13.3 Mechanism for Acid-Catalysed Hydrate Formation

The equilibrium constant $\left( {{K_{eq}}} \right)$ for hydrate formation depends on the relative stabilities of the carbonyl compound and the hydrate. We have seen that the electron donating alkyl groups make the carbonyl compound more stable (less reactive), so acetone is more stable than formaldehyde. However, alkyl groups have the opposite effect on the stability of the hydrate, they make the hydrate less stable, so the hydrate of acetone is less stable than that of the formaldehyde.

Relative stabilities of carbonyl compounds

$$C{H_3}COC{H_3} > C{H_3}CHO > HCHO$$

Relative stabilities of hydrates of carbonyl compounds

Steric interaction between the alkyl groups are responsible for the decreased stability of the hydrate. The electron cloud of the alkyl groups in the carbonyl compound do not interfere with each other, since the bond angle between them is 120°. However, the bond angle in the hydrate is reduced to 109° 5’. So the alkyl groups are closer to each other. Hydrates of aldehydes and ketones are generally too unstable to be isolated and exist only in an aqueous solution.

The reaction co-ordinate diagrams in figure shows how the opposing effects of substituents on the stabilities of the carbonyl compounds and the hydrate can have a dramatic effect on the equilibrium constant for hydrate formation.

If the amount of hydrate formed from the reaction of ${H_2}O$ with a ketone is too small to detect, how do we know that the reaction has even occurred? We can prove that it occurs by treating the ketone with $^{18}O$-labelled water and isolating the ketone after the equilibrium has been established. Finding that the labeled ${O^{18}}$ is incorporated into the ketone proves that the reaction has occurred.