Fatty acids ester exchange reactions

As discussed earlier, the preparation of alkyd resins is essentially an esterification process in which the polybasic acids and polyhydric alcohols are reacted with various oils or fatty acids and modifying agents. When a fatty acid is used, the process chiefly involves direct esterification. When an oil is the fatty acid source, an alcoholysis or ester-exchange reaction between the oil and the polyhydric alcohol is usually carried out before the esterification step. 

Enzymic reaction catalyzed by PEG-lipase prweeded under mild conditions at room temperature in a homogeneous reaction system. Therefore, esterification of unstable substrates proceeded successfully without extensive oxidation of unsaturated substrates [61] PEG-hpase catalyzed an ester exchange reaction from retinyl acetate and saturated or unsaturated fatty acid in benzene solution. 

The products of the base treatment reaction are recovered by phasing into chloroform and into water as described earlier in this and the previous chapter. The chloroform-soluble fraction will contain the methyl esters of the long-chain fatty acids, and the glycerophosphoinositols will be recovered in the water-rich fraction. It is important that the latter fraction be acidified and that the inorganic cations be removed by passage through an ion exchange column (e.g., Dowex-50). 

Figure 9-1- Role of carnitine in fatty acid oxidation. Long-chain fatty acids are activated as the thioester of CoA on the cytoplasmic side of the mitochondrial membrane. The fatty acyl group is then transferred to form the corresponding carnitine ester in a reaction catalyzed by carnitine palmitoyltransferase I (CPT ]) The acylcarnitine then enters the mitochondrial matrix in exchange for carnitine via the carnitine-acylcarnitine translocase. The acyl group is transferred back to CoA in the matrix by carnitine palmitoyltransferase II (CPT II). The intramitochondrial acyl-CoA can then undergo P-oxidation.

Reactions converting acids to esters or vice versa and the exchange of ester groups are among the most widely used in fatty acid and hpid chemistry (Figure 4). They find applications from microscale preparation of methyl esters for GC analysis to the industrial production of oleochemicals and biodiesel. The exchange of groups attached to the fatty acid carboxyl is usually an equihbrium process driven to one product by an excess of one reactant or the removal of one product, and it is usually... 

The fatty acid or alcohol groups present in an ester can be exchanged in a number of ways by reaction with an excess of other fatty acids (acidolysis), alcohols (alcoholysis), or other esters (interesterification). Generally, the starting point will be a triacylglycerol, and these reactions provide routes by which the composition and properties of oils and fats can be modified. 

Transesterification, in which fatty acid exchange occurs between esters. For example, the transesterification of tripalmitin (PPP) and triolein (OOO) would produce all positional isomers of POO and PPO. The reaction can be conducted using a single intact fat as the reactant, in which case shuffling... 

Enzymatic Methods The use of enzymes to produce fatty acids and fatty acid-derived products has been a focus in both academic and industrial circles. Lipases may catalyze esterification, hydrolysis, or exchange of fatty acids in esters (115). These processes can be selected by choosing appropriate substrates and reaction conditions. Lipase-catalyzed processes have attracted attention because of the mild reaction conditions under which they occur and the selectivity displayed by these catalysts. In both respects, they differ from typical chemical reactions. As enzymatic reactions occur under mild temperature and pH conditions and at ambient pressure, they generally require less energy and are conducted in equipment of lower capital cost than many other chemical processes. Another advantage of enzymatic process is related to the selectivity of many lipases, which allows obtaining products that are difficult to produce by more conventional chemical reactions. 

Acylation reactions of cellulose with tty acids have been accomplished in the absence of solvent with the help of a co-reagent and the solvent exchange technique as a pretreatment for cellulose. The latter included soaking of cellulose with water, followed by washing with ethanol and finally with the tty acid. In the present work, we propose a new technique for the synthesis of cellulose and starch tty esters by esterification with fatty acids without the use of co-reagent or organic solvent. This technique passes through an emulsion to accomplish an intimate contact betwe the polysaccharide and the fatty acid. 

