Carboxylic acids from carboxylate ions

A very different situation exists m an aqueous solu tion maintained at pH = 7 0 from the situation m pure water We saw earlier that almost all the acetic acid m a 0 1 M solution m pure water was nonion ized At pH 7 0 however hardly any nonionized acetic acid remains it is almost completely converted to its carboxylate ion... 

FIGURE 19 6 Space filling model of a micelle formed by association of car boxylate ions derived from a long chain carboxylic acid The hydrocarbon chains tend to be on the inside and the carboxylate ions on the surface where they are in contact with water mole cules and metal cations... 

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 ... 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

Alkoxide ion abstracts the acidic proton from the carboxylic acid and yields a carboxylate ion. 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

The reaction between carbonium ions and carbon monoxide affording oxocarbonium ions (acyl cations) is a key step in the well-known Koch reaction for making carboxylic acids from alkenes, carbon monoxide and water ... 

Many other examples of synthetic equivalent groups have been developed. For example, in Chapter 6 we discussed the use of diene and dienophiles with masked functionality in the Diels-Alder reaction. It should be recognized that there is no absolute difference between what is termed a reagent and a synthetic equivalent group. For example, we think of potassium cyanide as a reagent, but the cyanide ion is a nucleophilic equivalent of a carboxy group. This reactivity is evident in the classical preparation of carboxylic acids from alkyl halides via nitrile intermediates. 

A two-step mechanism must be assumed for this very valuable reaction of carboxylic acids with CDI.[9] Obviously the first step is a nucleophilic attack of the carboxylic acid or —depending on the acidity —the carboxylate ion on the carbonyl group of CDI, leading after elimination of imidazole to a mixed anhydride of imidazole-iV-carboxylic acid and the attacking carboxylic acid. This intermediate must have a very short life-time since it has not been detected down to — 50 °C. Rapid cleavage of CO2 from this mixed anhydride involves exclusively the carbonyl group linked to the imidazole unit If... 

Xiao-Hua Yang et al. [ 1 ] determined nanomolar concentrations of individual low molecular weight carboxylic acids (and amines) in seawater. Diffusion of the acids across a hydrophobic membrane was used to concentrate and separate carboxylic acids from inorganic salts and most other organic compounds prior to the application of ion chromatography. Acetic propionic acid, butyric-1 acid, butyric-2 acid, valeric and pyruvic acid, acrylic acid and benzoic acid were all found in reasonable concentrations in seawater. 

The trends in IpK in passing from water to the less polar alcohols can be explained in part by considering the equilibria for carboxylic acid and aminium ion dissociation shown in Equations (4) and (5). Carboxylic acid dissociation creates two opposite charges, while aminium ion dissociation simply relocates (H+) from the amine to the solvent as ROH, which may be... 

Arachidic acid monolayers were prepared from a benzene solution on the water subphase of pH5.8(pure water) and 12.6(adjusted by addition of NaOH) at Tsp of 303 K below Tm(=328 K) of the monolayer [31]. The ionic dissociation state of hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transform-infrared attenuated total reflection, FT-IR ATR measurements. 70 arachidic acid monolayers were transferred on germanium ATR prism, resulting in the formation of the multi-layered film. Transfer on the prism was carried out at surface pressures of 25 or 28 mN-nr1. Infrared absorption measurements revealed that almost carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pH 12.6. 

Formation of 12 tol6-membered lactones in 66 to 75% yield from the corresponding u-bromo carboxylic acids can be carried out by addition of a solution of ex situ formed (33) to a dilute solution of the carboxylic acid at low temperature (—60°G) [116]. A crucial point in the selective reaction seems to be R4N+, which stabilizes the carboxylate ion and slows down bimolecular reactions for steric reasons. The yield of the cycKc product increases with increasing size of R [116]. 

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