Chemistry
Reactions of Esters
The reactions of esters involve processes such as hydrolysis, which breaks esters into alcohols and carboxylic acids, and transesterification, where the ester functional group is exchanged with another alcohol. Additionally, esters can undergo reduction to form alcohols or undergo nucleophilic acyl substitution reactions. These reactions are important in organic synthesis and the production of various compounds.
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3 Key excerpts on "Reactions of Esters"
- Kenneth Williamson, Katherine Masters(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 519 Chapter 40 ■ Esterification and Hydrolysis In the latter reaction, an organic base such as pyridine is usually added to react with the hydrogen chloride. Other methods can also be used to synthesize the ester group. Among these are the addition of 2-methylpropene to an acid to form t -butyl esters, the addition of ketene to make acetates, and the reaction of a silver salt with an alkyl halide. 3-Methylbutyl acetate As noted previously, Fischer esterification is an equilibrium process. Consider the reaction of acetic acid with 1-butanol to give n -butyl acetate. The equilibrium constant is as follows: K For primary alcohols reacting with unhindered carboxylic acids, K eq < 4. If equal quantities of 1-butanol and acetic acid are allowed to react at equilibrium, the theoretical yield of ester is only 67%. To upset the equilibrium we can, by Le Châtelier’s principle, increase the concentration of either the alcohol or acid. If either one is doubled, the theoretical yield increases to 85%. When one is tripled, the yield goes to 90%. But note that in the example cited, the boiling point of the relatively nonpolar ester is only about 8°C higher than the boiling points of the polar acetic acid and 1-butanol, so a difficult separation problem exists if the product must be isolated by distillation after the starting materials are increased in concentration. Other ester syntheses Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
eBook - PDFCarboxylic Ortho Acid Derivatives: Preparation and Synthetic Applications
Preparation and Synthetic Applications
- Robert H. Dewolfe(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
C H A P T E R 2 Reactions of Ortho Esters Which Result in Carbon-Oxygen and Carbon-Halogen Bond Formation In this chapter are considered reactions of trialkyl orthocarboxylates and tetraalkyl orthocarbonates which result in the formation of new carbon-oxygen or carbon-halogen bonds. In most of these reactions the ortho ester (or an alkoxycarbonium ion derived from it) functions as an alkylating agent. Many of these reactions are useful synthetic tools. These include formation and hydrolysis of cyclic ortho ester derivatives of diols and triols (which serve to protect the hydroxyl groups of the polyols, and provide means for their mild monoacylation), alkylation of phenols, enols and acids (to form ethers and esters), and conversion of carbonyl compounds to acetals and ketals. Other reactions, such as the alkylation of halogens by ortho esters, are of only limited synthetic utility, while still others, such as the acid-catalyzed hydrolyses of ortho esters, are of interest primarily because of the light they shed on the mechanisms of reactions of ortho esters with nucleo-philic reagents. I. HYDROLYSIS OF CARBOXYLIC ORTHO ESTERS Carboxylic ortho esters are inert, or nearly so, to aqueous alkali. Under acidic conditions, however, they hydrolyze rapidly to alcohols and ordinary 134 HYDROLYSIS OF CARBOXYLIC ORTHO ESTERS 135 carboxylate esters [Eq. (1)]. They are more reactive toward acid hydrolysis than almost any other class of compounds. RC(OR) 3 + H 2 0 -5^> RC0 2 R' + 2 R'OH (1) The high hydrolytic reactivity of orthocarboxylates complicates their synthesis and storage. To prevent hydrolysis, ortho esters must be isolated under alkaline or anhydrous conditions. In spite of reasonable care to exclude atmospheric moisture, ortho ester samples which have been stored for any length of time are usually contaminated by hydrolysis products.
eBook - PDFBiotransfrmtns Prepartv Organic Chemistry
The Use of Isolated Enzymes and Whole Cell Systems in Synthesis
- H. G. Davies, Ralph Green, D. R. Kelly, Stanley M. Roberts(Authors)
- 2012(Publication Date)
- Academic Press(Publisher)
—2— Hydrolysis and Condensation Reactions In this Section the enzyme catalysed hydrolysis Reactions of Esters, amides, epoxides and nitriles are reviewed. The formation of esters and amides using enzymes is also discussed. 2.1. CLEAVAGE AND FORMATION OF CARBOXYLIC ACID ESTER BONDS 2.1.1. Esterases and Lipases Of the wide range of esterases available commercially, very few have been widely utilized in organic transformations. The most commonly used ester-ases have been pig liver esterase (E.C. 3.1.1.1), porcine pancreatic lipase (E.C. 3.1.1.3) and -chymotrypsin (E.C. 3.4.21.1); others, such as the lipase from the yeast Candida cylindracea, are gaining popularity. Various micro-organisms have been employed for certain hydrolyses and these are included in the following discussion where they perform similar reactions but in better yield, or where they afford better enantiomeric excesses than the reactions catalysed by isolated, partially purified, esterases or lipases. Generally, esterases and lipases have been used for two basic transform-ations. (i) Cleavage of a racemic ester to afford an optically active ester and an optically active acid. Thus by chemical hydrolysis of the resulting optically active ester both the (R) and the (S) acids may be obtained. This method has been utilized to provide starting materials for elegant syntheses of many natural products. (ii) Removal of the acyl group from a racemic acylate to produce an optically active alcohol. Similarly, the recovered acylate may then be chemically hydrolysed to the chiral alcohol, thereby enabling both optically active alcohols to be available for further synthesis. In addition, pro-chiral diesters have been hydrolysed to give high yields of optically active mono-esters. Increasingly, enzymes are becoming used as 25 26 2. HYDROLYSIS AND CONDENSATION REACTIONS catalysts in esterification and transesterification.
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