Esters formation, general

Note also that if another ester, of general formula R-COOCjHj, were used in place of benzaldehyde in the above reaction, a similar complex would be formed, and on acidification would give an unstable p-hydroxy-P-ethoxy ester, which would very readily lose ethanol with the formation of a 3-keto-ester. 

SOME GENERAL METHODS FOR PHOSPHATE ESTER FORMATION REMOVAL OF PROTECTIVE GROUPS FROM PHOSPHORUS... 

In perfumery, acetates are the most important aliphatic esters formates do not keep well. Animal and fatty notes become more pronounced in esters of higher fatty acids. Acetates of alcohols up to C(, are used principally for fruity notes, whereas the acetates of Cg, Cio, and C12 alcohols are employed for blossom fragrances and for flower notes in general. Lauryl acetate in particular is also used for conifer notes. 

These include thiobenzoates, thiocarbonylimidazolides, and phenyl thionocarbonate esters.6 The S-methyl xanthate ester is a particularly convenient intermediate to prepare because of its ease of formation and the low cost of the reagents. Its use is precluded, however, by the presence of base-labile protecting groups and, in such cases, the thiocarbonylimidazolide or phenyl thionocarbonate ester will generally prove satisfactory. Additional methods for the radical deoxygenation of alcohols are described in a review by Hartwig.7... 

Orf/io-substituents in general reduce the rates of acid-catalyzed ester formation and hydrolysis. Some typical data are collected in Table 20. In each series the rate coefficient decreases as the size of the ortho substituent increases. Only the smallest substituent of all, the fluorine atom, does not have a marked effect. 

It is generally assumed that the ionized carboxyl group R COO- is not susceptible to nucleophilic attack, and essentially all the available evidence supports this assumption. A solitary exception is discussed below (p. 160). Thus ester formation with acyl-oxygen cleavage, and oxygen exchange with the solvent, are not expected under alkaline conditions. There is no reason, however, why they should not occur in neutral solution, where a significant proportion of the undissociated acid (R COOH) is present, and both reactions are well established under these conditions. 

Alcohols undergo a large number of reactions. However, these reactions may be grouped into a few general types. Reactions of alcohols may involve the O—H or C—O bonds. Ester formation and salt fonnation are examples of the former class, while conversion to halides is an example of the latter type. 

The relative rate of amide and ester formation depends on the nature of the zeolite but we observe generally a rapid stabilization of the amide concentration while the ester concentration shows a linear dependence against tii.ie. A typical example is described for a HM (Si/A1 =8) (Fig. 3). The signi-... 

The reactions of sydnones with acetylenic esters are generally thermally induced concerted processes which are allowed on orbital symmetry considerations.8 On the basis of Huisgen s mechanistic study,805 the conversion of 71 into 73 proceeds through the formation of 72 in a slow rate-determining step, followed by rapid loss of carbon dioxide, giving the pyrazole 73. 

While still useful for large-scale esterification of fairly robust carboxylic acids, Fischer esterification is generally not useful in small-scale reactions because the esterification depends on an acid-catalyzed equilibrium to produce the ester. The equilibrium is usually shifted to the side of the products by adding an excess of one of the reactants—usually the alcohol—and refluxing until equilibrium is established, typically several hours. The reaction is then quenched with base to freeze the equilibrium and the ester product is separated from the excess alcohol and any unreacted acid. This separation is easily accomplished on a large scale where distillation is often used to separate the product from the by-products. For small-scale reactions where distillation is not a viable option, the separation is often difficult or tedious. Consequently Fischer esterification is not widely used for ester formation in small-scale laboratory situations. In contrast, intramolecular Fischer esterification is very effective on a small scale for the closure of hydroxy acids to lactones. Here the equilibrium is driven by tire removal of water and no other reagents are needed. Moreover the closure is favored entropically and proceeds easily. 

Figure 13.50 outlines how esters in general (not shown) and especially lactones (shown) can be prepared for a one-step aldol condensation with an aldehyde they are exposed to a mixed ( crossed ) Claisen condensation with formic acid methyl ester (cf. Figure 13.59, first line). Like all Claisen condensations (Section 13.5.1), this also first leads to the formation of the enolate of the acylated ester. Unlike other Claisen condensations, this enolate is isolated. 

Here are the two conformations drawn out for ethyl acetate. When the ethyl group (= R) and O are cis, not only can one oxygen lone pair interact with the C-O ft, but the other lone pair can also donate into the O of the C=0 bond. This is not possible when Et and O are trans they are no longer anti-pcripla-nar. The cis conformation of esters is generally the preferred one, even in formate esters, where the alkyl group ends up in what is clearly a more sterically hindered orientation. 

Methyl acetate from methanol and acetic acid General process for ester formation... 

It is generally stated that biocatalysis in organic solvents refers to those systems in which the enzymes are suspended (or, sometimes, dissolved) in neat organic solvents in the presence of enough aqueous buffer (less than 5%) to ensure enzymatic activity. However, in the case of hydrolases water is also a substrate and it might be critical to find the water activity (a ) value to which the synthetic reaction (e.g. ester formation) can be optimized. Vahvety et al. [5] found that, in some cases, the activity of Candida rugosa lipase immobihzed on different supports showed the same activity profile versus o but a different absolute rate. With hpase from Burkholderia cepacia (lipase BC), previously known as lipase from Pseudomonas cepacia, and Candida antarctica lipase B (CALB) it was found that the enzyme activity profile versus o and even more the specific activity were dependent on the way the enzyme was freeze dried or immobihzed [6, 7]. A comparison of the transesterification activity of different forms of hpase BC or CALB can be observed in Tables 5.1 and 5.2, respectively. 

Is nil. As reaction conditions deteriorate and alkylate quality decreases, the rate of dimer and trimer ester formation increases. Ester production, relative to alkylate quality, is shovm by Figure 3 Generally ester production from feed contaminants is more difficult to predict. Data obtained from commercial operations and plotted in Figure 4, show the combined effect of ester production from these two sources. 

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