contradictory (Berglund 2001). However, very good papers have been published in tuning lipase enantioselectivity by reaction medium engineering (Wehtje and Adlercreutz 1997 Matsumoto et al. 2001 Bomscheuer 2002). 

A diamine like hexamethylenediamine has a functionality of 2 in amide-forming reactions such as that shown in Equation 1.2. However, in esterification reactions a diamine has a functionality of zero. Butadiene has the following structure ... 

As is the case in esterification reactions, the presence of unreacted (1) can cause problems since the amine reacts with this as quickly as it does with the acylimidazole, forming urea byproducts that can be difficult to separate. Use of exactly one equivalent of (1) is difficult due to its moisture sensitivity, and also because of the tendency of some peptides or amino acids to form hydrates. Paul and Anderson solved this problem by use of an excess of (1) to form the acylimidazole, then cooling to —5 °C and adding a small amount of water to destroy the unreacted (1) before addition of the amine. ... 

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