Final answer:
Increasing alkyl groups on a carbonyl carbon in compounds like aldehydes and ketones makes the molecule more hydrophobic and introduces steric hindrance, both of which inhibit hydrate formation.
Step-by-step explanation:
The reason why increasing the number of alkyl groups on the carbonyl carbon decreases the amount of hydrate at equilibrium in carbonyl compounds such as aldehydes and ketones is related to the hydrophobic character introduced by the alkyl groups. As alkyl groups are added, the entire molecule becomes less polar and more hydrophobic, which hinders the formation of the hydrate that requires strong interaction with water (a polar solvent). A hydrate is a compound formed when water is added to the carbonyl group, and this reaction is reversible. Larger alkyl groups can hinder the approach of water molecules to the carbonyl carbon, making it less likely to form the hydrate.
In addition, these alkyl groups introduce steric hindrance which can inhibit the nucleophilic addition reaction needed to form the hydrate. This is observed in other nucleophilic additions such as the reduction of aldehydes and ketones where hydride (H-) or other nucleophiles add to the electrophilic central carbon. Moreover, the presence of alkyl groups can stabilize the carbonyl group's partial positive charge, making the carbonyl carbon less susceptible to attack by nucleophiles like water.