Ionization of carbon acids

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. 

The partial ionization of carbonic acid produces hydronium ion, H+, driving the indicator equilibrium to the weak acid form. A colorless solution results. As the water in the ink evaporates, the white residue of sodium carbonate remains. 

Rate constants for ionization of carbon acids (chloroform-r and acetophenone-r) in alkaline aqueous sulfolane have been determined and their dependence on solvent composition and temperature has been interpreted for this highly basic medium. °... 

Exercise 18-3 The Ka for the first ionization of carbonic acid, 0=C(0H)2 + H20 0=C(0H)0e + H30 , is about 1000 times smaller than Ka for ethanoic acid. Show how this fact can be rationalized by considering the expected relative stabilization energies of carbonic acid and the hydrogen carbonate ion compared to those of ethanoic acid and ethanoate anion. 

We define the reference state in the usual way, so that y(H2C03) goes to unity as m(H2C03) goes to zero. This definition defines the standard state of the assumed species. The change of state for the first ionization of carbonic acid may then be written as... 

As long as the base is in excess both stages of the ionization of carbonic acid can proceed to completion because of the removal of H+ ions (see Ionization of Polybasic Acids, Chapter III, page 116),... 

Jones, J. R. (1973). The Ionization of Carbon Acids. Academic Press, London and New York. 

Various aspects of proton transfer to and from carbon have been reviewed previously. Bell s classic work [1] covers the whole field of proton transfers and discusses in detail the mechanisms of acid and base catalysed reactions. The structure and stabilization of carbanions has been described [2] and a general review of the ionization of carbon acids has been published [3], The role of proton transfers in the mechanisms of chemical and biological reactions has also been described [4]. 

The amount of unionized carbonic acid in the solution can be calculated in the same way. The equilibrium expression for the ionization of carbonic acid, HoCO, into hydrogen ion and hydrogen carbonate ion is... 

Write the steps in the complete ionization of carbonic acid. 

Rov, R. N. et ul. 1983. The first ionization of carbonic acid in aqueous solutions ol potassium chloride including the activity coefficients of potassium bicarbonate. J. Cheni. Thermo. 15 37-47. 

The nature of the interaction mechanisms that stabilize the carb-anion profoundly influences the rates of reactions that lead to the generation of carbanions. For the most thoroughly studied carbanion-forming reactions, the ionization of carbon acids, this fact has been recognized for a long time and discussed in a number of reviews (1-7). 

Write balanced chemical equations for the two successive ionizations of carbonic acid in water. Identify the conjugate-base pair in each of the equations. 

This sequence of reactions illustrates the buffering of the blood by the carbonic acid-bicarbonate conjugate acid-base system. The protons generated at the tissue level by the ionization of carbonic acid are removed at the lungs when bicarbonate ion reenters the red blood cell and reacts with them to produce carbonic acid. H2CO3 then rapidly dissociates, under the influence of carbonic anhydrase, and the resultant CO2 diffuses into the alveolar space because its concentration in the plasma is higher than in the alveoli. 

I. P. Beletskaya, L. S. German, and V. R. Polishchuk, J. Organometallic Ckem., 1970,25, 11. D. Holtz, Progr. Phys. Org. Chem., 1971, 8,1 (note that the acidities of fluorocarbon hydrides have been discussed recently by J. R. Jones in The Ionization of Carbon Acids , Academic Press, London and New York, 1973). 

To illustrate, the ionization of carbonic acid corresponds to that just described for A—B. A neutral molecule (H2CO3) dissociates to a cation (H ) and an anion (HC03 ). 

TABLE 3. Bond extensions estimated by ISM for the ionization of carbon acids in watei ... 

Kinetic Isotope Effects in Proton-Transfer Reactions 263 Table 25. isotope effects in the ionization of carbon acids in water at 25°C and /cb = velocity constants for transfer of protons and deuterons respectively, dm mol" s Ap = p sH P BH ApK = statistically corrected value of ApK ... 

In contrast to carbocations, the ionization of carbon acids to generate carbanions has been somewhat more difficult to demonstrate. This appears to be due largely to the difficulty of finding solvents of suitable dielectric constant to support ionization but yet not too acidic (allowing proton transfer) or otherwise too reactive. Thus, while diethyl ether (ether [(CH3CH2)20]) and other similar solvents are, in principle. 

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