Carboxylic acids acid strength

In Chapter 8 we saw how increasing the number of electronegative substituents on a carboxylic acid decreased the acid s pKa, that is, increased its acidity. Acid strength is a measure of the equilibrium constant for this simple reaction. 

Perfluorinated carboxylic acids are corrosive liquids or solids. The acids are completely ionized in water. The acids are of commercial significance because of their unusual acid strength, chemical stabiUty, high surface activity, and salt solubiUty characteristics. The perfluoroaLkyl acids with six carbons or less are hquids the higher analogues are soHds (Table 1). 

Equation 20 is the rate-controlling step. The reaction rate of the hydrophobes decreases in the order primary alcohols > phenols > carboxylic acids (84). With alkylphenols and carboxylates, buildup of polyadducts begins after the starting material has been completely converted to the monoadduct, reflecting the increased acid strengths of these hydrophobes over the alcohols. Polymerization continues until all ethylene oxide has reacted. Beyond formation of the monoadduct, reactivity is essentially independent of chain length. The effectiveness of ethoxylation catalysts increases with base strength. In practice, ratios of 0.005—0.05 1 mol of NaOH, KOH, or NaOCH to alcohol are frequendy used. 

Indolmycin, biosynthesis of, 864 Inductive effect. 37, 562 alcohol acidity and. 604 carboxylic acid strength and. 758 electronegativity and, 37 electrophilic aromatic substitution and, 562... 

Bifunctional catalysis in nucleophilic aromatic substitution was first observed by Bitter and Zollinger34, who studied the reaction of cyanuric chloride with aniline in benzene. This reaction was not accelerated by phenols or y-pyridone but was catalyzed by triethylamine and pyridine and by bifunctional catalysts such as a-pyridone and carboxylic acids. The carboxylic acids did not function as purely electrophilic reagents, since there was no relationship between catalytic efficiency and acid strength, acetic acid being more effective than chloracetic acid, which in turn was a more efficient catalyst than trichloroacetic acid. For catalysis by the carboxylic acids Bitter and Zollinger proposed the transition state depicted by H. 

In general acid catalysis, the rate is increased not only by an increase in [SH ] but also by an increase in the concentration of other acids (e.g., in water by phenols or carboxylic acids). These other acids increase the rate even when [SH ] is held constant. In this type of catalysis the strongest acids catalyze best, so that, in the example given, an increase in the phenol concentration catalyzes the reaction much less than a similar increase in [H30 ]. This relationship between acid strength of the catalyst and its catalytic ability can be expressed by the Breasted catalysis equation ... 

The acid strengths of a series of phosphonic acid derivatives in a variety of solvents have also been used to estimate Hammett constants. In contrast to carboxylic acids, the phosphonic acids are stronger in ketonic solvents than in hydroxylic solvents, which may be attributed to the dissociation of phosphonic acids without the necessity to disrupt the dimeric nature of the acid (see Scheme 3). 

The catalytic constants measured in 95% aqueous dioxan have been compared with piT-values in water. The twenty-four acids referred to in Table 3 are mainly carboxylic acids, but also include nitric acid, o-chloro-phenol and water. Two oximes show large positive deviations, and saccharin has considerably less catalytic activity than anticipated these substances have not been included in the correlation. A number of strong acids gave closely similar catalytic constants— HCl (3-05), HBr (2-30), CoHb.SOsH (2-30), MeSOsH (2-15), HCIO (1-25)—and the minor variations within this series are not in the expected order of acid strengths HCIO4 > HBr > HCl > ObHb. SO3H > MeSOsH. Presumably all these acids are converted in solution to the hydronium ion, the catalytic power of which is somewhat modified by ion-pairing with different anions in the solvent of low dielectric constant. The catalytic constants observed are consistent with the conventional value pJT = —1-74 for H36+. 

The acid strengths of the quinoxaline monocarboxylic acids can be correlated with the electron densities at the carbon atoms attached to the carboxyl group. Quinoxaline 2-carboxylic acid has a pAa of 2.88, compared with 4.03 and 3.64 for the 5- and 6-carboxylic acids. The calculated 7r-electron densities at C-2, C-5 and C-6 in the parent heterocycle are 0.866, 0.938, and 0.933, and thus the carboxyl group attached to C-2, the ring carbon that carries least electronic charge, is the most easily ionized.246... 

The apparent acid strength of boric acid is increased both by strong electrolytes that modify the structure and activity of the solvent water and by reagents that form complexes with B(OH) 4 and/or polyborate anions. More than one mechanism may be operative when salts of metal ions are involved. In the presence of excess calcium chloride the strength of boric acid becomes comparable to that of carboxylic acids, and such solutions may be titrated using strong base to a sharp phenolphthalein endpoint. Normally titrations of boric acid are carried out following addition of mannitol or sorbitol, which form stable chelate complexes with B(OH) 4 in a manner typical of polyhydroxy compounds. Equilibria of the type ... 

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