Alcohols fatty-ester synthesis

Further improvement in the technology of methyl fatty ester synthesis can be achieved by dual esterification [4], This takes advantage of the fact that the sulfated zirconia catalyst has similar activity for normal alcohols, over the series C1-C8. However, methanol manifests about twice the activity [20], The removal of water produced by the esterification with methanol is solved simply, by employing a heavy alcohol immiscible with water, such as 2-ethyl-hexanol, which acts simultaneously as a reactant and an entrainer. As a result, the two fatty esters are obtained in the bottom product in the desired ratio by adjusting the feeds. For example, in a preferable operation mode the ratio of fresh feed reactants is acid methanol 2-ethyl-hexanol 1 0.8 0.2. 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Ester synthesis of fatty acid ethyl ester. The lipase-catalyzed esterification of fatty acid and alcohol is well-known. It was also favorable for the esterification of poly unsaturated fatty acids under mild conditions with the enzyme. However, the activity of native lipase is lower in polar organic solvents, i.e. ethanol and methanol. The synthesis of Ae fatty acid ethyl ester was carried out in ethanol using the palmitic acid-modified lipase. As shown in Figure 7, the reactivity of the modified lipase in this system was much higher than that of the unmoditied lipase. 

Ring opening of epoxidized fatty esters with alcohols has been studied to explore the synthesis of vicinal hydroxy ethers, which are potential lubricants725 [Eq. (5.274)]. In a comparative study of ring-opening with methanol, Nafion SAC-13 showed superior activity (TOF=lmin 1 versus 0.04 min-1 after 0.5 h at 60°C) when compared to Amberlist 15 (TOF = 0.04 min-1) both exhibiting equally high selectivity (>98%). 

Chemical esterification methods use an alcohol and a carboxylic acid in the presence of a mineral acid as catalyst. Sulfuric acid, which is commonly used, leads to the formation of undesirable byproducts, requiring a difficult separation step (3). Moreover, in this case, the starting material is a high-value component (fatty acid). Consequently, researchers are interested in the alcoholysis reaction using a vegetable oil with low cost and largely produced in Brazil as a raw material for ester synthesis. 

The complementary approach to ADMET for the synthesis of plant oil-based polyesters is the SM of fatty acids, esters, or alcohols, followed by classic polycondensation of the generated ot,co-difunctional compounds. In 2001, Warwel and coworkers showed the self-metathesis of different co-unsaturated fatty esters and their subsequent polycondensation in the presence of diols and Ti(OBu)4 or Ca... 

In the interesterification of fats, 1,3-positional specific lipases catalyze reactions in which only the fatty acids in the a-positions of the triglycerides take part, whereas positional nonspecific lipases are able to catalyze reactions in which the fatty acids from both the a- and / -positions take part. In transesterification between two types of fat, the positional non-specific lipase is therefore able to randomize the fatty acids, resulting in the same fatty acid composition in the triglycerides as obtained in the commercially important chemical randomization process. In ester synthesis, positional non-specific lipases catalyze the reaction with both primary and secondary alcohols whereas positional specific lipases are more or less specific for primary alcohols. 

Lipase A seems to be responsible for the interesterification characteristics of the immobilized crude lipase preparation, including its unique specificity towards saturated fatty acids. Further, Lipase A can explain some of the activity found in ester synthesis with long-chain alcohols. On the other hand, Lipase B is responsible for the activity in ester synthesis of short-chain alcohols, and for some of the activity on long-chain alcohols. 

Fig. (5). Lepidopteran pheromone biosynthetic pathways utilize fatty acid synthesis, desatiindiun, specific chain-shortening enzymes, and/or functional modification of tlie carbony l carbon to produce species-apecific acetate ester, aldehyde, alcohol, or hydrocarbon pheromone blends. Unsaturated hydrocarbons can be further modified to epoxides (adapted from ref. [21]).

Fatty acid synthesis has been engineered as well. Two pathways to very long chain polyunsaturated fatty acids were realized in P. pastoris to demonstrate their feasibility for future reengineering in oilseed crops [163]. Fatty acids and their esters are also interesting potential biofuels. Fatty acid esters with branched chain alcohols are potential low-viscosity biodiesels, and were successfully synthesized in Escherichia coli and P. pastoris by metabolic engineering [164]. 

