Base catalysis transesterification

Methylphenol is converted to 6-/ f2 -butyl-2-methylphenol [2219-82-1] by alkylation with isobutylene under aluminum catalysis. A number of phenoHc anti-oxidants used to stabilize mbber and plastics against thermal oxidative degradation are based on this compound. The condensation of 6-/ f2 -butyl-2-methylphenol with formaldehyde yields 4,4 -methylenebis(2-methyl-6-/ f2 butylphenol) [96-65-17, reaction with sulfur dichloride yields 4,4 -thiobis(2-methyl-6-/ f2 butylphenol) [96-66-2] and reaction with methyl acrylate under base catalysis yields the corresponding hydrocinnamate. Transesterification of the hydrocinnamate with triethylene glycol yields triethylene glycol-bis[3-(3-/ f2 -butyl-5-methyl-4-hydroxyphenyl)propionate] [36443-68-2] (39). 2-Methylphenol is also a component of cresyHc acids, blends of phenol, cresols, and xylenols. CresyHc acids are used as solvents in a number of coating appHcations (see Table 3). 

Figure 7 depicts a simplified block flow diagram (BFD) for a typical biodiesel production process using base catalysis. In the first step, methanol and catalyst (NaOH) are mixed with the aim to create the active methoxide ions (Figure 4, step 1(b)). Then, the oil and the methanol-catalyst solution are transferred to the main reactor where the transesterification reaction occurs. Once the reaction has finished, two distinct phases are formed with the less dense (top) phase containing the ester products and unreacted oil as well as some residual methanol, glycerol, and catalyst. The denser (bottom) layer is mainly composed of glycerin and methanol, but ester residues as well as most of the catalyst, water, and soap can also be found in this layer. 

On the other hand, formation of methyl benzoate was also found to occur in methanol, indicating that, together with a general base-catalysis to produce benzamide and with the intramolecular, orthoester mechanism (see p. 110) to give the nitrogenated sugars, a transesterification reaction takes place in which the alkoxide ions play an important role. This can be exemplified by the following sequence. 

In the United States, the most utilized process is the base catalyzed transesterification of oil with alcohol. The base catalysis is popular because of its... 

Base-catalyzed transesterification proceeds faster than the acid-catalyzed reaction and requires lower temperatures. For this reason, combined with the fact that the alkaline catalysts are less corrosive than acidic compounds and lead to the formation of fewer artifacts, they are more frequently used. This type of catalysis is also recommended for samples containing short-chain FAs or labile FAs (polyunsaturated, conjugated unsaturations). 

Transesterification of triglycerides can be achieved via either acid catalysis or base catalysis to produce biodiesel. 

Acid-catalyzed transesterification this transformation also works with base catalysis R 0... 

In the case of base-catalyzed reactions the substrate comes into contact with either HO or any other highly electron-rich catalyst (e.g., alcoholates, strongly basic amines, metal alkyls). Again, the substrate is activated, typically via the intermediate formation of carbanion species. A technically important example of base catalysis is the transesterification of natural oils to fatty acid methyl esters (FAME, better known as biodiesel ), a process typically catalyzed by methanolate salts. 

The easiest acrylates to produce industrially are the epoxy acrylates their preparation (see Scheme 16.28) starts with an epoxide-functional resin (see Section 16.4.2). In principle any epoxide-functional material can be chosen. In this reaction (meth)acrylic resin is added to the epoxide at elevated temperatures, (around 90-130°C). The (meth)acrylic acid adds to the epoxide in a ring-opening reaction resulting in an ester alcohol group. Basically this reaction is similar to the reactions used in the preparation of epoxy resins (see Section 16.4). The reactions can be either acid- or base-catalyzed base catalysis is the more frequently used, since it limits the number of possible side reactions (for instance, transesterifications). Although these reactions can be carried out in solvent, industrially they are most frequently performed in bulk. Generally these preparations are performed in a batch-type process. 

Fig. 24.29 (a) Chemical structure of l,3-guanidinocalix[4]arene 77. (b) Bifunctional general acid-base catalysis of monoprotonated guanidinocalix[4]arene in the transesterification of HPNP. (c) Diribonucleotide phosphate cleavage [91, 92]... 

Baltzer s group has recently described a fully-synthetic protein that is also capable of hydrolysing p-nitrophenyl esters the polypeptide, which contains 42 amino acids, was designed to fold into a hairpin helix-loop-helix motif that dimerises into a four-helix bundle. The dimer is predicted to present on its surface a shallow reactive site containing several histidine residues. The spectroscopic properties of the peptide are consistent with the predicted folded structure, and the molecule does indeed catalyse ester hydrolysis (and transesterification) more effectively than 4-methylimidazole does. However, there is little substrate selectivity, and not much turnover. The histidine array does not seem to act via general acid-base catalysis, but rather to bind and stabilise ester oxygens in the transition state. We return to this molecule below. 

FIGURE 3.3 Reaction mechanism of RNase A-catalyzed hydrolysis of RNA. His-12 and His-119 are the catalytic residues of RNase A that perform concerted acid—base catalysis in the first (transesterification) and the second (hydrolysis) steps. B denotes the nucleotide 3 to the cleavage site (Py). 

RNase Tl cleaves P-05 ester bonds in ssRNA, specifically at the 3 -P of the guanylic acid residues. As in RNase A (see Fig. 3.3), the catalysis occurs by a two-step mechanism, i.e., the formation of a terminal guanosine 2, 3 -cyclic phosphate intermediate (transesterification step) and the hydrolysis of the cyclic ester to guanosine 3 -monophosphate (hydrolysis step). The transesterification step involves a general acid—base catalysis. 

Liu, Q., et al., 2014a. The application of Zr incorporated Zn—Al dehydrated hydrotalcites as solid base in transesterification. Catalysis Today 234, 161—166. Available at http //www. sciencedirect.com/science/article/pii/S0920586114001540 (accessed 23.04.15.). 

Fatty acid esters of sugars are also very important biodegradable and biocompatible surfactants that are prepared either by transesterification of methyl ester with sugar on basic catalysts or by esterification of fatty acids with sugar on acidic catalysts. Liquid acids and bases have been replaced by enzymatic catalysis with lipase, giving a higher yield of monoester [43, 44], but solid catalysts have not been used extensively so far. 

However, in contrast to the production know-how , the scientific knowledge on the details of phase equilibria, kinetics, mechanisms, catalysis and mass-transport phenomena involved in polycondensation is rather unsatisfactory. Thus, engineering calculations are based on limited scientific fundamentals. Only a few high-quality papers on the details of esterification and transesterification in PET synthesis have been published in the last 45 years. The kinetic data available in the public domain are scattered over a wide range, and for some aspects the publications even offer contradicting data. 

One of the best examples of the utility of enzymatic synthesis in catalyzing reactions that cannot be accomplished by any other route is the synthesis of substituted oxazolidine diesters. The oxazolidine ring is extremely water sensitive, the oxazolidine rapidly reverting back to the alkanolamine and aldehyde in the presence of water. Bis-oxazolidines have been used as hardeners for polymer coatings but the diester based on the hydroxyethyl oxazolidine and adipic acid cannot be synthesized directly with chemical catalysis because of the rapid rate of reaction of the oxazolidine ring with either the water from the esterification or the alcohol from transesterification. ... 

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