Methyl esters,trans-esterification

Fatty acid methyl esters (FAME) are currently manufactured mainly by trans-esterification with an alcohol, using a homogeneous base catalyst (NaOH/KOH). Methanol is more suitable for biodiesel manufacturing, but other alcohols can in principle also be used, depending on the feedstock available. The... 

Methyl esters undergo trans-esterification with the quaternary ammonium salts at high temperature and the reaction has been used with some effect for the preparation of, for example, n-butyl esters by heating the methyl ester with tetra-n-butylammo-nium chloride at 140°C [31]. Optimum yields (>75%) are obtained in HMPA or in the absence of a solvent. A two-step (one-pot) trans-esterification under phase-transfer catalysed conditions in which the carboxylate anion generated by initially hydrolysis of the ester is alkylated has been reported for Schiff s bases of a-amino acids [32] and for A-alkoxycarbonylmethyl [1-lactams [33]. Direct trans-esterification of methyl and ethyl esters with alcohols under basic catalytic conditions occurs in good yield in the presence of Aliquat [34, 35]. 

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. 

The process involves reacting the degummed oil with an excess of methyl alcohol in the presence of an alkaline catalyst such as sodium or potassium methoxide, reaction products between sodium or potassium hydroxide and methyl alcohol. The reaction is carried out at approximately 150°F under pressure of 20 psi and continues until trans-esterification is complete. Glycerol, free fatty acids and unreacted methyl alcohol are separated from the methyl ester product. The methyl ester is purified by removal of residual methyl alcohol and any other low-boiling-point compounds before its use as biodiesel fuel. From 7.3 lb of soybean oil, 1 gallon of biodiesel fuel can be produced. See FIGURE 12-5. 

BASIC PROTOCOL I PREPARATION OF FATTY ACID METHYL ESTERS FROM LIPID SAMPLES CATALYZED WITH BORON TRIFLUORIDE IN METHANOL In this method, lipid samples are first saponified with an excess of NaOH in methanol. Liberated fatty acids are then methylated in the presence of BF3 in methanol. The resulting fatty acid methyl esters (FAMEs) are extracted with an organic solvent (isooctane or hexane), and then sealed in GC sample vials for analysis. Because of the acidic condition and high temperature (100°C) used in the process, isomerization will occur to those fatty acids containing conjugated dienes, such as in dairy and ruminant meat products, that contain conjugated linoleic acids (CLA). If CLA isomers are of interest in the analysis, Basic Protocol 2 or the Alternate Protocol should be used instead. Based on experience, this method underestimates the amount of the naturally occurring cis-9, trans-11 CLA isomer by -10%. The formulas for the chemical reactions involved in this protocol are outlined in Equation D1.2.1 Saponification RCOO-R + NaOH, RCOO-Na + R -OH v 100°C DC Esterification RCOO-Na + CH,OH r 3 v RCOO-CH, + NaOH ioo°c ... 

This acid is a mixture of cis- and trans-isomers. By esterifying the acid to the methyl ester with diazomethane a two compound mixture is formed, whereas ester formation under acidic conditions (MeOH/HCl) gives a single compound. This single compound 115 is thought to be the trans-isomer, because isomerization of the mixture of esters obtained by reaction with diazomethane to a single compound is apparently possible by an esterification relactonization mechanism by treating the mixture with acid in methanol. These results are consistent with the formulation of the ester 115 as the trans-isomer. 

The effect of the degree d xrlarization of the ester linkages in low-motecular-weight esters on their reactivity has been examined by studying the reaction of alcoholysis, with ceUulose, of methyl esters of benzoic, p-chloro-, p-hydroxy, p-hydroxy-, p- and o-nitrobenzoic acids, and those of phenyl-, phenoxy-, 2,4-dichlorophenoxy- and monodiloro-acetic adds. The trans-esterification reaction was carried out in non-aqueous dinrethylformamide in the presence of catalysts (sodium methylate, cadmium acetate, p-toluenesulphonic add) at 110-140° C. 

In the DOD, phytosterols are present in both the free and esterified forms with fatty acids. Therefore, the first step in the extraction of phytosterols is conversion of phytosterol fatty esters into free phytosterols. This is achieved either by hydrolysis or trani-esterification. Hydrolysis could be carried out under strong basic conditions (saponification with further acidulation), under strong acidic conditions, or under chemical or enzyme (specific or nonspecific) catalyzation. Re-esterification of phytosterols occurs during methyl ester distillation as a result of the high temperatures involved therefore, a further trani-esterification step for free sterols is required. Esterification of phytosterols or trani-esterification of sterol fatty acid esters is the second step in this process. Methanol is the most commonly used alcohol, and it leads to methyl esters, which are characterized by a higher volatility, however, other Ci to C4 alcohols may also be used. Esterification and trans-esterification of fatty acids or phytosterols can be catalyzed by metal alcoholates, or hydroxide, by organic catalysts, or by enzymes (Table 7). 

