Esterifications transesterification

KTB and KTA are superior to alkaU metal hydrides for deprotonation reactions because of the good solubiUties, and because no hydrogen is produced or oil residue left upon reaction. Furthermore, reactions of KTA and KTB can be performed in hydrocarbon solvents as sometimes requited for mild and nonpolar reaction conditions. Potassium alkoxides are used in large quantities for addition, esterification, transesterification, isomerization, and alkoxylation reactions. 

The variety of enzyme-catalyzed kinetic resolutions of enantiomers reported ia recent years is enormous. Similar to asymmetric synthesis, enantioselective resolutions are carried out ia either hydrolytic or esterification—transesterification modes. Both modes have advantages and disadvantages. Hydrolytic resolutions that are carried out ia a predominantiy aqueous medium are usually faster and, as a consequence, require smaller quantities of enzymes. On the other hand, esterifications ia organic solvents are experimentally simpler procedures, aHowiag easy product isolation and reuse of the enzyme without immobilization. 

Esterification of tertiary alcohols poses several problems and expensive catalysts, like dimethylamino pyridine, are recommended. While esterification/transesterification/hydrolysis involving primary and secondary alcohols has been reported both with chemocatalysts and biocatalysts, terf-alcohol based esters have not found success. Recent work of Yeo et al. (1998) reports successful results for /er/-butyl octonoate using a new strain of lipase. This is a significant finding as the production of esters based on fert-alcohols (and reciprocally with hindered acids) may well be possible with biocatalysts, avoiding expensive catalysts and allowing easier separation. 

The lipases demonstrated very high stability in media partially or totally composed of organic solvent. In such media, the lipases catalyze esterification, transesterification, and resolution of enantiomers [19,45,75,97-100]. Nevertheless, several biphasic systems (organic-aqueous) are used for hydrolysis of lipid and fats [7,34,101]. Kinetic studies in biphase media or in inverted micelles demonstrate that the lipase behavior is different... 

Table 2.12 Publications on the modelling of batch and continuous esterification, transesterification, polycondensation, and recycling of PET... 

Inorganic add catalysts are more suitable for the esterification-transesterification reaction. 

This catalyst showed good activity in the esterification-transesterification of a number of unrefined vegetable oils (Table 10.4), used oils and non-edible oils at 150 °C with an oil-to-methanol ratio of 1 15. The catalyst performance is not influenced by the water content and it can be reused after separation by centrifugation without any further purification [38, 39]. 

Table 10.4 Esterification—transesterification of different oils in the presence of Fe-Zn double metal cyanide.

After pre-esterification, transesterification is carried out using a typical base-catalyzed process as shown in Figure 7. 

The application of lipases in synthetic biotransformations encompasses a wide range of solvolytic reactions of the carboxyl group, such as esterification, transesterification (alcoholysis), perhydrolysis, and aminolysis (amide synthesis) [103]. Transesterification and amide synthesis are preferably performed in an anhydrous medium, often in the presence of activated zeolite, to suppress unwanted hydrolytic side reactions. CaLB (which readily tolerates such conditions [104,105]), PsL, and PcL are often used as the biocatalyst [106]. 

Lipases still offer the potential for an important range of applications since they are able to carry out the reactions of esterification, transesterification (acidolysis or alcoholysis), inter-esterification, or hydrolysis, often with high specificity or selectivity, suitable for the production of high-added-value molecules as shown in Example 1 above (stereospecific alkylation, acylation, or hydrolysis for the resolution of racemic mixtures of acids, alcohols or esters). 

The physical properties of most acids (esters) and alcohols allow the reaction to be carried out either in the liquid or in the vapour phase. In the liquid phase, the effects of solvents and of transport phenomena may play a more important role than in the vapour phase. On the other hand, the side reactions (mainly the ether and/or olefin formation from the alco- TABLE 20 Reactants and inorganic catalysts used in kinetic studies of esterification (transesterification) ... 

The reactants and inorganic catalysts used in kinetic studies of heterogeneous catalytic esterifications (transesterifications) are summarised in Table 20. As can be seen, no systematic comparative study with more than one catalyst (with the exception of paper [406]) has been performed by any one worker. The greatest attention was paid to silica gel [407— 411]. The reactants were usually low molecular weight acids and alcohols a typical pair of reactants is acetic acid—ethanol. Only in one study [126] was the structure of the reactants systematically varied in order to establish the effect on the reactivity. 

