Esterification base-catalyzed

Kinetic Considerations. Extensive kinetic and mechanistic studies have been made on the esterification of carboxyHc acids since Berthelot and Saint-GiHes first studied the esterification of acetic acid (18). Although ester hydrolysis is catalyzed by both hydrogen and hydroxide ions (19,20), a base-catalyzed esterification is not known. A number of mechanisms for acid- and base-catalyzed esterification have been proposed (4). One possible mechanism for the bimolecular acid-catalyzed ester hydrolysis and esterification is shown in equation 2 (6). 

The Ingold248 classification of esterification and hydrolysis reactions is reported in Table 4. Basic compounds are seldom used as catalysts for esterifications, at least in diluted media. Thus, in Table 4 all arrows are oriented right to left. However, some authors (Naudet193, Kutepov27 ) carried out base-catalyzed esterifications in concentrated media and proposed mechanisms. 

Base-catalyzed esterification of D-glucurono-6,3-lactone proceeds smoothly at room temperature.242 The acetylated glycuronic acids, and the per-O-acetylglycosyluronic halides of their methyl esters, are important precursors for the synthesis of glycosiduronic acids. D-Glucofuranurono-6,3-lactone... 

The mechanism of high-temperature, base-catalyzed esterification involves cyclic anhydride formation by the polycarboxylic acid, followed by base-catalyzed reaction of anhydride groups with cellulosic hydroxyls. Fumaric acid, which cannot form a cyclic anhydride because of the trans arrangement of the two carboxyl groups, fails to react with cotton in the presence of weak bases and heat. 

Many oil products are available that cannot be esterified directly using alkali catalysts as they have sufficient fatty acid content to neutralize an alkali catalyst, yet have high ester contents. An acid esterification step may be employed in advance of base catalyzed esterification. These pre-esterification methods need only provide sufficient acid and alcohol to esterify the fatty acids present (3, 68, 71), although... 

A review of synthetic methods for the most popular photoactive compounds used in diazoquinone resists—DNQ-5-sulfonate and DNQ-4-sulfonate—has been provided by Ershov et al. The synthesis typically begins with naphthalene derivatives, and proceeds via introduction of a sulfonic acid group, followed by diazotization and reaction with thionyl chloride to yield the sulfonic acid chloride (Scheme 7.2). In the next step, the chloride is reacted in a base-catalyzed esterification with a suitable ballast group or backbone, which usually is a multifunctional phenol, less frequently a monofunctional phenol or an aliphatic alcohol. ... 

Figure 7.21 Synthesis of sorbitan esters by intramolecular dehydration of sorbitol in the presence of an acid at 150-200°C and subsequent base-catalyzed esterification with fatty acids at 200-250°C.

To illustrate how gas chromatography and mass spectrometry can be used to characterize the products of an organic reaction, we consider the synthesis of methyl benzoate using a base-catalyzed esterification of benzoic add.The reaction proposed for this synthesis involved deprotonation of benzoic add by n-butyllithium in dry tetrahydrofuran (THF), followed by the addition of methyl iodide, with dimethylformamide (DMF) added to promote the Sn2 displacement of iodide by the benzoate anion ... 

Direct, acid catalyzed esterification of acryhc acid is the main route for the manufacture of higher alkyl esters. The most important higher alkyl acrylate is 2-ethyIhexyi acrylate prepared from the available 0x0 alcohol 2-ethyl-1-hexanol (see Alcohols, higher aliphatic). The most common catalysts are sulfuric or toluenesulfonic acid and sulfonic acid functional cation-exchange resins. Solvents are used as entraining agents for the removal of water of reaction. The product is washed with base to remove unreacted acryhc acid and catalyst and then purified by distillation. The esters are obtained in 80—90% yield and in exceUent purity. 

In this definition ko is the rate constant for CH3COOR and k is the constant for RCOOR thus = 0 for R = CH3. Table 7-11 lists some values. Taft s Es steric constants are in some instances based on averages of several different reactions, so MacPhee et al. have defined a steric constant Es by Eq. (7-52) for a single reaction, namely, the acid-catalyzed esterification of carboxylic acids in methanol at 40°C. Es values are also given in Table 7-11. Additional Es and Es values are available. 

One of the most important characteristics of IL is its wide temperature range for the liquid phase with no vapor pressure, so next we tested the lipase-catalyzed reaction under reduced pressure. It is known that usual methyl esters are not suitable for lipase-catalyzed transesterification as acyl donors because reverse reaction with produced methanol takes place. However, we can avoid such difficulty when the reaction is carried out under reduced pressure even if methyl esters are used as the acyl donor, because the produced methanol is removed immediately from the reaction mixture and thus the reaction equilibrium goes through to produce the desired product. To realize this idea, proper choice of the acyl donor ester was very important. The desired reaction was accomplished using methyl phenylth-ioacetate as acyl donor. Various methyl esters can also be used as acyl donor for these reactions methyl nonanoate was also recommended and efficient optical resolution was accomplished. Using our system, we demonstrated the completely recyclable use of lipase. The transesterification took place smoothly under reduced pressure at 10 Torr at 40°C when 0.5 equivalent of methyl phenylthioacetate was used as acyl donor, and we were able to obtain this compound in optically pure form. Five repetitions of this process showed no drop in the reaction rate (Fig. 4). Recently Kato reported nice additional examples of lipase-catalyzed reaction based on the same idea that CAL-B-catalyzed esterification or amidation of carboxylic acid was accomplished under reduced pressure conditions. ... 

