Carboxylate anion, reactions

This IS because carboxylic acids are converted to their corresponding carboxylate anions which are stable under the reaction conditions... 

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... 

In most biochemical reactions the pH of the medium is close to 7 At this pH car boxylic acids are nearly completely converted to their conjugate bases Thus it is common practice m biological chemistry to specify the derived carboxylate anion rather than the carboxylic acid itself For example we say that glycolysis leads to lactate by way of pyruvate... 

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. 

The carbon-car bon bond-fonrring potential inherent in the Claisen and Dieckmann reactions has been extensively exploited in organic synthesis. Subsequent transfonnations 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- conesponding carboxylate anions as well, lose carbon dioxide so easily that they tend to decarboxylate under the conditions of their formation. 

Carboxyl-related and acyl substituents. Included here are cyano, protonated amidinium ion, thionoacyl, acyl (Ar—CO, H—CO, Alkyl—CO), carboxamido, carboaryloxy, carboalkoxy, carboxy (unionized), amidino (unionized), and carboxylate anion, listed approximately in order of decreasing electron attraction or activation. The relative activation by some of these groups (e.g., ketone, aldehyde, nitrile) will change upon reversible interaction with the nucleophile, which will vary with the group and with the nucleophile (e.g., MeO , N3, NCS ). Irreversible interaction will be obvious when the reaction products in kinetic studies are characterized. 

Methyl ketones 1, as well as acetaldehyde, are cleaved into a carboxylate anion 2 and a trihalomethane 3 (a haloform) by the Haloform reaction The respective halogen can be chlorine, bromine or iodine. 

Perhaps the most useful reaction of carboxylic acids is their conversion into esters. There are many methods for accomplishing the transformation, including the S -2 reaction of a carboxylate anion with a primary alkyl halide that we saw in Section 11.3. 

Conversion of Acid Halides into Anhydrides Nucleophilic acyl substitution reaction of an acid chloride with a carboxylate anion gives an acid anhydride. Both symmetrical and unsymmetrical acid anhydrides can be prepared in this way. 

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,... 

Heteroarylphenylalanines could be smoothly obtained via microwave-promoted Suzuki reaction of heteroaryl halides with 2-amino-3-[4-(dihy-droxyboryl)phenyl]propanoic acid (Scheme 28) [46]. Interestingly, the free amino acid could be used without any protection of the amine and carboxylic acid fimctionahty. When 4-(dihydroxyboryl)-L-phenylalanine was used as organometallic partner no racemization was observed. The carboxylate anion and free amino group seem to shield the a-C - H from deprotonation and thus hmit racemization. 

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... 

The oxygen nucleophiles that are of primary interest in synthesis are the hydroxide ion (or water), alkoxide ions, and carboxylate anions, which lead, respectively, to alcohols, ethers, and esters. Since each of these nucleophiles can also act as a base, reaction conditions are selected to favor substitution over elimination. Usually, a given alcohol is more easily obtained than the corresponding halide so the halide-to-alcohol transformation is not used extensively for synthesis. The hydrolysis of benzyl halides to the corresponding alcohols proceeds in good yield. This can be a useful synthetic transformation because benzyl halides are available either by side chain halogenation or by the chloromethylation reaction (Section 11.1.3). 

Two methods for converting carboxylic acids to esters fall into the mechanistic group under discussion the reaction of carboxylic acids with diazo compounds, especially diazomethane and alkylation of carboxylate anions by halides or sulfonates. The esterification of carboxylic acids with diazomethane is a very fast and clean reaction.41 The alkylating agent is the extremely reactive methyldiazonium ion, which is generated by proton transfer from the carboxylic acid to diazomethane. The collapse of the resulting ion pair with loss of nitrogen is extremely rapid. 

Especially for large-scale work, esters may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl halides or tosylates. Carboxylate anions are not very reactive nucleophiles so the best results are obtained in polar aprotic solvents45 or with crown ether catalysts.46 The reactivity order for carboxylate salts is Na+ < K+ < Rb+ < Cs+. Cesium carboxylates are especially useful in polar aprotic solvents. The enhanced reactivity of the cesium salts is due to both high solubility and minimal ion pairing with the anion 47 Acetone is a good solvent for reaction of carboxylate anions with alkyl iodides48 Cesium fluoride in DMF is another useful... 

Carboxylate anions derived from somewhat stronger acids, such as p-nilrobcnzoic acid and chloroacetic acid, seem to be particularly useful in this Mitsunobu inversion reaction.53 Inversion can also be carried out on sulfonate esters using cesium carboxy-lates and DMAP as a catalyst in toluene.54 The effect of the DMAP seems to involve complexation and solubilization of the cesium salts. 

A variety of reagents could be used to carry out such a conversion (18,19). We chose to react the alkoxide ion with succinic anhydride (SA), because the alkoxide ion could be converted quantitatively to the carboxylate ion when excess of SA is used, and also because no side reactions are reported (19). The carboxylate anion, 3, thus formed was used to polymerize PVL giving the masked poly(oxyethylene)-b-po y(pivalolactone) co-polymeric salt, 4. The salt, 4, was converted to the teiechelomer, 5, by acid hydrolysis.. ... 

The several kinetic rates are a consequence of Reactions 2-5 The first two represent monomeric additions the third describes all rates by which two polymeric molecules smaller than j form a molecule of size j. The molecule A AC contains a carboxylic anion the second molecule P. supplies aS oxirane and, in general, is any polymeric molecule of iSze j-n. 

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