Dehydration of carbonic acid

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...

To determine the speed of response of the instrument, a step change in the pH of the fluid around the electrode would have been ideal. It was not possible to do this experimentally in less than 0.030 sec, owing to the time required to wash out the electrode chamber. However, it was possible to estimate this speed using a ramp input, namely, the linear increase in pH produced by the dehydration of carbonic acid (I, 2, 5, 9) ... 

Van Eldick R (1987) High pressure studies of inorganic reactions. In Van Eldick R, Jonas J (eds) High Pressure Chemistry and Biochemistry. Reidel, Dordrecht, The Netherlands, p 333-356 Van Eldiek R, Palmer DA (1982) Effects of pressure on the kinetics of the dehydration of carbonic acid and the hydrolysis of CO2 in aqueous solution. J Solution Chem 11 339-346 Van Hook WA (1972) Vapor pressure isotope effect in aqueous systems. III. The vapor pressure of HOD (-60 to 200°C). J Phys Chem 76 3040-3043... 

The cyclic CO2 dimer is expected to require veiy low, possibly matrix isolation, temperatures for its observation, and a low-temperature photochemical synthesis by extmsion of CO2 from the cyclic trimer was suggested. The trimer and the acyclic species may be isolable at somewhat higher temperatures, and may be pteparable by dehydration of carbonic acid. 

Garg, L.C., Maren, T.H. (1972). The rates of hydration of carbon dioxide and dehydration of carbonic acid at 37°C. Biochimica et Biophysical Acta, 261, 70-76. 

This reaction occurs spontaneously and is accelerated by anions and by certain nitrogenous bases. Nevertheless, the nonenzymatic hydration of CO2 and the dehydration of carbonic acid are very slow compared with the reactions that require or produce one of these forms, and carbonic anhydrase is found in many tissues, where it performs essential physiological functions. Its presence in red cells is associated with the necessity of transferring CO2 efficiently both in the removal of CO2 from body tissues and in the elimination of CO2 in lungs or gills. This activity has also been found in secretory cells and in green plants, but its function in these places is not established. The formation of carbamic acid from CO2 and ammonia and the subsequent formation of carbamyl phosphate by the kinase are greatly accelerated by the presence of carbonic anhydrase. 

The first-order rate constant for the dehydration of carbonic acid... 

The first-order rate constant for tlie dehydration of carbonic acid H2CO3------------------------------------ CO2 + H2O... 

Van Eldik, R. Palmer, D. A. 1982. Elfects of pressure on the kinetics of the dehydration of carbonic acid and the hydrolysis of CO2 in aqueous solution. Journal of Solution Chemistry, 11, 339-346. 

It follows that liquids of high boiling point should not be distilled from drying agent systems which have appreciable vapour pressures. An extreme case of this action is the dehydration of oxalic acid dihydrate by distillation over toluene or over carbon tetrachloride. 

Carbon monoxide, CO, is produced when carbon or organic compounds burn in a limited supply of air, as happens in cigarettes and badly tuned automobile engines. It is produced commercially as synthesis gas by the re-forming reaction (Section 14.3). Carbon monoxide is the formal anhydride of formic acid, HCOOH, and the gas can be produced in the laboratory by the dehydration of formic acid with hot, concentrated sulfuric acid ... 

The dehydration of malonic acid, H02C-CH2-C02H, produces C302 (known as tricarbon dioxide or carbon suboxide). Draw the structure for C302 and describe the bonding in terms of resonance structures. 

Alternatively, carbon monoxide may be prepared by action of steam either on natural gas or on hot coke or coal. In the laboratory, CO may be produced by heating CaCOs with zinc dust or by dehydration of formic acid ... 

Selenium oxychloride may be prepared by several methods (1) by passing chlorine gas into a suspension of selenium dioxide in carbon tetrachloride, (2) by heating thionyl chloride, SOCE, with selenium dioxide, (3) by dehydration of dichloroselenious acid, H2Se(CE)02, and (4) by fusion of selenium dioxide, selenium, and calcium chloride. 

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). 

In the 1940 s, in addition to these operations, two other processes became important. Acetic acid was made by reacting methanol with carbon monoxide, and acetic anhydride was being made by the ethylidene diacetate process, which in effect is the dehydration of acetic acid to the anhydride by the use of acetylene. Fermentation ethyl alcohol was converted to acetic acid via acetaldehyde as well as by the direct oxidation of ethyl alcohol. A new operation on the Gulf Coast was also based on acetaldehyde. However, the acetaldehyde is made by the direct oxidation of liquefied petroleum gas. A further process for the production of these materials, in which acetaldehyde is oxidized in one step to a mixture of anhydride and acid, was also begun. 

Attempted dehydration of 95% acid to anhydrous formic acid caused rapid evolution of carbon monoxide. 

Malonic anhydride cannot be made directly from malonic acid because attempted dehydration of the acid leads to the exotic molecule carbon suboxide C3O2. 

Other carboxylic acid derivatives, for example ortho esters, ortho carbonates,or hy-drazono esters.are obtained via the corresponding acid chloride. Anhydrides are formed by dehydration of carboxylic acids with phosphorus peiitoxide. ... 

The oxides of carbon include the very stable and abundant CO and CO2, the unstable but isolable C3O2, and more complicated carbon oxides derived from organic backbones by removal of all hydrogen, for example, C12O9, from the complete dehydration of meUitic acid (benzenehexacarboxyhc acid). Oxocarbon anions, C 0 , with apparent aromatic properties are also known. The properties of these carbon-oxygen compounds are sununarized below. 

Carbon suboxide, C3O2, 0=C=C=C=0 , also called l,2-propadiene-l,3-dione, is an evil-smelling unstable gas, bp +6.8 °C, which is obtained by the dehydration of malonic acid with P4O10 in vacuum at 140-150 °C (equation 6). Carbon suboxide polymerizes readily at room temperature to a yellow solid, and above 100 °C to a ruby-red water-soluble solid. Photolysis of C3O2 gives C2O ( C=C=0 ) as a reactive intermediate, which reacts with alkenes by carbon atom insertion (equation 7). 

Carbon monoxide is usually prepared by the dehydration of formic acid by means of sulphuric acid ... 

Carbon monoxide, CO, though made by the dehydration of formic acid, is too insoluble to be considered as the acid anhydride. The gas is produced in large quantities by the reaction between carbon and COg at high temperatures ... 

Derivation (1) Made almost pure by placing a mixture of oxygen and carbon dioxide in contact with incandescent graphite, coke, or anthracite. (2) Action of steam on hot coke or coal (water gas) or on natural gas (synthesis gas). In the latter case, carbon dioxide is removed by absorption in amine solution, and the hydrogen and carbon monoxide separated in a low-temperature unit. (3) By-product in chemical reactions. (4) Combustion of organic compound with limited amount of oxygen, as in automobile cylinders. (5) Dehydration of formic acid. 

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