Ester formation is associated with yeast growth in the early phase of fermentation. Acetate esters are produced via the reaction between an alcohol and acetyl Co-A, which is catalysed by the enzyme alcohol acetyl transferases (ATFl and ATF2). Ethanol, branched-chain alcohols and 2-phenylethanol are the common moieties of acetate esters. Ethyl esters of medium-chain fatty adds are formed through the reaction between ethanol and respective fatty acyl Co-A, which is catalysed by the enzyme alcohol acyl transferases. Saccharomyces cerevisiae strains also produce esterases that hydrolyse esters, and thus the final concentration of esters in beers is the net balance between ester synthesis and hydrolysis. Strains of brewing yeasts produce predominantly ethyl esters of fatty acids, particularly ethyl octanoate, with relatively little formation of acetate esters. Ester production in beer is regulated by a number of factors such as yeast strain, temperature, hydrostatic pressure, wort composition, sugar type and concentration, type and amount of yeast-assimilable nitrogen, aeration, and unsaturated fatty acids (Hiralal, Olaniran, PiUay, 2014 Pires et al., 2014). 

Wax ester biosynthesis probably involves an acyl transfer mechanism. The high thioesterase activity found in crude plant extracts makes it difficult to demonstrate acyl-CoA involvement in wax ester synthesis. However, partial purification of an acetone powder extract from the leaves of B. oleracea gave a protein fraction that catalyzed an acyl-CoA-dependent esterification of fatty alcohols (222). Additionally the acetone powder extract from B. oleracea leaves appeared to catalyze the direct transfer of acyl moieties from phospholipids to fatty alcohols. The leaf extract also catalyzed under appropriate conditions the esterification of fatty alcohols to free fatty acids. The transacylase mechanism is likely to be the main mechanism of wax ester synthesis in vivo. The fact that labeled wax esters were synthesized by a membrane-bound microsomal fraction from Hordeum vulgare leaves following incubation with radioactive alcohols, but not after incubation with free fatty acids (17), is consistent with the proposed acyl transfer mechanism. In E. gracilis the acyl-CoA reductase is functionally coupled to the acyl transferase (227). Both of these activities were solubilized from the microsomes... 

Synthesis of amino acid surfactants has also been achieved using certain lipases. Studies with immobilized lipases from Candida antarctica and Rhizopus miehei have shown that the enzymes could accept At-Cbz amino acids as acyl donors and catalyse the esterification with long-chain fatty alcohols with high yields [55, 56]. Removal of water produced during the reaction was essential to shift the equilibrium towards ester synthesis. Synthesis of At-acyl amino acids was also done by lipase-catalysed direct transacylation of amino acids with triglycerides or vegetable oils (e.g. soya bean, palm oil) [57-59]. 

Hydroxylations of fatty acids by cytochrome P450119 compound increase in rate with chain length and show no intermolecular KE in buffer. With glycerol, the rate of reaction of lauric acid increases, and a KIE is observed. Reversible formation of a non-reactive complex of a fatty acid with the cytochrome and its isomerization to a reactive one is proposed. A tandem oxidative cyclocondensation process is reported for the synthesis of 3,4-dihydropyrimidin-2(l//)-one or -thione derivatives from primary aryl alcohols, -keto esters, and urea or thiourea in the presence of aluminium nitrate nonahydrate as oxidant catalyst. ... 

Figure 9 Ester synthesis fiom alcohols and fatty acids with various carbon numbers in benzene by PEG-lipases from P. fluorescens, P. fragi, and C. cylindracea. Left-side and right-side panels ester synthesis with varying carbon number of alcohol and fatty acid, resj tively.

Higher molecular primary unbranched or low-branched alcohols are used not only for the synthesis of nonionic but also of anionic surfactants, like fatty alcohol sulfates or ether sulfates. These alcohols are produced by catalytic high-pressure hydrogenation of the methyl esters of fatty acids, obtained by a transesterification reaction of fats or fatty oils with methanol or by different procedures, like hydroformylation or the Alfol process, starting from petroleum chemical raw materials. 

The triacylglycerols (Figure 14—6) are esters of the tri-hydric alcohol glycerol and fatty acids. Mono- and di-acylglycerols wherein one or two fatty acids are esteri-fied with glycerol are also found in the tissues. These are of particular significance in the synthesis and hydrolysis of triacylglycerols. 

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