Carbonic acid, dehydration

B. Kinetics of C02 Hydration and Carbonic Acid Dehydration The Chemistry of Metal Carbonates... 

The final oxidation products of organic compounds are invariably acetic and carbonic acids, which release protons. Biological fluids must, however, maintain a constant pH of 7.4. A few milligrams of burned food would lower the pH of the body fluids of a 70 kg man to 6, and if the acids were released into the blood the resultant changed protein conformations and subsequent swelling would be fatal. This does not happen because the acetic acid is immediately es-terified and the major oxidation product—carbonic acid—dehydrates spontaneously in an aqueous environment to form the gas carbon dioxide. This is breathed out into the atmosphere, and no protons are released in the body. 

Figure 6-9. The Bohr effect. Carbon dioxide generated in peripheral tissues combines with water to form carbonic acid, which dissociates into protons and bicarbonate ions. Deoxyhemoglobin acts as a buffer by binding protons and delivering them to the lungs. In the lungs, the uptake of oxygen by hemoglobin releases protons that combine with bicarbonate ion, forming carbonic acid, which when dehydrated by carbonic anhydrase becomes carbon dioxide, which then is exhaled.

However, this simple picture only applies to gases that do not undergo reactions in the boundary layers. For gases that do react, for example through hydration and acid-base reactions, the net flux depends on the simultaneous movement of all the solutes involved, and the flux will not be the simple function of concentration expressed in Equation (3.25). An example is CO2, which reacts with water to form carbonic acid and carbonate species-H2C03, HCOs and COs . The situation is complicated because the exchange of H+ ions in the carbonate equilibria results in a pH gradient across the still layer, and it is therefore necessary to account for the movement of H+ ions across the still layer as well as the movement of carbonate species. The situation is further complicated in the case of CO2 by the kinetics of hydration and dehydration, which may be slow in comparison with transport. 

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

The hydration rate constant of C02, the dehydration rate constant of carbonic acid (H2C03), and p pK2 values (pTf, =6.03, pTf2 = 9.8 at 25 °C, 7=0.5 M) (63) are such that nearly 99% of dissolved carbon dioxide in water at pH < 4 exists as C02. However, these four different species may be considered as the reactive species under different pH conditions which can react with aqua metal ions or their hydroxide analogues to generate the metal carbonato complexes. The metal bound aqua ligand is a substantially stronger acid than bulk H20 ( )K= 15.7). Typical value of the p of H20 bound to a metal ion may be taken to be 7. Hence the substantial fraction of such an aqua metal ion will exist as M-OH(aq)(ra 1) + species at nearly neutral pH in aqueous medium. A major reaction for the formation of carbonato complex, therefore, will involve pH controlled C02 uptake by the M-OH(" 1)+ as given in Eq. (17). 

Applications of the bipolar pulse technique have demonstrated its utility in a variety of experiments, but it is particularly useful in monitoring reaction kinetics [18]. The technique has been shown to be useful on the stopped-flow time scale by the investigation of the dehydration of carbonic acid [20]. The study of this widely used text reaction demonstrates the accuracy and precision of the method. A sample data set from a single experiment is shown in Figure 8.16, and the excellent precision obtainable in such experiments is evident. The... 

Figure 8.16 Reaction curve for the dehydration of carbonic acid by conductometric detection. Lower dots, every tenth experimental point solid line, fitted exponential curve. Middle dots, logarithmic plot, every tenth point solid line, least-squares line. Upper residuals (G - Gcajcd). [From Ref. 20, reprinted with permission. Copyright 1978 American Chemical Society.]...

An interesting example of kinks in kinetic data is obtained from Scheurer and co-workers (133), who studied the dehydration of carbonic acid. Figure 6 is a plot of the log (dehydration rate) vs. reciprocal of absolute temperature. Note the relatively abrupt change near 31 °C. 

Figure 6. Rate of dehydration of carbonic acid from data of Scheurer...

Dehydrations produce olehns from alcohols by the acid-catalyzed elimination of a water molecule from between two carbons. Acid-catalyzed dehydrations often give mixtures of products because the intermediate carbocation is prone to cationic rearrangements to more stable carbocations prior to formation of the olefin product. Moreover, even when the intermediate carbocation is not subject to skeletal rearrangement, as in file case of tertiary alcohols, mixtures of regioisomers are often produced during file loss of a proton from file carbocation. As a consequence, the acid-catalyzed dehydration of alcohols is generally not a viable synthetic method. 

NCAs are well known forms of a-amino acids which present the amino group protected and the carboxylic group activated for a peptide coupling step. The most common access to NCAs is based on dehydration procedures of a-amino acids involving carbonic acid equivalents, which has limited the method to proteinogenic... 

As the last example of an SN reaction at the carboxyl carbon of a carbonic acid derivative, consider the synthesis of dicyclohexylurea in Figure 6.39. In this synthesis, two equivalents of cyclohexylamine replace the two methoxy groups of dimethyl carbonate. Dicyclohexylurea can be converted into the carbodiimide dicyclohexylcarbodiimide (DCC) by treatment with tosyl chloride and triethylamine. The urea is dehydrated. The mechanism of this reaction is identical to the mechanism that is presented in Figure 8.9 for the similar preparation of a different carbodiimide. 

Compound produced by dehydration of a carbonic acid general formula is R--C--O-C-R Acidic Salt... 

As in all of these more complex syntheses, other routes to the target compound are possible. This route was chosen because the Grignard reaction introduces a double bond without removing functionality at carbon 3. Dehydration occurs in the desired direction to produce a double bond conjugated with the carboxylic acid carbonyl group. 

Furan is synthesized by l< ss of carbon mono3cide <acidic dehydration of the penlotie sugars found in oat liulls and comcoba. 

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