Homogeneous acid-catalyzed

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). 

Scheme 10.2 Homogeneous acid-catalyzed reaction mechanism for the transesterification of triglycerides (1) protonation of the carbonyl group by the acid catalyst (2) nucleophilic attack of the alcohol, forming a tetrahedral intermediate (3) proton migration and breakdown of the intermediate. The sequence is repeated twice.

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

Section 3 surveys the application of EGA to organic synthesis in comparison with other conventional homogeneous acid-catalyzed reactions. 

In summary, pre-esterification presents a viable strategy in dealing with high FFA feedstocks. Homogeneous acid catalyzed esterification of FFA proceeds readily and under mild conditions. A disadvantage of this approach is that the acid catalyst will have to be neutralized before proceeding to the transesterification step. In addition, methanol from the esterification step will have to be dried before it can be recycled. Heterogeneous catalysts also... 

The homogenous acid catalyzed hydrolysis of sucrose uses food approved mineral acids (sulfuric acid, hydrochloric acid) at elevated temperature. The degree of inversion could be adjusted by the point of neutralization (sodium or potassium hydroxide). This step leads inevitably to the formation of the respective salts, thereby causing high ash contents in the product. A further disadvantage of this method is the applied elevated temperature in combination with the low pH, thus causing by-product formation. 

Alkyl levulinate esters 72 can be prepared by homogeneous acid-catalyzed Fischer esterification of LA [188], under solid acid catalysis [189, 190], by acid-catalyzed... 

Transparent, homogeneous hybrids using a 50 50 PVAc-to-TEOS mixture and an acid-catalyzed reaction have been produced and characterized by dsc and dms (46). Dsc indicated only a slight increase in the T of the hybrid with incorporation of sihca. Dynamic mechanical tan 8 responses indicate a strong interaction between the organic and inorganic phases and, hence, weU-dispersed phases that lead to high modulus mbbery plateaus. 

Transparent, homogeneous hybrids using a 50 50 PVP-to-TEOS mixture and an acid-catalyzed reaction have been reported, but only tga data were presented in the way of characterization (46). 

Acidic, high area silica-almnina had received substantial attention in ICC 1, 52-58). Perhaps the most dramatic change in the subsequent catalytic literature was the debut of zeolites. Why acid catalyzed reactions are so much faster on zeolites than on silica-alumina has been extensively discussed but probably not conclusively. One should be able to know the exact structures of catalytic sites in zeolites, but initial hopes that this would do wonders for mechanistic imderstanding have not been fully realized. Super acids and carbonium ions came into heterogeneous catalysis from homogeneous chemistry and in special cases reaction via carbonium ions seems to occur. 

Method and device for production of phenol and acetone by means of acid-catalyzed, homogeneous decoposition of cumd-hydroperoxid, WO 01/30732, Phenol-chemie GmbH, Priority 22.10.99. 

We encounter homogeneous catalysts in both step-growth and chain-growth polymerization processes. We saw several examples of these types of reactions in Chapter 2. For example, the acid catalyzed polymerization of polyesters occurs via a homogeneous process as do some metallocene catalyzed polymerization of polyolefins. 

Rate data for hydrolysis reactions in homogeneous aqueous solutions have been reviewed (79), but application of these data to environmental conditions involving mineral surfaces remains difficult due to the unknown effects sorption may have. Several studies have demonstrated that acid-catalyzed reactions are promoted if the substrate is sorbed at clay surfaces (70-74 and other works reviewed by Theng, 8), but inhibition may also occur if substrate hydrolysis is base-promoted (74). 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

The zero slope found for transesterification (series 45) can be explained in accordance with the general view on acid-catalyzed reactions of organic acids and esters. The first step is the protonation of the acid or ester, which is followed by interaction with the alcohol (or water in ester hydrolysis). The absence of any observable influence of the alcohol structure on rate indicates that the rate-determining step must be the protonation of the ester. This is in contrast to the homogeneous reaction, in which this step is usually very rapid. The parallel dehydration of the alcohols exhibited a large structure effect on rate (Case 7 from Table II), confirming the independence of the two reaction routes. 

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