Burton, 2009 Helwani et al., 2009). A typical form of this reaction is shown in Chapter 2. The fatty acid chains present in the feedstock usually range from 12 to 22 carbon atoms. In these types of reactions, the triglycerides are transformed into straight-chain molecules by exchanging the alcohol from an ester with another alcohol in a process similar to hydrolysis, except that an alcohol is used instead of water. The final product is always similar in size to the molecules of the species present in the diesel fuel (Rajan and Senthilkumar, 2009 Sinha et al., 2008). Therefore, such products can be used in normal engines without any modification. By transesterification, oil molecular weight can be reduced to approximately one-third and the viscosity to about one-seventh, and the flash point and in some cases the volatility can be reduced as well (Demirbas, 2009b). 

The catalytic action of the lipase may not be the same in the wet solvent as it is in water. The enzyme from Candida cylindrocarpon catalyses the hydrolysis of a different range of esters in the solvent, and it also catalyses ester transfer. This transesterification reaction has some potential for the upgrading of food oils, whose value is dependent on the nature and the position of the fatty acids which esterify the glycerol. Some cocoa butter equivalents are now manufactured from palm oil by exchanging a proportion of its esterified... 

A mixture of ester is obtained, and the ratio of monoester to diester is controlled by ratios of the compounds charged to the reactor. Excess polyoxyethylene is used to maximize monoester production (5), and excess fatty acid is used to maximize diester formation (6). Because of the existing equilibrium, it is important that water be removed with an azeotroping agent such as toluene, xylene, etc., and/or by use of an inert-gas sparge to carry off water as it is formed to force the equilibrium toward the desired product. Catalysts such as sulfuric acid (7), benzene sulfonic acid, and other aromatic sulfonic acids (5, 8, 9), as well as cationic ion-exchange resins such as polystyrene-sulfonic acids (5, 9), are used. The latter compounds have the advantage of easy removal from batch reactions and of use in a fixed bed for continuous processes. Metals such as tin, iron, and zinc, as well as their salts in powdered form, have been used as catalysts (10,11). Catalysts can improve the yield of monoester. Of course, use of a monohydroxyl-functional polyoxyethylene, such as that from methanol-started ethylene oxide polymers (methoxy-polyoxyethylene), can be esterified with fatty acids to yield effectively all monoester. 

Enzymatic acylation to yield bioactive compounds with additional properties and/or altered functionahties generally takes place via esterification or transesterification reactions. In simple esterification, the bioactive compound reacts with a fatty acid (or alcohol, based on its structure) to yield an ester and a molecule of water. In contrast, transesterification reactions involve acyl exchange between an ester and alcohol to yield structurally different ester and alcohol species. Both reactions are typically catalyzed by lipase and can, therefore, benefit from milder reaction conditions, substrate and/or regiospecificity (Chebil et al., 2006). 

Fatty acid alkyl esters (FAAE) can be prepared from transesterification of vegetable oils or animal fats with ahphatic alcohols (Knothe et al., 2005). The transesterification reaction, also known as alcoholysis, is the exchange of alkoxy group of an ester compound (TAG) with an aliphatic alcohol (the acyl acceptor) in the presence of a catalyst. The overall reaction is a sequence of three consecutive and reversible reactions in which DAG and MAG are formed as intermediate products (Ma and Hanna, 1999). The general reaction scheme is given in Figure 14.5. 

Transesterification is a reversible reaction in which there is an exchange of the alkyl group of an ester with alkyl group of an alcohol as shown in Fig. 6.1 (Section 6.1), and can be catalyzed by an acid or base catalyst. This reaction also can be accomplished by addition of enzymes (biocatalysts) particularly lipases (Fukuda et al., 2001 Meher et al., 2006 Garcia et al., 2008). Biodiesel is commonly composed of fatty acid methyl ester (FAME) and can be produced by triglycerides from vegetable oils by transesterification of methanol, as illustrated in Fig. 6.4. 

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