Esterification of 5 -nucleoside derivatives with N-(aminoacyl)-imidazoles yields 2 - and 3 -(aminoacyl) derivatives and proceeds with apparently little selectivity169,170 this is not surprising in view of the facile, base-catalyzed, ester migration that may occur in 2 (3 )-esters of the D-ribofuranose system (see Section VII,1). 

Several total syntheses of antirhine (11) and 18,19-dihydroantirhine (14) have been developed during the last decade. Wenkert et al. (136) employed a facile route to ( )-18,19-dihydroantirhine, using lactone 196 as a key building block. Base-catalyzed condensation of methyl 4-methylnicotinate (193) with methyl oxalate, followed by hydrolysis, oxidative decarboxylation with alkaline hydrogen peroxide, and final esterification, resulted in methyl 4-(methoxycar-bonylmethyl)nicotinate (194). Condensation of 194 with acetaldehyde and subsequent reduction afforded nicotinic ester derivative 195, which was reduced with lithium aluminum hydride, and the diol product obtained was oxidized with manganese dioxide to yield the desired lactone 196. Alkylation of 196 with tryptophyl bromide (197) resulted in a pyridinium salt whose catalytic reduction... 

As noted below, such deprotonation/protonation chemistry occurs repeatedly without any loss of the hyperbranched graft as measured by ellipsometry (vide infra). This stability to acid and base mirrors the stability noted above for these hyperbranched grafts in continuous extractions and acid-catalyzed esterification chemistry and is thought to be a consequence of the light cross-linking that occurs during the grafting process. 

Much longer times are required to effect formation of high polymer polyesters in uncatalyzed esterifications than for acid or base-catalyzed systems. For catalyzed systems, since the added acid or base is a catalyst, its apparent concentration does not change with time thus, it is not included in the kinetic rate expression. In such cases the reaction follows the rate expression... 

Sorbitol. Sorbitol is the sugar alcohol obtained by reduction of glucose and it can be dehydrated to either isosorbide or to 1,4- and 2,5-sorbitan in acid or base catalyzed processes, respectively. Using sulfonic acid functionalized MCM-41 type materials lauric acid esters of isosorbide can be achieved quite selectively starting from sorbitol (>95% selectivity towards isosorbide dilaurate at 33% lauric acid conversion) in a dehydration-esterification... 

To produce biodiesel grade methyl esters, the oil must be trans-esterified with methyl alcohol. This process cleaves the triglyceride to yield glycerol and the individual methyl ester compounds. Most biodiesel trans-esterification is base catalyzed and has the operational advantage of low temperature, low pressure, and high conversion rates. 

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

When substrate activities are used instead of substrate concentrations in studies of enzyme kinetics in organic media, solvent effects due to substrate solvation disappear. Remaining solvent effects should be due to direct interactions between the enzyme and the solvent. In a study of lipase-catalyzed esterification reactions, it was found that Km values based on activities were indeed more similar tban those based on concentrations in different solvents, but still some differences remained [49]. 

Resolution of branched alkanoic adds. Hydrolase-catalyzed esterification of 2-methylalkanoic acids can be fairly efficient, especially for acids with long chains, provided that the conditions are carefully adjusted by immobilization of the enzyme (in some cases), by control of the water activity, and by proper choice of the appropriate alcohol as nucleophile as well as the correct solvent [134]. The alcohol concentration does also influence the E-value [133]. It is important to note that the esterifications are reversible, thus preventing easy access to the remaining substrate in high ees. Some representative examples are given in Table 4.4. A procedure based on iterative resolutions can be used to provide both enantiomers of 2-methyloctanoic acid in high ees (>99%) and reasonable yields (25% for S- and 43% for R-acid based on the starting racemic acid) [137]. 

In addition to their acidic properties, carboxylic acids also can act as weak bases when the carbonyl oxygen accepts a proton from a strong acid, such as H2S04, HC104, or HSbF6 in S02 (Equation 18-4). Such protonation is an important step in acid-catalyzed esterification, as discussed in Section 15-4D ... 

Like the AVADA and the AlkyClean processes, these two processes also replace the liquid acid/base catalysts with solid acids and bases [192]. Although the reaction mechanism for the heterogeneous acid-catalyzed esterification is similar to the homogeneously catalyzed one [207,208], there is an important difference concerning the relationship between the surface hydrophobicity and the catalyst s activity. This is especially true for fatty acids, which are very lipophilic compounds. One can envisage three cases First, if there are isolated Bronsted acid sites surrounded by a... 

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