Basic esters, chemical hydrolysis

Hydrolysis of vinyl acetate is catalyzed by acidic and basic catalysts to form acetic acid and vinyl alcohol which rapidly tautomerizes to acetaldehyde. This rate of hydrolysis of vinyl acetate is 1000 times that of its saturated analogue, ethyl acetate, ia alkaline media (15). The rate of hydrolysis is minimal at pH 4.44 (16). Other chemical reactions which vinyl acetate may undergo are addition across the double bond, transesterification to other vinyl esters, and oxidation (15—21). 

As esters of sulfuric acid, the hydrophilic group of alcohol sulfates and alcohol ether sulfates is the sulfate ion, which is linked to the hydrophobic tail through a C-O-S bond. This bond gives the molecule a relative instability as this linkage is prone to hydrolysis in acidic media. This establishes a basic difference from other key anionic surfactants such as alkyl and alkylbenzene-sulfonates, which have a C-S bond, completely stable in all normal conditions of use. The chemical structure of these sulfate molecules partially limits their conditions of use and their application areas but nevertheless they are found undoubtedly in the widest range of application types among anionic surfactants. 

Stereoinversion Stereoinversion can be achieved either using a chemoenzymatic approach or a purely biocatalytic method. As an example of the former case, deracemization of secondary alcohols via enzymatic hydrolysis of their acetates may be mentioned. Thus, after the first step, kinetic resolution of a racemate, the enantiomeric alcohol resulting from hydrolysis of the fast reacting enantiomer of the substrate is chemically transformed into an activated ester, for example, by mesylation. The mixture of both esters is then subjected to basic hydrolysis. Each hydrolysis proceeds with different stereochemistry - the acetate is hydrolyzed with retention of configuration due to the attack of the hydroxy anion on the carbonyl carbon, and the mesylate - with inversion as a result of the attack of the hydroxy anion on the stereogenic carbon atom. As a result, a single enantiomer of the secondary alcohol is obtained (Scheme 5.12) [8, 50a]. 

We have studied this reaction in considerable detail (88) and have found that when one uses quinine (eq. [25]) or any one of the chiral bases, a variety of aldehydes react with ketene to form the corresponding p-lactones in excellent chemical and nearly quantitative enantiomeric yields. Equation [25] exemplifies the reaction. Note that mild basic hydrolysis of the lactone furnishes a trichlo-rohydroxy acid that was prepared earlier by McKenzie (89). If one uses quinidine as catalyst, the process furnishes the natural (S)-malic acid. Note that ketene first acylates the free hydroxyl group of quinine, so that the actual catalyst is the alkaloid ester. 

Nature often exploits large pJQ shifts in enzymes to effect chemical catalysis similarly, we hoped to apply the large shifts in the effective basicities of encapsulated guests to reaction chemistry. Initial studies focused on the hydrolysis of orthoformates, a class of molecules responsible for much ofthe formulation ofthe Bronsted theory of acids almost a century ago [98]. While orthoformates are readily hydrolyzed in acidic solution, they are exceedingly stable in neutral or basic solution [99]. However, in the presence of a catalytic amount of 1 in basic solution, small orthoformates are quickly hydrolyzed to the corresponding formate ester [38]. Addition of NEt4 to the reaction inhibited the catalysis but did not affect the hydrolysis rate measured in the absence of 1. With a limited volume in the cavity of 1, substantial size selectivity was observed in the orthoformate hydrolysis. Orthoformates smaller than tripentyl... 

The chemical properties of PVAc are diose of an aliphatic ester. Thus, acidic or basic hydrolysis produces poly(vinyl alcohol) and acetic acid or die acetate of the basic cation. 

The chemical modes of bimolecular ester hydrolysis, as represented by Ingold (26) and his school, are related to either acidic or basic catalysts. They involve attack at the carbonyl portion of the ester group, giving as intermediates or transition states derivatives of the ortho acid, which may undergo reversible (4) or practically irreversible (B) changes ... 

The preparation of a solution of soap by the reaction of fat with water in the presence of base was probably one of the earliest chemical processes discovered by humans. Although the details of this discovery are lost in antiquity, we can imagine early humans finding that water that had been in contact with wood ashes from the campfire could be used to remove grease from hands and other objects and that this water became a more effective cleaning agent as it was used. The water leaches some alkaline compounds from the ashes, and this basic water hydrolyzes the esters of the fat or grease to alcohols and soap. This is why the hydrolysis of esters under basic conditions is called saponification (the Latin word for soap is sapo). 

Alkylation products of pseudoephedrine amides are readily transformed in a single operation into highly enantiomerically enriched carboxylic acids, aldehydes, ketones, lactones or primary alcohols. Alkylated pseudoephedrine amides can be hydrolyzed under acidic or basic conditions to form carboxylic acids. Simply heating a pseudoephedrine amide at reflux in a 1 1 mixture of sulfuric acid (9-18 N) and dioxane affords the corresponding carboxylic acid in excellent chemical yield with little or no epimerization (eq 7). Under these conditions, the substrate initially undergoes a rapid N— -0 acyl transfer reaction followed by rate-limiting hydrolysis of the resulting (3-ammonium ester intermediate to form the carboxylic acid. ... 

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). 

The presence of other cathodic and anodic peaks points to electrochemical activity on other oxygen species existing on the carbon surface (see Table 4). Additionally, they may be overlapped by a significant capacitive current [153]. However, it should be remembered that the real chemical structure of an oxidized carbon surface [101] depends on the hydrolysis of lactone-, ester- or ether-like anhydrous systems and the ionization of some functionalities at extreme pH values (acidic or basic environments) [91]. These phenomena influence the surface density of species that can take part in charge-transfer processes, which explains the observed differences in height of reduction peak in different environments (see Fig. 18). These relationships can account for the reactions, e.g. [7,14,148],... 

Alumina is probably the most commonly used adsorbent. It has the advantages of being white, insoluble, reasonably chemically inert, a good adsorbent for many compounds, and readily available. Ordinary alumina often contains some sodium carbonate or sodium bicarbonate, and in many cases this is a disadvantage since the base may promote aldol condensations, hydrolysis of esters and lactones, and similar reactions. The basic impurities may be removed by washing the alumina... 

To ensure safety during lab development and scale-up, a complete chemical hazard assessment must be done. Not all reactions need to be thoroughly subjected to analysis For instance, the basic hydrolysis of an ester in water under relatively dilute conditions would not be expected to pose an extreme safety hazard. Safety testing can determine how exothermic a reaction is and whether the reaction can be conducted safely on scale [4, 5]. For instance, by knowing the amount of heat evolved in a reaction and having assessed the system s ability to remove heat from a reactor, it is possible to calculate the amount of time needed on scale to add a reagent that produces an exothermic reaction. 

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