Hydration-dehydration equilibria

The reaction is reversible and its stereochemical requirements are so pronounced that neither the cis isomer of fumaric acid (maleic acid) nor the R enantiomer of malic acid can serve as a substrate for the fumarase catalyzed hydration-dehydration equilibrium... 

Mechanistic questions in the hydration-dehydration equilibrium center around the acid-base relationships and the precise sequence of events in the addition or elimination of the water molecule. Investigations have relied primarily on kinetics of aldehyde hydration to elucidate the mechanistic details ... 

Thus, we need to prepare 180-labeled ethyl alcohol from the other designated starting materials, acetaldehyde and 180-enriched water. First, replace the oxygen of acetaldehyde with 180 by the hydration-dehydration equilibrium in the presence of 180-enriched water. 

If the equilibrium constant of the chemical reaction (such as complex stability constant, hydration-dehydration equilibrium constant, or the piCa of the investigated acid-base reaction) is known, limiting currents can be used to calculate the rate constant of the chemical reaction, generating the electroactive species. Such rate constants are of the order from 104 to 1010 Lmols-1. The use of kinetic currents for the determination of rate constants of fast chemical reactions preceded even the use of relaxation methods. In numerous instances a good agreement was found for data obtained by these two independent techniques. 

We have seen that both the forward and reverse reactions represented by the hydration-dehydration equilibrium are useful synthetic methods. 

The qualitative reasoning expressed in Le Chatelier s principle is a helpful guide a system at equilibrium adjusts so as to minimize any stress applied to it. For hydration-dehydration equilibria, the key stress factor is the water concentration. Adding water to a hydration-dehydration equilibrium mixture causes the system to respond by consuming water. More alkene is converted to alcohol, and the position of equilibrium shifts to the right. When we prepare an alcohol from an alkene, we use a reaction medium in which the molar concentration of water is high—dilute sulfuric acid, for example. 

How, then, do we control which product will predominate Recall that LeChatelier s principle states that a system in equilibrium will respond to a stress in the equilibrium by counteracting that stress. This response allows us to control these two reactions to give the desired product. Large amounts of water (achieved with the use of dilute aqueous acid) favor alcohol formation, whereas a scarcity of water (achieved with the use of concentrated acid) or experimental conditions by which water is removed (for example, heating the reaction mixture above 100 °C) favor alkene formation. Thus, depending on the experimental conditions, it is possible to use the hydration-dehydration equilibrium to prepare either alcohols or alkenes, each in high yields. 

This hydration-dehydration equilibrium illustrates a very important principle in the study of reaction mechanisms—the principle of microscopic reversibility. According to this principle, the sequence of transition states and reactive intermediates (i.e., the mechanism) for any reversible reaction must be the same, but in reverse order, for the reverse reaction as for the forward reaction. 

Polarographic data yield ki2 = 1.3 X lO W" sec, which agrees well with specific rates of similar reactions shown in Table II. The specific rate kn of the much slower dehydration reaction has been determined by both the temperature and pressure jump methods to be about 0.5 sec at pH 3 and 25 °C with some general acid-base catalysis. While the hydration-dehydration equilibrium itself involves no conductivity change, it is coupled to a protolytic reaction that does, and a pressure jump determination of 32 is therefore possible. In this particular case the measured relaxation time is about 1 sec. The pressure jump technique permits the measurement of chemical relaxation times in the range 50 sec to 50 tisec, and thus complements the temperature jump method on the long end of the relaxation time scale. 

---