Hydration, of acetaldehyde

The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. For acetaldehyde, the half-life of the exchange reaction is on the order of one minute under neutral conditions but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 M s . Acid catalysis involves either protonation or hydrogen bonding at the carbonyl oxygen. 

The following data give the dissociation constants for several acids that catalyze hydration of acetaldehyde. Also given are the rate constants for the hydration reaction catalyzed by each acid. Treat the data according to the Bronsted equation, and comment on the mechanistic significance of the result. 

The hydration of acetaldehyde (Scheme XI) constitutes a process in which an oxygen atom exchanges between the solvent and the solute. 

The reactions are accompanied by a considerable volume change, and a dilatometric method was employed by Bell and Higginson (1949), who added acetaldehyde-water mixtures (containing about equal quantities of MeCHO and MeCH(OH)2) to an excess of acetone, and thus measured kj, in presence of a large number of acid catalysts. The direct hydration of acetaldehyde in aqueous buffer solutions is inconveniently fast at room temperatures, but ( (j + A ) was measured dilatometrically at 0°C by Bell and Darwent (1950), who established the existence of general acid-base catalysis. 

The rate of heat evolution can be used to follow reactions with half-lives down to a second or less. This method was first applied by Bell and Clunie (1952b) to the hydration of acetaldehyde in aqueous acetate buffers at 0° C, and a more detailed study was made at 25° C by Bell et al. (1956). A similar method was later used by Gruen and McTigue (1963b) for other aldehydes. 

A recent report (Pocker and Meany, 1964) states that the enzyme carbonic anhydrase is a powerful catalyst for the reversible hydration of acetaldehyde, but no details have yet been published. 

Very recently the isolation of still active carbonic anhydrase from the biocrystalline layer of hens egg shells has been reported. Crude egg shell extracts exhibited no enzyme activity. however, after removal of Ca2+ and simple gel filtration of the extracts, carbonic anhydrase activity could be measured. Further purification steps led to the isolation of two isoenzymes. The possibility of inhibition of this enzyme activity by shell components cannot be excluded although recombination of shell components after separation did not inhibit that enzyme. The isolated enzymes has an apparent molecular size of 28,000 daltons. Carbonic anhydrase from egg shells catalyzes the reversible hydration of CO2 + HOH H+ + HCO3-. This probably is the primary action of the enzyme of the shell. Moreover, egg shell carbonic anhydrase catalyzes the hydration of acetaldehyde and pyridine aldehyde. Furthermore, the same enzymes have esterase activity (hydrolysis of p-nitrophenyl acetate). Whether the latter activities play a role in the egg shell cannot be judget at the present time. 

The hydration of acetaldehyde is catalyzed by zinc(II) and the catalysis is enhanced by the presence of anions such as acetate and hydroxide.523 Various processes were considered (Scheme 44) to account for the hydroxide-dependent and -independent pathways involving zinc(II). Zinc(II)... 

This enzyme, which occurs in animals, plants and certain microorganisms, is a most effective catalyst of the reversible hydration of C02 and dehydration of HC03. 479-482 It also catalyzes reactions of a number of compounds which undergo hydrolysis or hydration, for example the hydrolysis of 4-nitrophenyl acetate and the hydration of acetaldehyde.480... 

I. Kinetic studies of the enzyme catalysed hydration of acetaldehyde. Biochemistry 4, 2535-2541 (1965). 

The rate-determining step is 3, the transfer of the OH proton to the conjugate base A . For the reverse process, the hydration of acetaldehyde, reaction 4 is rate-determining. For the base-catalyzed reaction the proposed mechanism is very similar ... 

This complex has been shown to be an excellent structural and functional model for the zinc hydrolytic enzymes, particularly carbonic anhydrase but also carboxypeptidase and the zinc phosphate esterases (24-26). The same complex also catalyzes the hydration of acetaldehyde and hydrolysis of carboxylic esters. These reactions appear to progress via a mechanism similar to that proposed for carbonic anhydrase. The rates are slower for [Zn([12]aneN3)OH] than for the enzyme but an order of magnitude faster than for existing model systems such as [(NH3)5Co(OH)]2+ (26). 

For the last step a reductive elimination, whether through a carbonium ion intermediate by heterolysis [12] or by a concerted reaction [23] forming the hydrate of acetaldehyde, is more a philosophical question (eqs. (21) and (22)). Both routes would explain the findings of Moiseev and Vargaftik [22] that in... 

Lauder (1948) also gives data for the rate of hydration of acetaldehyde based on dilatometric and refractometric measurements. He did not realize the catalytic nature of the reaction, and worked in unbuffered solutions which presumably contained small but variable quantities of acetic acid this may account for the erratic nature of his results and for the fact that his rates decrease with increasing temperature. It is more difficult to explain the fact that most of his recorded rates are considerably lower than the minimum (water-catalysed) velocity obtained by other methods. He used concentrated solutions of acetaldehyde, which may have contained considerable quantities of the hemihydrate (MeCH0H)20. The presence of this species has been recently suggested by Ahrens and Strehlow (1965) on the basis of N.M.R. spectra there is evidence for the existence of analogous species in aqueous formaldehyde solutions (Bezzi et al., 1951), and the hemihydrate (CH2CI. CH0H)20 can be isolated as a solid (Natterer, 1882). 

In aqueous solution, acetaldehyde (ethanal) is about 50% hydrated. Draw the structure of the hydrate of acetaldehyde. Under the same conditions, the hydrate of W,W-dimethylformamide is undetectable. Why the difference ... 

The last step, the release of acetaldehyde, can be interpreted as a reductive elimination to give the hydrate of acetaldehyde (Eqs. (9.19) and (9.20)) where Eq. (9.19) represents merely the completion of the complex ligation sphere at the central Pd by a solvent molecule. A reductive elimination is a common reaction of group 8 metal compounds. For this case, it has been first proposed in [20]. Keith etal. [21] derived such a pathway but chloride assisted from theoretical considerations. The barrier heights of the transition states for other pathways, for example, P-hydride elimination, were found to be too high. The route according to Eq. (9.20) would also be valid for chloride-free Pd compounds. 

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