Ester from carboxylate ions

The generation of radicals from carboxylate ions at the anode (Section 2.17.6, p. 115), and their coupling to form new carbon-carbon bonds is illustrated by the synthesis of hexacosane (Expt 5.11). The method has been usefully applied to the preparation of esters of long-chain carboxylic acids, from which of course the free acids may be prepared by hydrolysis. 

Whereas all the alkylations in Figure 2.26 take place in basic or neutral solutions, carboxylic acids can be methylated as such with diazomethane (Figure 2.27). The actual nucleophile (the carboxylate ion) and the actual methylating agent (H3C—N+=N) Fig. 2.27. Preparation of are then produced from the reaction partners by proton transfer, methyl esters from carboxylic acids and di azomethane. 

Animal fats and vegetable oils are triacylglycerols, or triesters, formed from the reaction of glycerol (1,2, 3-propanetriol) with three long-chain fatty acids. One of the methods used to characterize a fat or an oil is a determination of its saponification number. When treated with boiling aqueous KOH, an ester is saponified into the parent alcohol and fatty acids (as carboxylate ions). The saponification number is the number of milligrams of KOH required to saponify 1.000 g of the fat or oil. In a typical analysis, a 2.085-g sample of butter is added to 25.00 ml of 0.5131 M KOH. After saponification is complete, the excess KOH is back titrated with 10.26 ml of0.5000 M HCl. What is the saponification number for this sample of butter ... 

Esters are usually prepared from carboxylic acids by the methods already discussed. Thus, carboxylic acids are converted directly into esters by SK2 reaction of a carboxyfate ion with a primary alkyl halide or by Fischer esterification of a carboxylic acid with an alcohol in the presence of a mineral acid catalyst. In addition, acid chlorides are converted into esters by treatment with an alcohol in the presence of base (Section 21.4). 

Today the coupled product is described as being formed by union of two alkyl radicals fonned by loss of one electron and carbon dioxide from the carboxylate ion. Extensive early use of the Kolbe reaction was made for the synthesis of long chain a,co-dicarboxylate esters starting from the half esters of shorter chain a,03-diacids [49]. 

Esters can undergo hydrolysis using either an acid or a base as a catalyst. Hydrolysis always produces an alcohol from the alkyl portion of the ester. During acid hydrolysis, the acid portion of the ester yields a carboxylic acid. During base hydrolysis of an ester, which is called saponification, the acid portion of the ester yields the carboxylate ion. 

In the reaction, one mole of hydroxide generates one mole of alcohol and one mole of carboxylate ion from one mole of ester. Based on this stoichiometry (the mole relationship as defined by the balanced chemical equation), if the number of moles of base is known, then the amount of ester is known. 

The saponification (the base-catalyzed hydrolysis of an ester) of fats has been important since ancient times. This process frees the glycerin and releases the fatty acids as carboxylate ions. The carboxylate ions, along with sodium or potassium ions from the base, create a soap (refer to Figure 16-28). 

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The differences in alkaline lability of compounds I-IX results from the varying stability of their enolic intermediate. The alkaline stability of compounds I, II, and V results from resonance stabilization of the carboxylate ion. Esters and amides (R2) do not show such resonance,... 

If R2 is an ester or an amide group, the release of electrons from the enolic ion to the glycosyloxy linkage will be favored also, resonance stabilization of the carboxylate ion would be eliminated. 

On the pages which follow, general methods are illustrated for the synthesis of a wide variety of classes of organic compounds including acyl isocyanates (from amides and oxalyl chloride p. 16), epoxides (from reductive coupling of aromatic aldehydes by hexamethylphosphorous triamide p. 31), a-fluoro acids (from 1-alkenes p. 37), 0-lactams (from olefins and chlorosulfonyl isocyanate p. 51), 1 y3,5-triketones (from dianions of 1,3-diketones and esters p. 57), sulfinate esters (from disulfides, alcohols, and lead tetraacetate p. 62), carboxylic acids (from carbonylation of alcohols or olefins via carbonium-ion intermediates p. 72), sulfoxides (from sulfides and sodium periodate p. 78), carbazoles... 

A second method to efficientiy produce mediyl esters of carboxylic acids is to heat die acid with potassium carbonate and mediyl iodide. The mediyl ester is produced under mild conditions and is easily separated from die reaction byproducts. This method is somewhat different in tiiat die ester is formed by a nucleophilic displacement of iodide by die carboxylate ion. Normally carboxy-lates are not thought of as good nucleophiles—and tiiey are not—but mediyl iodide is a quite reactive electrophile which matches die poor nucleophilicity of die carboxylate satisfactorily. 

Carboxylic esters. The molecular ion of the ester 1R-C022R is usually observed in those cases where the alkyl group, 2R, is smaller than C4. The characteristic ions in the spectrum arise from McLafferty rearrangements, which can occur with either the acyl- or alkoxy-alkyl group, providing they are at least three or two carbon atoms long respectively. 

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

Soldenhoff, K. H. (1987) Solvent-Extraction of Copper(Ii) from Chloride Solutions by Some Pyridine Carboxylate Esters, Solvent Extr. Ion Exch., 5(5), 833-851. 

Carboxylic amides, carboxylic esters, and carboxylic acids react with acid-stable heteroatom nucleophiles in a neutral solution much more slowly via the mechanism of Figure 6.2 than in an acidic solution via the mechanism of Figure 6.5. In an acidic solution, their car-boxonium ion derivatives, which result from the reversible protonation of the carboxyl oxygen, act as precursors of the tetrahedral intermediate. According to the discussion earlier in... 

You might indeed have guessed from our previous example, the hydrolysis of esters, where the transition states for formation and breakdown of the tetrahedral intermediate had about the same energies, that in the hydrolysis of amide the second step becomes rate-determining. This offers the opportunity for further base catalysis. If a second hydroxide ion removes the proton from the tetrahedral intermediate, the loss of NH j is made easier and the product is the more stable carboxylate ion. 

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