Acetylation catalysts

Cm ORINE OXYGEN ACIDS AND SALTS - DICm ORINE MONOXIDE, HYPOCm OROUS ACID, AND HYPOCm ORITES] (Vol 5) -as acetylation catalyst [ACETIC ACID AND DERIVATIVES - ACETIC ACID] (Vol 1)... 

Nearly all commercial acetylations are realized using acid catalysts. Catalytic acetylation of alcohols can be carried out using mineral acids, eg, perchloric acid [7601-90-3], phosphoric acid [7664-38-2], sulfuric acid [7664-93-9], benzenesulfonic acid [98-11-3], or methanesulfonic acid [75-75-2], as the catalyst. Certain acid-reacting ion-exchange resins may also be used, but these tend to decompose in hot acetic acid. Mordenite [12445-20-4], a decationized Y-zeohte, is a useful acetylation catalyst (28) and aluminum chloride [7446-70-0], catalyzes / -butanol [71-36-3] acetylation (29). 

Perchloric acid is a weU-known acetylation catalyst, especially in the fibrous method of preparing cellulose triacetate. Unlike sulfuric acid, perchloric acid does not combine with cellulose (78), ie, it does not form esters, and therefore virtually complete acetylation (DS 3.0, 44.8% acetyl) occurs. However, the extremely corrosive nature of perchloric acid and explosive nature of its salts have precluded its use industrially as an acetylation catalyst. 

Zinc chloride is a Lewis acid catalyst that promotes cellulose esterification. However, because of the large quantities required, this type of catalyst would be uneconomical for commercial use. Other compounds such as titanium alkoxides, eg, tetrabutoxytitanium (80), sulfate salts containing cadmium, aluminum, and ammonium ions (81), sulfamic acid, and ammonium sulfate (82) have been reported as catalysts for cellulose acetate production. In general, they require reaction temperatures above 50°C for complete esterification. Relatively small amounts (<0.5%) of sulfuric acid combined with phosphoric acid (83), sulfonic acids, eg, methanesulfonic, or alkyl phosphites (84) have been reported as good acetylation catalysts, especially at reaction temperatures above 90°C. 

Figure 6.10 Active sites of lipase (1), triflinctional (thio)urea derivatives (38 39) mimicking the acive site of serine hydrolases (2), and acetyl-catalyst intermediate of the biomimetic transesterification between vinyl trifluoroacetate methanol and 2-propanol, respectively (3).

The nitroalcohols are esterified by the usual acylating agents in the presence of acidic catalysts. With the nitroglycols of the sugar series, acetic anhydride containing a trace of sulfuric acid gives rapid and complete acetylation. Basic acetylation catalysts, such as pyridine or sodium acetate, are not satisfactory, presumably due to interaction with the nitro group. 

Sulfuric acid is a powerful esterification catalyst. It has been widely applied with mixtures of acetic acid and acetic anhydride to promote acetylations of numerous substances. Use of this catalyzed reaction for starch acetylation, however, has not risen to pre-eminence among starch acetylation methods as it has done among cellulose acetylations, although both reactions were discovered at the same time. The underdevelopment of this reaction in the starch field may be due to the following causes (1) sulfuric acid, a powerful acetylation catalyst, strongly catalyzes the hydrolysis of starch molecules and cannot be used for starch acetylations in the concentrations found most effective for cellulose reactions (2) most investigations of this reaction have been made on whole granules... 

Sodium acetate, a mild acetylation catalyst, has been extensively used in the acetylation of carbohydrate material. When used in conjunction with acetic anhydride or with acetic anhydride and acetic acid at reflux temperatures, it is capable of producing only a slow acetylation of untreated starch granules. After four days of refluxing with acetic anhydride and sodium acetate, com starch has been acetylated to 43.5% acetyl content, but the acetylation is non-uniform since the product can be fractionated into parts having different acetyl contents. The slow acetylation of ordinary starch in these cases is probably due to the compact structure of the starch granules, and a much more rapid and complete acetylation would be expected to occur with a starch pretreated as described in Section II. 

Sulfuryl chloride and magnesium perchlorate have been suggested as acetylation catalysts for starch. Sulfur dioxide in acetic acid/ sulfur trioxide in acetic anhydride, and sulfonated fatty acid or sulfonated salicylic acid in a mixture of acetic anhydride and acetic acid have also been reported to acetylate starch. 

Acetylation. A comparative student experiment with salicylic acid and acetic anhydride demonstrates that common acetylation catalysts fall into the following order of relative effectiveness coned. H2SO4 > boron trifluoride etherate > pyridine > sodium acetate. Although it is a relatively weak catalyst, sodium acetate is completely nondestructive and can be employed in much larger than truly catalytic amounts. An example is the acetylation of furylcarbinol. A mixture of the reactants and solvent benzene was heated on the steam bath with stirring to prevent caking... 

Acetylation catalyst. Whitman and Schwenk showed that a number of steroid iilcohols can be acetylated in high yield by suspending 1 g. of steroid in 10 ml. of acetic acid and 3 ml. of acetic anhydride, cooling to 18°, and adding 0.1 ml. of 5A1... 

Reactions of an alcohol with acetic anhydride are greatly accelerated by acid catalysts, such as sulfuric acid, zinc chloride, phosphorus pentoxide, ferric chloride, etc. However, such catalysts cannot be used with sensitive alcohols, such as linalool, which are isomerized or otherwise affected by them. The effective acid strength of a catalyst in an anhydrous medium is the predominant factor in determining its activity. The acidities of mineral acids in glacial acetic acid correspond remarkably well with their activities as acetylation catalysts. Sulfuric and perchloric acids in acetic acid solution have been termed superacid solutions because of thdr exceptional strength as compared with the strength of other acids. Most anhydrides react more rapidly with an alcohol in the presence of a base. The base can be sodium hydroxide, the sodium salt of the acid, or a tertiary amine, which can also be the solvent for the reaction. ... 

A comparison of cellulose acetate solution properties as a function of wood-pulp source and acetylation catalyst level is shown in Table 11.2. 

A more basic acetylation catalyst was used by Adams, again after making amino acid n-propyl esters, who used a freshly prepared mixture of acetone, triethyl-amine and acetic anhydride in the proportions of 3 2 1 at 60 °C for 30 seconds. Excess reagents were blown off cautiously to avoid volatilization losses, and the residue was dissolved in ethyl acetate for injection into the gas chromatograph [23]. 

To confirm the turnover of a catalyst, substrate must be added in large excess to the catalyst. Variation of p-nitrophenol concentration with time in the hydrolysis of p-nitrophenyl acetate under this condition ([substrate] >> [catalyst]) is shown in Figure 1. It shows that after reaction for 5 hours 3-CD hydrolyzed p-nitrophenyl acetate only 1/4 molar quantity of the catalyst, but 3-CD-histamine acted more than 4 times. After the reaction for 24 hours, 3-CD-histamine was separated from the substrate and products by HPLC, and this recovered catalyst reacted at the same rate as the original catalyst. This result reveals that the catalyst is not modified by p-nitrophenol or p-nitrophenyl acetate. Under this condition [S] >> [E], the deacylation of acetyl-catalyst is the rate-determing step for hydrolysis of p-nitrophenyl acetate (scheme 1). In the case of 3-CD, the rate constant of deacylation (k )... 

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