Carboxylate anion, conjugation

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... 

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

The result of this inductive effect is that the electron density on the carboxylate anion is reduced, the negative charge is distributed over more atoms, and the chloroacetate anion is stabilized relative to acetate. Because the chloroacetate anion is more stable than the acetate ion, its conjugate acid, chloroacetic acid, is a stronger acid than the conjugate acid of the acetate ion, acetic acid (Table 3.1). 

The enolization constants of carboxylic acids to form enediols are generally still lower than those of ketones. The pAE of acetic acid is about 20.35 Due to the relatively high acidity of 1,1-enediols, the enol content of carboxylate anions is somewhat higher. When the carboxylate is attached to cyclopenta-dienyl, a strong mesomeric electron acceptor, the conjugate acid of the enol, fulvene-l,l-diol, becomes a strong acid, pAa = 1.3, and the pAE of the enol anion is reduced to 5.0 (Almstead JIK and Wirz J, Unpublished data).36,37... 

One important reaction of carboxylic acids is their ability to act as acids. Carboxylic acids are medium-strong acids, with pKa values typically between 4 and 5. The conjugate bases of carboxylic acids are named by adding the suffix -oate to the parent name (Figure 11.47). One application of carboxylate anions is to serve as biologically inert anions for positively charged medications. 

SDS has several advantages over soap. Unlike the carboxylate anions in soap, the dodecylsulfate anion does not form insoluble precipitates with hard water ions (e.g., Ca2+), leading to the formation of soap scum. Also, the dodecylsulfate anion is the conjugate base of a very strong acid (H2S04). Therefore, SDS is a weak base, and solutions of SDS are pH neutral. 

The cleaning action of soap is due to the dual nature of the conjugate base of the fatty acid molecule. On one end is the ionic carboxylate anion group the rest of the molecule consists of a nonpolar hydrocarbon chain. The tonic part, called the head is attracted to a polar solvent such as water and is hydrophilic, whereas the long hydrocarbon tail is hydrophobic. In water, soap molecules tend to group together in clusters called micelles, with their ionic heads oriented toward the water molecules and their hydrocarbon tails in the interior of the cluster so that unfavorable interactions with water are avoided. Non-... 

Transfer of a hydride from the dianion to the second aldehyde molecule leads to the carboxylate anion and the conjugate base of the alcohol, which then obtains a proton from the solvent. Although the dianion is less stable than the initial anion, it is a stronger hydride donor because of its two negative charges. 

On the other hand, for groups such as the carboxyl, with a neutral acid form and an anionic conjugate base, a represents the fraction uncharged. The fractional charge is then ... 

Carboxylic acids and their conjugate bases, the carboxylate anions, feature extensively in homogeneous catalysis systems.1"7 In addition, the carboxylate anions are very versatile ligands capable of coordinating in unidentate,... 

An electron-donor group destabilizes a conjugate base by donating electron density onto a negatively charged carboxylate anion. A benzoic acid substituted by an electron-donor group has a higher pATa than benzoic acid (pATa = 4.2). 

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