Mechanisms esterification

Table 4, Classification of esterification mechanisms and of ester hydrolysis mechanisms according to Ingold248)... 

Esters can be hydrolyzed to carboxylic acids under either acidic or basic conditions. Under acidic conditions the mechanism is the exact reverse of the Fischer esterification mechanism shown in Figure 19.3. Again, because the acid and the ester have comparable reactivities, some method must be used to drive the equilibrium toward the desired product—the acid in this case. This can be accomplished by using water as the solvent, providing a large excess of this reagent that, by Le Chatelier s principle, shifts the equilibrium toward the carboxylic acid. 

Butyl esters can be cleaved by reaction with dilute acid under milder conditions than those required to hydrolyze a methyl ester. The reaction follows an SNI mechanism, rather than the reverse of the Fischer esterification mechanism, because of the stability of the /-butyl carbocation ... 

First all three ester bonds and both amide bonds are hydrolyzed to carboxylic acid groups by the aqueous acid. The mechanisms for these reactions are discussed in Section 19.5. The ester hydrolyses follow the exact reverse of the Fischer esterification mechanism shown in Figure 19.3, and the amide hydrolysis occurs by a very similar mechanism. The product of these hydrolysis steps has three carboxylic acid groups and one amino group. Two of these acid groups are attached to the same carbon so that one can be eliminated as carbon dioxide by the cyclic mechanism described in Section 20.4 for the malonic ester synthesis ... 

The Fischer esterification mechanism (Key Mechanism 20-2) is an acid-catalyzed nucleophilic acyl substitution. The carbonyl group of a carboxylic acid is not sufficiently electrophilic to be attacked by an alcohol. The acid catalyst protonates the carbonyl group and activates it toward nucleophilic attack. Attack by the alcohol, followed by loss of a proton, gives the hydrate of an ester. 

The mechanism of the Fischer esterification would seem long and complicated if you tried to memorize it, but we can understand it by breaking it down into two simpler mechanisms (1) acid-catalyzed addition of the alcohol to the carbonyl and (2) acid-catalyzed dehydration. If you understand these mechanistic components, you can write the Fischer esterification mechanism without having to memorize it. 

The Fischer esterification mechanism is a perfect example of acid-catalyzed nucleophilic acyl substitution. You should understand this mechanism well. 

Most of the Fischer esterification mechanism is identical with the mechanism of acetal formation. The difference is in the final step, where a carbocation loses a proton to give the ester. Write mechanisms for the following reactions, with the comparable steps directly above and below each other. Explain why the final step of the esterification (proton loss) cannot occur in acetal formation, and show what happens instead. 

I. Dhimitruka, J. Santa Lucia Jr, Investigation of the Yamaguchi esterification mechanism. Synthesis of a lux-s enzyme inhibitor using an improved esterification method, Org. Lett. 2006, 8, 47-50. 

The formation of esters continues throughout the aging process thanks to the presence of many different acids in wine, together with large quantities of ethanol. Research into esterification mechanisms in wine (Ribereau-Gayon et al 1982)... 

In the recent years, various esterification mechanisms were proposed for homogeneous and heterogeneous systems. Base on the mechanism, numerous kinetic models have been developed to represent the kinetic behaviors of esterification, such as simple orders or the power-law model, the pseudo-homogeneous model, the L-H model, the E-R model, etc. 

Ethyl oleate was synthesized by the esterification of and ethanol catalyzed by SnCb 2H2O (Cardoso et al., 2008). Under the circumstance of excess ethanol, the effects of the concentration of the catalyst and oleic acid, and temperature on the reaction rate were investigated. A related esterification mechanism was presented and described as follows in presence of Sn2+( SnCh 2H2O) catalyst, the carbonyl of the fatty acid is polarized to activate of substrate, which makes the nucleophilic attack to the molecules by ethanol become more favorable. Cardoso et al. investigated the effect of different carbonic chain of alcohol (methyl alcohol, ethyl alcohol, n-p>ropyl alcohol, n-butyl alcohol) on the conversion of oleic acid into respective ester. The results showed that the conversion rate was down with the increase of carbon chain of alcohol, which indicated that high bulk hindrance occurs on the hydroxyl of the alcohol, and the efficient attack of them to the p>olarized carbonyl of oleic acid is reduced. However, it is not clear how the carbonyl is p>olarized by Sn2+. 

This chapter discussed esterification mechanisms, and evaluated the kinetics objectively and quantitatively, which provided a most effective way to select catalyst and design reactor for different esterification systems. It is discovered that some new catalysts (such as lipases, room temperature ionic liquids) have being used in esterification nevertheless, there are few research on the case. Herein, it is worthy to be investigated deeply. 

Carboxylic acids are converted into esters when treated with an alcohol in the presence of an acid catalyst. This process is called the Fischer esterification (Mechanism 21.6). 

FIGURE 17.20 (a) The first step in the Fischer esterification mechanism is protonation of the carbonyl oigrgen to give a resonance-stabilized intermediate, (b) The analogous process with an aldehyde or ketone. 

When the evolving gas flow is relatively small, the nucleation of bubbles may become a limiting factor, particularly in deep liquid layers. This may be a problem when the chemical reaction is reversible (e.g., esterifications). Mechanical stirring or stripping with an inert gas may be helpful (section 4.6.1.3). Another alternative is the use of a liquid film reactor (section 4.6.3.1). 

One of the chemical treatments of triglycerides of different vegetable and algae oils and animal fats produces the biofuel known as biodiesel. This development was based on the knowledge of the catalytic processes as well as on the evolution of instrumental techniques, which permitted the elucidation of the esterification mechanisms and consequently, the study of transesterification. Homogeneous and heterogeneous processes were studied focusing on basic, acid, and enzymatic catalysis [13, 15, 16, 22-36], Usually,... 

---