Acid-catalyzed transesterification

A monoacetate can be isolated by continuous extraction with organic solvents such as cyclohexane/CCI4. Monoacylation can also be achieved by ion exchange resin or acid-catalyzed transesterification. 

Dialkyl 4-pyrimidinylaminomethylenemalonates (1426, n = 2,3,5) were prepared in 70-80% yields in the base-catalyzed transesterification of diethyl 4-pyrimidinylaminomethylenemalonates (1426, n = 1) with the appropriate alkanol at ambient temperature for 120 hr in the presence of sodium hydride. Acid-catalyzed transesterification was unsuccessful (84JHC247). 

The homogeneous acid-catalyzed transesterification process does not enjoy the same popularity in commercial application as its counterpart, the base-catalyzed process, one of the main reasons being that it is about 4000 times slower, due to the different mechanism [10]. Thus, in the reaction sequence triglyceride is converted stepwise to diglyceride, monoglyceride and finally glycerol with formation of one molecule of methyl ester at each step (Scheme 10.1). 

Acid-Catalyzed Synthesis. - 3.2.1 The Fundamentals. Homogeneous acid catalysts, such as sulfuric acid, phosphoric acid, hydrochloridric acid, organo sulfonic acids and others, can be used to catalyze the transesterification of TGs and the esterification of FFAs to produce biodiesel type monoesters. Nevertheless, because the acid-catalyzed transesterification is about 3 orders of magnitude slower than the alkali-catalyzed reaction for comparable amounts of catalyst, base catalysts have received the most attention in both the academic and industrial sectors. In addition, the corrosiveness of strong liquid acids and... 

The accepted chemical mechanism for the homogeneous acid-catalyzed transesterification is illustrated in Figure The sequence of steps can be... 

Compared to the base-catalyzed synthesis of biodiesel, fewer studies have dealt with the subject of acid-catalyzed transesterification of lipid feedstocks. Among acid catalysts, sulfuric acid has been the most widely studied. In the previously mentioned work of Freedman et al., the authors examined the transesterification kinetics of soybean oil with butanol using sulfuric acid. The three reaction regimes observed (in accordance with reaction rate) for base-catalyzed reactions were also observed here. A large molar ratio of alcohol-to-oil, 30 1, was required in this system in order to carry out the reaction in a reasonable time. As expected, transesterification followed pseudo-first-order kinetics for the forward reactions (Figure 2), while reverse reactions showed second-order kinetics. 

In general, alcohols, such as methanol, ethanol, propanol, butanol, amyl alcohol, etc., can be used with acid-catalyzed transesterifications to obtain high biodiesel yields. " Methanol is preferred due to its low cost and wide availability, but bioderived ethanol would be ideal for the synthesis of a fully biogenerated fuel. On the other hand, the use of heavier alcohols like butanol... 

Transesterification Reactions. The heterogeneous acid-catalyzed transesterification of TGs has not been investigated as much as its counterpart, the base-catalyzed reaction. Various solids are available with sufficient acid strength to be effective catalysts for the named reaction. Among the solid acids available are functionalized polymers, such as the acid forms of some resins, as well as inorganic materials, such as zeolites, modified oxides, clays, and others. Some of these solids have already been found to be effective in transesterification reactions of simple esters and (3-ketoesters. 

Regioselective deacylation has also been observed189 in the acid-catalyzed transesterification of some 3, 5 -di-0-acetyl-2 -deoxy-2 -halo-uridines in methanol to give the corresponding 3 -0-acetyl derivatives in yields of 36-66%. The presence of the 2-halogen substituent seems essential for the selectivity in the case of 2, 3, 5 -tri-0-acetyl-uridine, no significant differences in the reaction rates of the acetyl groups towards transesterification were apparent. 

The polymerization of spiroorthocarbonate 255 and spirotetrathioorthocarbonate 256 has previously been studied. The synthesis of spirotetrathioorthocarbonate 258 was performed by acid-catalyzed transesterification of tetra fr-(methylthio)methane with o-xylenedithiol <1997MM6721> (Scheme 78). 

Figu re 14.7 Mechanism of acid-catalyzed transesterification R1 -glyceride fragment, R2-fatty-acid carbon chain. 

Complete the mechanism for this acid-catalyzed transesterification by drawing out all the individual steps. Draw the important resonance contributors for each resonance-stabilized intermediate. 

Acid-catalyzed transesterification requires extra proton transfers before and after the major steps. The overall reaction takes place in two stages. The first half of the reaction involves acid-catalyzed addition of the nucleophile, and the second half involves acid-catalyzed elimination of the leaving group. 

Acid-catalyzed transesterification and Fischer esterification take place by nearly identical mechanisms. Transesterification can also take place by a base-catalyzed mechanism, but all attempts at base-catalyzed Fischer esterification (using OR", for example) seem doomed to failure. Explain why Fischer esterification cannot be catalyzed by base. 

Although acid catalyzed transesterifications exhibit slow kinetics (Ataya et al., 2007), acid catalysts have the advantage of concurrently converting FFA to esters. The process can be performed under pressure in the presence of sulfuric acid and excess methanol (Luxem, 2004), in which case esterification of FFA and transesterification of free oils proceed simultaneously, though at different rates (Scheme 6.2). 