Fats and oils are renewable products of nature. One can aptly call them oil from the sun where the sun s energy is biochemically converted to valuable oleochemicals via oleochemistry. Natural oleochemicals derived from natural fats and oils by splitting or tran -esterification, such as fatty acids, methyl esters, and glycerine are termed basic oleochemicals. Fatty alcohols and fatty amines may also be counted as basic oleochemicals, because of their importance in the manufacture of derivatives (8). Further processing of the basic oleochemicals by different routes, such as esterification, ethoxylation, sulfation, and amidation (Figure 1), produces other oleochemical products, which are termed oleochemical derivatives. 

Biodiesel fuel can be produced from jatropha oil using a fairly simple chemical reaction known as trans-esterification. In this process, the oil reacts with a simple alcohol (methanol/ethanol) in the presence of a catalyst (caustic soda/potash) and, under specified conditions such as a temperature of 65°C and normal atmospheric pressure, yields a mixture of methyl or ethyl esters which is the biodiesel. This fuel... 

Of the methods of synthesis of cellulose esters, the one that has been most thoroughly studied is the reaction of trans-esterification, and this method is widely used for the synthesis of low-molecular-weight esters. The alcoholysis of a low-molecular-weight ester (methyl- and n -propyl-borate) with hydroxyl groups of cellulose was first used (37) for die preparation of cellulose borate. This was followed by the trans-esterification, with cellulose, of the esters of phosphorous acids (see above), i.e. mono-, di- and trimethylphosphites (71, 72, 75), esters of phosphonic acids (76), and also phenyl-/ -chloroethyl- and / -fluoroethylphosphites (77, 78). Of considerable interest is the reaction of alcoholysis, with cellulose, of the esters of aryl- and naphthalenesulphonic add, which results in the formation of cellulose ethers, rather than esters (79-81). 

Although HMFA is more stable than DHMF, its still limited thermal stability is reflected in the fact that only two scientific papers and one patent application can be found describing HMFA-based polyesters. Whereas Hirai et al were only able to obtain macrocyclic oligo-esters by solution polymerization in pyridine [58,59], Moore and Kelly used trans-esterification of the methyl ester of HMFA, also giving materials in low yield and with low molecular weights [60]. 

Cleavage of the peptide from the resin by trans-esterification can be achieved readily with most alcohols. The resultant C-terminal-protected molecules can be used in fragment condensation to assemble longer peptides. The protocol described here involves cleavage with methanol in the presence of a tertiary base. The methyl ester produced can subsequently be removed by saponification. 

A formal total synthesis of the proteasome inhibitor omuralide (71) has been reported by Kobayashi and co-workers involving a highly diastereoselective U-SCR." The condensation of y-ketoacid 64, p-methoxybenzylamine (65) and isonitrile 66 resulted in the y-lactam 67 as a single diastereomer. The source of the stereoselectivity in the reaction is rationalized by axial attack of the isonitrile with the intermediate iminium ion. In addition, the novel isonitrile 66 was developed and used in the reaction in order to avoid the difficulty associated with subsequent selective trans-esterification of the more hindered amide (C9 vs. Cl) in the Ugi product. Conversion of 68 to the 7V-acylindole 69 was accomplished under mildly acidic conditions. This intermediate was then treated with methanol and triethylamine to readily afford the neo-pentyl methyl ester 70 without observable methanolysis of the lactam functionality. 

Such distannoxanes have been found to be highly active and selective catalysts for (trans)esterification reactions of methyl esters or acids with a twofold excess of alcohol and 0.5 mol% of distannoxane, either solventless or in toluene.A similar 100% yield in esters could be observed with a stoichiometric amount of alcohol in fluorous solvent that permitted recycling of the catalyst without loss of its performances. " The proposed mechanism consists in the activation of the alcohol via Sn-X (Y) ligands exchange to Sn-OR ones (Scheme 21.13b), while the ester is coordinated through the ketonic o>ygen atom to an adjacent tin centre (Scheme 21.13c). A similar dinuclear activation was reported for the melt transesterification of a variety of polyesters and polyacetates in the presence of such distannoxane catalysts. 

A range of carboxylic esters undergo mild and neutral transesterification when treated with iodotrimethylsilane followed by an alcohol. The reaction is limited to primary and secondary aliphatic alcohols, and yields between 40 and 98% are obtained. Another non protic trans-esterification method involves treatment of the methyl esters of various carboxylic acids with tetra-n-alkylammonium halide at 140 °C, leading to the production of the corresponding n-alkyl esters. The generality of this method clearly depends on the availability of the required tetra-n-alkylammonium halide. Lactones react with iodotrimethylsilane in the presence of an alcohol to provide a convenient route to iodoalkyl esters in good yield. ... 

Scheme 4.2 Synthesis of sucrose esters by base-catalysed trans-esterification with fatty acid methyl esters (R COOMe), usually carried out in solvents (e.g. dimethyl formamide), microemulsions or solvent free.

Scheme 4.7 Synthesis of methyl glucoside ester by base-catalysed (K2CO3) trans-esterification of methyl glucoside with fatty acid methyl ester (R COOMe) at 120-160 °C.

All feedstocks are refined through trans-esterification, a process by which the triglycerides react with alcohol and a catalyst, thus producing biodiesel (which contains fatty acid methyl esters — FAME), along with glycerol as a byproduct. 

---