Carboxylate anion formation

The pKa of the protonation of the nitrogen in position 8 has been reported as 6.02 and the pKa for the carboxylate anion formulation has been reported as -0.94. These were determined by Staroscik and Sulkowska by a spectrophotometric method.(14) Further study by the same workers on the partition equilibria of nalidixic acid between water and various organic solvents led to calculations of the pKa values of 5.99 + 0.03 for N-protonation and -0.86 4- 0.07 for carboxylate anion formation.(12) Takasugi and co-workers reported the apparent pKa of nalidixic acid to be 5.9 at 28° by a spectrophotometric method.(13)... 

Carboxylate anions are better nucleophiles for allylation. The monoepoxide of cyclopentadiene 343 is attacked by AcOH regio- and stereoselectively via tt-aliylpalladium complex formation to give the m-3,5-disubstituted cyclopen-tene 344[212]. The attacks of both the Pd and the acetoxy anion proceed by inversion (overall retention) to give the cis product. 

In base the tetrahedral intermediate is formed m a manner analogous to that pro posed for ester saponification Steps 1 and 2 m Figure 20 8 show the formation of the tetrahedral intermediate m the basic hydrolysis of amides In step 3 the basic ammo group of the tetrahedral intermediate abstracts a proton from water and m step 4 the derived ammonium ion dissociates Conversion of the carboxylic acid to its corresponding carboxylate anion m step 5 completes the process and renders the overall reaction irreversible... 

The carbon-carbon bond forming potential inherent m the Claisen and Dieckmann reac tions has been extensively exploited m organic synthesis Subsequent transformations of the p keto ester products permit the synthesis of other functional groups One of these transformations converts p keto esters to ketones it is based on the fact that p keto acids (not esters ) undergo decarboxylation readily (Section 19 17) Indeed p keto acids and their corresponding carboxylate anions as well lose carbon dioxide so easily that they tend to decarboxylate under the conditions of their formation... 

Ba.sic Hydrolysis. Throughout most of history, soap was manufactured by boiling an ester with aqueous alkaU. In this reaction, known as saponification, the ester is hydroly2ed with a stoichiometric amount of alkaU. The irreversible formation of carboxylate anion drives the reaction to completion. 

Finally a proton transfer leads to formation of carboxylate anion 3. Of particular interest is the benzilic acid rearrangement of cyclic diketones such as 4, since it... 

First, the acid anhydride is produced by the reaction of the free acid with DCC. NucleophiUc attack by 4-pyrroUdinonepyridine on the anhydride results in the corresponding, highly reactive acylpyridinium carboxylate this leads to the formation of cellulose ester, plus a carboxylate anion. The latter imdergoes a DCC-mediated condensation with a fresh molecule of acid to produce another molecule of anhydride. N,N-carbonyldiimidazole (CDl) may substitute DCC for acid activation, the intermediate being N-acyhmidazol,... 

A carboxylic acid can be represented as R — CO2 H. Many different carboxylic acids participate in organic chemistry and biochemishy. Although carboxylic acids react in many different ways, breaking the C—OH bond is the only reaction that is important in polymer formation. A carboxylic acid is highly polar and can give up H to form a carboxylate anion, R — CO2. The carboxyl group (— CO2 H) also forms hydrogen bonds readily. These properties enhance the solubility of carboxylic acids in water, a particularly important property for biochemical macromolecules. 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

Although the reaction could proceed via intermediate 14 or 15, the authors favour a mechanism where the formation of 14 is rate-determining because the displacement of the acetate at Pb by carboxylate anions is known to be rapid. The large negative AS (—34 e.u./mol) observed for the oxidation reaction is consistent with formation of the pseudo-cyclic intermediate 14. Also, the small Hammett p value of 0.4 determined for a series of meta- and para-substituted mandelic acids indicates that there is very little charge development on the benzyl carbon in the transition state of the rate-determining step. This is also consistent with the proposed mechanism. 

In the case of benzal chloride, the carboxylation in conventional diaphragm systems fails, leading to poor yields in phenylacetic and mandelic add [180], At an Al anode, carboxylation occurs because the self-esterification of the first carboxylate anion onto the second chloride group is hindered by the formation of Al complex salts [181]. Yields of phenylmalonic and chlorophenylic acetic acids up to 30% each have been obtained [178],... 

Fersht and Kirby, 1968b. Formation of the anhydride is rate determining here. The reference reaction is that of phenyl acetate with a carboxylate anion of pK, 3.11 (see note a) y Kemp and Thibault, 1968. The reference reaction is that of RCOO- with phenyl benzoate (see note j)... 

FIGURE 8.21 Proposed mechanism for the BOP-Cl-mediated reaction of a carboxylate anion with a methylamino group. Formation of a mixed anhydride is followed by aminolysis that is facilitated by anchimeric assistance provided by the oxygen atom of the ring carbonyl.101 (van der Auwera Anteunis, 1987). BOP-C1 = fcw(2-oxo-3-oxazolidino)phosphinic chloride. 

Using two-dimensional NMR spectroscopy, the spatial location of various carboxylate anions relative to the polyene chain of the protonated Schiff base of all-fraws-retinal was determined. The observed intermolecular NOE cross-peaks between a proton on the counterion and a proton near the nitrogen atom indicate the existence of ion-pair formation between the protonated retinal Schiff base and various counterions in chloroform. The results suggest that the most likely site of the carboxylate group of the counterion is in the immediate vicinity of the positively charged nitrogen atom of the retinal Schiff base. 

In spite of the general lack of detailed understanding of mechanism, the procedure is superior to that using the cobalt catalyst both in the overall yields and in the specificity of the reaction to produce only mono-carbonylation products. Prolonged reaction times may lead, however, to the formation of benzyl esters of the acids, as a result of a catalysed reaction of the halide with the carboxylate anion. 

In a slightly less convenient procedure, but one which has general versatility, carbonylation of aryl (or vinyl) palladium compounds produces aryl, heteroaryl, and vinyl carboxylic acids. As with the other procedures, immediate upon its formation, the carboxylate anion migrates to the aqueous phase. Consequently, haloaromatic acids can be obtained from dihaloarenes, without further reaction of the second halogen atom, e.g. 1,4-dibromobenzene has been carbonylated (90% conversion) to yield 4-bromobenzoic acid with a selectivity for the monocarbonylation product of 95%. Additionally, the process is economically attractive, as the organic phase containing the catalyst can be cycled with virtually no loss of activity and ca. 4000 moles of acid can be produced for each mole of the palladium complex used [4],... 

---