Carboxylate anions water

This difference in behavior for acetic acid in pure water versus water buffered at pH = 7 0 has some important practical consequences Biochemists usually do not talk about acetic acid (or lactic acid or salicylic acid etc) They talk about acetate (and lac tate and salicylate) Why Its because biochemists are concerned with carboxylic acids as they exist in di lute aqueous solution at what is called biological pH Biological fluids are naturally buffered The pH of blood for example is maintained at 7 2 and at this pH carboxylic acids are almost entirely converted to their carboxylate anions... 

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... 

Wool is dyed from aqueous solutions. The majority of dyes used on wool are sodium salts of aromatic anions. Water solubiUty is usually provided by sulfonic acid groups, but in a few cases carboxyl or hydrophilic, nonionic substituents are used. 

Examples of polyfunctional carboxylic acids esterified by this method are shown in Table I. Yields are uniformly high, with the exception of those cases (maleic and fumaric acids) where some of the product appears to be lost during work-up as a result of water solubility. Even with carboxylic acids containing a second functional group (e.g., amide, nitrile) which can readily react with the oxonium salt, the more nucleophilic carboxylate anion is preferentially alkylated. The examples described in detail above illustrate the esterification of an acid containing a labile acetoxy group, which would not survive other procedures such as the traditional Fischer esterification. 

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. 

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). 

Montmorillonite is electronegatively charged (105 5 meq./ lOOg) in water and the carboxylate anions in CMHEC are repelled... 

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)... 

The rate of decarboxylation of activated carboxylate anions [e.g. (10)], shows strong solvent dependence. It is not surprising, therefore, that these reactions have been used to probe the microsolvent effects of micelles and CDs (Fendler and Fendler, 1975). In particular, it was anticipated that complexation with a CD might result in catalysis by providing an environment for the reaction that is less polar than water. 

A water-soluble dendrimer zinc porphyrin 6a, having 32 carboxylate anion... 

Reaction of the carbonium ion with water could be reduced if overlap occurred with the carboxylate anion of aspartic acid-52 either during or after the glycoside-cleavage step. Since the carboxylate anion would be held adjacent to the carbonium ion in the active site, equilibrium should be far to the side of the acylal. Reaction of acylal with H2O would then very probably be ratedetermining in the forward direction. Evidence has been obtained that the solvent is directly involved in the hydrolysis of the cyclic acylal 2-(p-nitrophenoxy)phthalide where steric factors are similar... 

In contrast to redox reactions, only proton transfer takes place in acid-base reactions (see also p.30). When an acid dissociates (1), water serves as a proton acceptor (i. e., as a base). Conversely, water has the function of an acid in the protonation of a carboxylate anion (2). 

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. Because each of these nucleophiles can also act as a base, reaction conditions must be selected to favor substitution over elimination. 

In comparison, photodecarboxylation of various other carboxylic acids has been studied extensively. For example, photodecarboxylation of 23 in water presumably involves electron transfer from the carboxylate anion to the phenyl ring (Scheme 16). The electron transfer is followed by decarboxylation to form the anion 24, which is protonated by the solvent. As shown in Scheme 16, in less polar aprotic solvents, homolytic cleavage leads to decarboxylation subsequent to charge transfer in 23. 

In a related study (78CC934,79HCA2763) Lehn and coworkers incorporated guanidinium groups into macrocycles (e.g. 29). The resulting polyguanidium salts form stable complexes in water with phosphate and carboxylate anions. Unlike polyammonium anion receptors, these species remain protonated over a wide pH range and hold considerable potential in the development of anion-selective electrodes. 

---