Equations reported as best fitting esterification (transesterification) data on inorganic catalysts... 

Enzymes are biocatalysts constructed of a folded chain of amino acids. They may be used under mild conditions for specific and selective reactions. While many enzymes have been found to be catalytically active in both aqueous and organic solutions, it was not until quite recently that enzymes were used to catalyze reactions in carbon dioxide when Randolph et al. (1985) performed the enzyme-catalyzed hydrolysis of disodium p-nitrophenol using alkaline phosphatase and Hammond et al. (1985) used polyphenol oxidase to catalyze the oxidation of p-cresol and p-chlorophenol. Since that time, more than 80 papers have been published concerning reactions in this medium. Enzymes can be 10-15 times more active in carbon dioxide than in organic solvents (Mori and Okahata, 1998). Reactions include hydrolysis, esterification, transesterification, and oxidation. Reactor configurations for these reactions were batch, semibatch, and continuous. 

Many enzymes are both active and stable in carbon dioxide and have been used to conduct a number of reactions. Several different types of reactions have been examined, including hydrolysis (Lee et al., 1993 Randolph et al., 1985 Zheng and Tsao, 1996), oxidation (Hammond et al., 1985 Randolph et al., 1988), and esterification/transesterification (Kamihira et al., 1987 Nakamura et al., 1986 Rantakyla and Aaltonen, 1994), but there are other types of reactions that would make worthwhile investigations in carbon dioxide. These include preparation of amides, reduction of ketones, preparation of cyanohydrins from aldehydes, aldol reactions, hydroxylation reactions, and Baeyer-Villiger oxidation. 

The most important examples of reactive separation processes (RSPs) are reactive distillation (RD), reactive absorption (RA), and reactive extraction (RE). In RD, reaction and distillation take place within the same zone of a distillation column. Reactants are converted to products, with simultaneous separation of the products and recycling of unused reactants. The RD process can be efficient in both size and cost of capital equipment and in energy used to achieve a complete conversion of reactants. Since reactor costs are often less than 10% of the capital investment, the combination of a relatively cheap reactor with a distillation column offers great potential for overall savings. Among suitable RD processes are etherifications, nitrations, esterifications, transesterifications, condensations, and alcylations (2). 

A number of enzymatic resolutions have been investigated to the chiral halo ester, including esterification, transesterification, and aminolysis.69246 250 Zeneca (ICI) has commercialized the approach that relies on a dehalogenase (Scheme 31.25).69,245,248 249... 

Extensively metabolized Undergoes hydroxylation, de-esterification, transesterification, demethylation, conjugation, reduction Unknown... 

The term reactive distillation (RD) refers to both catalyzed and uncatalyzed reaction systems. Catalytic distillation systems may use a homogenous or heterogenous catalyst to accelerate the reaction. Reactive distillation is a well-known example of reactive separation process, and is used commercially. The first patent and early journal articles deal mainly with homogenously catalyzed reactions such as esterifications, transesterifications, and hydrolysis.f Heterogenous catalysis with RD is a more recent development. The key advantages for a properly designed RD colunm are complete conversion of reactants and attainment of high selectivity. An example of the benefits of RD is the acid catalyzed production of methyl acetate by... 

Tung biodiesel was produced by the esterification/transesterification reactions using Amberlyst-15 and KOH from tung oil. Amberlyst-15 showed a good esterification efficiency for the pretreatment of tung oil. The AV of tung oil decreased from 9.55 to... 

Reactive rectifications without a separate reactor, in which the reaction takes place in the distillation column, are increasingly being used in industry. Typical classes of reactions are esterification, transesterification, acetal formation and cleavage, etherification, oxidation, and hydrogenation. With regard to the design of such processes, three cases can be distinguished ... 

Uses Esterification/transesterification/polycondensation catalyst tor prep, of polyesters, unsat. polyesters, alkyds suitable tor food and pharmaceutical contact applies., such as coatings, epoxies, hybrid resins food pkg. adhesives, closures with sealing gaskets tor food containers food-contact paper/paperboard... 

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