Carbonic acid, protonated

Carbon dioxide + water = carbonic acid Carbonic acid = proton + bicarbonate anion. 

In mixed solvents formed by two aliphatic carbonic acids, proton transfer does not take place and interaction is usually limited to a mixed associate formation, according to the equilibrium [9.2]. 

Protonated and diprotonated carbonic acid and carbon dioxide may also have implications in biological carboxylation processes. Although behavior in highly acidic solvent systems cannot be extrapolated to in vivo conditions, related multidentate interactions at enzymatic sites are possible. 

Our experience to this point has been that C—H bonds are not very acidic Com pared with most hydrocarbons however aldehydes and ketones have relatively acidic protons on their a carbon atoms pA s for enolate formation from simple aldehydes and ketones are m the 16 to 20 range... 

In all cases, water and carbonic acid, the latter of which is the source of protons, are the main reactants. The net result of the reaction is the release of cations (Ca " ), Mg ", K", Na" ) and the production of alkalinity via HCO. When ferrous iron is present in the lattice, as in biotite, oxygen consumption may become an important factor affecting the weathering mechanism and the rate of dissolution. 

The two-proton exchange in pairs of OH O fragments of various carbonic acid dimers... 

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

The enzyme carbonic anhydrase promotes the hydration of COg. Many of the protons formed upon ionization of carbonic acid are picked up by Hb as Og dissociates. The bicarbonate ions are transported with the blood back to the lungs. When Hb becomes oxygenated again in the lungs, H is released and reacts with HCO3 to re-form HgCOj, from which COg is liberated. The COg is then exhaled as a gas. 

This reaction converts ribulose-5-P to another ketose, namely, xylulose-5-P. This reaction also proceeds by an enediol intermediate, but involves an inversion at C-3 (Figure 23.31). In the reaction, an acidic proton located a- to a carbonyl carbon is removed to generate the enediolate, but the proton is added back to the same carbon from the opposite side. Note the distinction in nomenclature here. Interchange of groups on a single carbon is an epimerization, and interchange of groups between carbons is referred to as an isomerization. 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

Loss of an acidic proton from the alpha carbon takes place in ihe normal way to yield an enol intermediate. 

In the first step, the fairly acidic proton on CIO of the red biladiene-ac salt 6 is abstracted and, even in solution in polar solvents, the salts are converted into the corresponding yellow bilatriene-u/ic salts 7. With a base such as piperidine, the salts 7 form the green bilatriene-a/>e free base. For further reaction to the porphyrin it is important that the salts 7 are oxidized to the bilatriene enamines 8 which cyclize via the electrophilic carbon of the terminal pyrrole ring by the loss of the leaving group X to 9. Porphin (10) is finally obtained by the loss of... 

Throughout these sections it has been assumed that protonation and association equilibria are established on time scales much shorter than those for the kinetic steps. For the usual protonations and ion-pairings that assumption will always be true, except when very rapid reactions are being studied by certain techniques presented in Chapter 11. On the other hand, if carbon acids are involved, or any sluggish association reactions, the assumption of rapid prior equilibria may not hold true. 

The principal difference between a polyprotic acid and a monoprotic acid is that a polyprotic acid donates protons in a succession of deprotonation steps. For example, a carbonic acid molecule can lose one proton to form I ICO , and then that ion can donate the remaining proton to form CO, 2. We shall see how to take this succession of deprotonations into account when assessing the pH of the solution of a polyprotic acid or one of its salts. In addition, we shall see how the relative concentrations of the ions in solution, such as P043, HP042, and H,P04 depend on the pH of the solution. 

Carbonic acid is an important natural component of the environment because it is formed whenever carbon dioxide dissolves in lake water or seawater. In fact, the oceans provide one of the critical mechanisms for maintaining a constant concentration of carbon dioxide in the atmosphere. Carbonic acid takes part in two successive proton transfer equilibria ... 

Sometimes we need to know how the concentrations of the ions present in a solution of a polyprotic acid vary with pH. This information is particularly important in the study of natural waters, such as rivers and lakes (Box 10.1). For example, if we were examining carbonic acid in rainwater, then, at low pH (when hydronium ions are abundant), we would expect the fully protonated species (H2C03) to be dominant at high pH (when hydroxide ions are abundant), we expect the fully deprotonated species (C032 ) to be dominant at intermediate pH, we expect the intermediate species (HC03, in this case) to be dominant (Fig. 10.20). We can verify these expectations quantitatively. 

To see how the concentrations of species present in a solution vary with pH, we take the carbonic acid system as an example. Consider the following proton transfer equilibria ... 

FIGURE 10 JO The fractional composition of the species in carbonic acid as a function of pH. Note that the more fully protonated species are dominant at lower pH. 

The bottom portion of Table 8.1 consists of very weak acids pK. above 17). In most of these acids, the proton is lost from a carbon atom, and such acids are known as carbon acids. The pa s values for such weak acids are often difficult to measure and are known only approximately. The methods used to determine the relative positions of these acids are discussed in Chapter 5. The acidity of carbon acids is proportional to the stability of the carbanions that are their conjugate bases (see p. 227). 

Many carbon acids, upon losing the proton, form carbanions that are stabilized by resonance. Structural reorganization (movement of atoms to different... 

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