Organic acids direct esterification

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

An optimised enzymatic synthesis of methyl benzoate in an organic medium was reported by Leszczak and Tran-Minh [43]. Methyl benzoate is part of the aroma of some exotic fruits and berries. The ester has been produced by direct esterification of benzoic acid with methanol in hexane/toluene catalysed by lipase from Candida rugosa. 

Several modifications of the simple direct esterification procedure described above have been developed. For example, it is sometimes convenient to prepare an ester by heating the organic acid, the alcohol and sulphuric acid in a solvent such as toluene. This method is illustrated by the preparation of diethyl adipate for which the procedure is particularly well suited (Expt 5.144). 

Again, it can be concluded that the system used in our work, without solvent, is a less costly and more selective process to produce mono- and diglycerides by direct esterification of glycerol and fatty acid. Such a system avoids the problems of separation, toxicity, and flammability of organic solvents, permitting recovery of product without further complex purification or evaporation steps and lowering the cost of the final product. 

Polyesterification. High molecular weight linear polyester resins, such as poly(ethylene terephthalate) (PET), poly(propylene terephthalate) (PPT), and poly(butylene terephthalate) (PBT), can be produced by either transesterification of dimethyl terephthalate (DMT) with an excess of the corresponding diol or by direct esterification of terephthalic acid (TPA). Tetraalkyl titanates, such as TYZOR TPT or —TYZOR TBT, have been found to be excellent catalysts for either of these reactions. However, in the case of PET, the residual titanate catalyst reacts with trace quantities of aldehydic impurities produced in the polymerization process to generate a yellow discoloration of the polymer (468,469). In the case of PPT and PBT, where the color of polymer is not as critical, organic titanates are the catalyst of choice because of their greater reactivity than antimony or tin (470). Numerous processing variations have been described in the literature to minimize formation of tetrahydrofuran in the PBT process (471—472). 

Many acetate esters (such as those of isoamyl, benzyl, citroneUyl, and geranyl alcohols) are components of natural flavors. They can be obtained by Upase-cata-lyzed esteriflcation in organic solvents, but the major problem with enzymatic acetylations is deactivation of lipases by acetic acid [8, 9]. Most of the Upase-catalyzed syntheses of esters have been carried out by transesterification to avoid free acid toxicity and water formation. Claon and Akoh [10] found that immobilized lipases from Candida antarctica promote highly effective direct esterification of geraniol and citronellol with acetic acid. 

Lipases have been extensively used for the kinetic resolution of racemic alcohols or carboxylic acids in organic solvents. Chiral alcohols are usually reacted with achiral activated esters (such as vinyl, isopropenyh and trichloroethyl esters) for shifting the equilibrium to the desired products and avoiding problems of reversibility. For the same reasons, chiral acids are often resolved by using acidolysis of esters. In both cases, the overall stereoselectivity is affected by the thermodynamic activity of water of water favors hydrolytic reactions leading to a decrease in the optical purity of the desired ester. Direct esterifications are therefore difficult to apply since water formed during the reaction may increase the o of the system, favors reversibiUty, and diminishes the overall stereoselectivity. 

The resolution of the racemic mixture of 2-octanol and of other secondary alcohols (2-butanol, 2-pentanol, 2-hexanol, and 2-heptanol) by direct esterification in organic solvent was studied by using lyophiUzed mycelia of R. oryzae CBS112.07 as catalysts [18]. The profile of the resolution of (R,S)-2-octanol under optimized conditions (lg/1 of alcohol and equimolar butanoic acid as acylating agent in n-heptane, 30g/l of dry biocatalyst, and 30 °C) is shown in Figure 6.1. 

Under almost anhydrous conditions in organic medium, lipases can be used in the reverse mode for direct ester synthesis from carboxylic acids and alcohols, as well as transesterifications (acyl transfer reactions) which can be divided into alcoholysis (ester and alcohol), acidolysis (ester and acid), and interesterification (ester-ester interchange). The direct esterification and alcoholysis in particular have been most frequently used in asymmetric transformations involving lipases. The parameters that influence enzymatic catalysis in organic solvents have been intensively studied and discussed. ... 

Kinetic Resolution by Direct Esterification. This is the least common strategy for kinetic resolution and is most commonly executed on racemic alcohols with carboxylic acids in organic solvents. Reports include several alicyclic secondary alcohols such as menthol and various aliphatic secondary alcohols. Kinetic resolution of a variety of racemic saturated, unsaturated, and a-substituted carboxylic acids has also been effected by direct esterification with various alcohols. ... 

Although constituting a liquid phase reaction in which sulfuric acid is used as a catalyst, the direct esterification of organic acids such as acetic by addition of olefins presents some interesting phases of the usefulness of high pressure as a tool in organic synthesis. Thus, the production of... 

Esters of monoglycerides with organic acids such as acetic acid, lactic acid, diacetyltartaric acid, succinic acid and citric acid are widely used as food emulsifiers. The esters are produced in standard batch reactors normally by reacting monoglycerides with the organic acid or its anhydride. A direct esterification of fatty acids, glycerol and the organic acid is also possible for some products. 

Some indication of at least the maximum size of the silicate species in a solution of sodium silicate of 3.3 ratio SiOjrNajO has been obtained from the nature of the corresponding silicic acid ester (69). Silicic acid of low molecular weight is so unstable in aqueous solutions that any attempt to isolate it by evaporation of water, even at ordinary temperature, results in rapid polymerization to a gel. The direct esterification of silicic acid therefore remained impractical until the discovery of Kirk (70) of a method for transferring silicic acid of low molecular weight from aqueous solution to solution in an alcohol. This transfer is accomplished by extraction of the acid with a suitable polar organic solvent, simultaneously saturating the aqueous phase with sodium chloride in order to salt the silicic acid into the organic phase. An alcohol such as rt-butyl alcohol is then added, and esterification is effected by azeotropic distillation of water from the alcohol solution (29). 

FAME may become in the future a possible organic feedstock to be sulphonated to Fatty Acid Methyl Ester Sulphonate (FAMES). This feedstock is naturally renewable as it is produced from oils/fats or fatty acids. There are several possible process routes for the manufacture of FAME. Transesterification of fat triglycerides is the predominant method for manufacture of mixed fatty acid methyl esters, and direct esterification of fatty acids (FA) is practised if very selective cuts of product, in general as an intermediate detergent range alcohol, are desired. Methyl cocoate is a mobile, oily liquid above 25"C with a yellow tint and a characteristic fatty acid pungent odour. FAME sulphonation to FAMES is technically possible but hardly applied up to now (1990). 

The esterification methods used for the sugars (Chapter III) are applicable to the polyols. The preparation and properties of organic and inorganic esters of polyol anhydrides are considered in Chapter VII. Fully esterified derivatives are generally unobtainable by direct esterification with organic acids because internal anhydrides are formed. Partial esters, especially diesters, may be obtained by the use of amounts of benzoyl or p-toluenesulfonyl chlorides insufficient for complete esterification (93),... 

Hydrolases, especially lipases are ideal to perform acylations such as direct esterification of free acids, acylation of alcohols, or amines in low-water organic media [29, 30, 60, 79, 81, 83]. Accordingly, the majority of the examples of hydrolase-catalyzed acylations in continuous-flow mode are enantiomer selective KRs (Figures 9.7-9.9 and Tables 9.5-9.6). Most of the earlier studies were aimed at the biocatalytic production of chiral pharmaceutical intermediates and were performed at a relatively large scale using immobilized lipases in PER [60, 99-102]. 

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