Ataya, F., Dube, M. A., and Ternan, M. 2007. Acid-catalyzed transesterification of canola oil to biodiesel under single- and two-phase reaction conditions. Energy Fuels, 21, 2450-2459. 

Transesterification is catalyzed by acid (H2SO4 or dry HCI) or base (usually alkoxide ion). The mechanisms of these two reactions are exactly analogous to those we have already studied. For acid-catalyzed transesterification ... 

The assumption, that Eg is independent of the alcohol part of the ester and of solvent can also be questioned. Different scales of Es were obtained for acid catalyzed esterification of RCOOH with R OH (equation 81), for acid catalyzed transesterification of RCOOR1 with Rn0H (equation 82), and for acid hydrolysis (equation 53), although all have similar transition states (equations 81 and 82) OH +... 

An asymmetric synthesis of 2-amino-4-phosphonobutanoic acid has been developed from diethyl 3-oxo-4-(ethoxycarbonyl)propylphosphonate and L-( -nf/fro-(lS,27 )-l,2-diphenyl-2-hydrox-ylamine as chiral source. Formation of Schiff s base and acid-catalyzed transesterification are successfully performed in boiling n-BuOH. After reduction of the imino linkage with Al-Hg in moist DME, debenzylation [H2, PdiUll) 7( and hydrolysis with 4 M HCl, (7 )-(-)-2-amino-4-phosphonobutanoic acid is isolated in 66% yield and 67% ee. ... 

Write the mechanism for the acid-catalyzed transesterification reaction of methyl acetate with ethanol. 

The stoichiometry of the methanolysis of oils requires 3mol of alcohol per Imol of triglyceride to convert to 3mol of fatty acid methyl esters and Imol of glycerol. The most significant factor in alkali/acid-catalyzed transesterification was the molar ratio of the alcohol and oil. As transesterification involves reversible and consecutive reactions, increases in the molar ratio of methanol will result in high conversions [3, 7], However, in enzyme-catalyzed methanolysis, enzyme activity was shown to decrease in the methanolysis of oil as methanol concentration increased [13-14]. 

Route B does not strictly represent a fractionation protocol but is worthy of highlighting in the instances where an alternative rapid procedure is preferred for the analysis of total FA. Here, an acid-catalyzed transesterification can be undertaken, converting total ester-finked acyl residues directly to their methyl esters. As a byproduct, nonsaponifiable lipids are also retained in this fraction (hydrocarbons, isoprenoids) and analyzed simultaneously by chromatographic techniques. While more rapid, the technique yields a more complex sample for analysis. 

Two mechanisms can be envisioned for the Pechmann condensation (1) acid-catalyzed transesterification of the P-keto ester followed by acid-catalyzed cyclodehydration of the resulting aryl acetoacetate (4), or (2) acid-catalyzed Friedel-Crafts-like phenol alkylation ortho to the phenolic hydroxyl group) followed by intramolecular transesterification/cyclization of the resulting o-hydroxycinnamic acid ester (5). 

Alicata et al. [29] analyzed a series of copolymers with units of hydroxybutyrate (HB) and units of epsilon-caprolactone (C) by ESI-MS. The copolymers were obtained via the acid-catalyzed transesterification of the corresponding homopolymers (toluenesulfonic acid). In the years 1940-1950 it was proven [4] that this process yields polymers and copolymers with a polydispersity index which tends toward I = 2. The spectral quality was unsatisfactory, and thus, the authors fractionated the copolymer by SEC. The spectral quality of the SEC fraction was good. 

Miao, X Li, R Yao, H. Effective acid-catalyzed transesterification for biodiesel production. Energy Conversion and Management, 2009, v. 50 (10), 2680-2684. 

Soriano Jr, NU Venditti, R Argyropoulos, DS. Biodiesel synthesis via homogeneous Lewis acid-catalyzed transesterification. Fuel, 2009, v. 88 (3), 560-565. 

Before fatty acids are analyzed by GC, they must first be converted to the methyl esters. There is now a substantial body of work to confirm that acid-catalyzed transesterification is undesirable in general for the preparation of methyl esters because it can cause geometrical isomerization with an increase in the relative proportions of trans,trans isomers, and other unwanted side reactions (2,5,6). However, there appear to be no significant drawbacks to the use of base-catalyzed transesterification of lipids. For example with sodium methoxide in methanol, glycerolipids are transesterified in 10 min at 50°C. (A longer reaction time is necessary for cholesterol esters containing CLA.) Of course, with samples such as milk fat that contain a high proportion of short-chain fatty acids, it is advisable to use modified methods to minimize the loss of butyric and hexanoic adds, especially (7). 

Since the transesterification of glycerides is an amphoteric catalytic reaction, it can be carried out in presence of either an acid catalyst or a base catalyst. For an alkali-catalyzed transesterification, the raw materials must be anhydrous and with low FFAs less than 1% to avoid the production of soap (348). If the content of such compounds is greater, the acid-catalyzed transesterification can be utilized (349). However, the use of a liquid acid catalyst is costly, since it needs a distillation process for water removal, a methanol recycling unit, purification of ester, and separation of the homogeneous catalyst (350,351). 

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