Hydration of aldehydes

PRINCIPLES OF NUCLEOPHILIC ADDITION HYDRATION OF ALDEHYDES AND KETONES... 

Principles of Nucleophilic Addition Hydration of Aldehydes and Ketones... 

Electronic and steric effects operate m the same direction Both cause the equilib rium constants for hydration of aldehydes to be greater than those of ketones... 

Hydration of aldehydes and ketones is a rapid reaction quickly reaching equilibrium but faster in acid or base than in neutral solution Thus instead of a single mechanism for hydration we 11 look at two mechanisms one for basic and the other for acidic solution... 

In the first stage of the hydrolysis mechanism water undergoes nucleophilic addi tion to the carbonyl group to form a tetrahedral intermediate This stage of the process IS analogous to the hydration of aldehydes and ketones discussed m Section 17 6... 

The hydrates of aldehydes and ketones are considerably more acidic than normal alcohols (pAl Fs 16-19). How would you account for this fact Some reported values are shown below. Explain the order of relative acidity. 

Montmorillonite K10 was also used for aldol the reaction in water.280 Hydrates of aldehydes such as glyoxylic acid can be used directly. Thermal treatment of K10 increased the catalytic activity. The catalytic activity is attributed to the structural features of K10 and its inherent Bronsted acidity. The aldol reactions of more reactive ketene silyl acetals with reactive aldehydes proceed smoothly in water to afford the corresponding aldol products in good yields (Eq. 8.104).281... 

There is a limited amount of information on the extent of hydration of aldehydes in deuterium oxide. Gruen and McTigue (1963a) concluded that for five aliphatic aldehydes at 25° C is 16 % to 26 % smaller in D2O... 

In the broader scope of this definition, we can also include the hydration of aldehydes or alkenes, where the C=0 or C=C double bond is broken by incorporating the elements of water ... 

Many addition and elimination reactions, e.g., the hydration of aldehydes and ketones, and reactions catalyzed by lyases such as fumarate hydratase are strictly reversible. However, biosynthetic sequences are often nearly irreversible because of the elimination of inorganic phosphate or pyrophosphate ions. Both of these ions occur in low concentrations within cells so that the reverse reaction does not tend to take place. In decarboxylative eliminations, carbon dioxide is produced and reversal becomes unlikely because of the high stability of C02. Further irreversibility is introduced when the major product is an aromatic ring, as in the formation of phenylpyruvate. 

The reaction, by either mechanism, is fast at room temperature. However, because water is a relatively weak nucleophile, the equilibrium does not favor the product in most cases. As a result, the hydrates of aldehydes and ketones usually cannot be isolated because removal of the water solvent also drives the equilibrium back to the left. Therefore, the hydration reaction is not useful in synthesis. 

Mechanism The mechanism of the Cr( VI) oxidation of aldehydes has been studied in detail in Scheme 7.14. A hydrate of aldehyde A is formed first, which reacts with chromium species to form a chromate ester B. A base abstracts a proton from the chromate ester B and Cr species leaves (E2 elimination) to give carboxylic acid. 

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