Question

Which compound forms the higher concentration of hydrate at equilibrium, PhCOCHO or PhCH₂CHO ? Explain your reasoning.

Short answer

PhCOCHO has a higher hydrate concentration at equilibrium because two carbonyl groups are present adjacent to each other.

Step-by-step solution

Effect of the group near carbonyl on the amount of hydrate at equilibrium

Electron-donating groups near the carbonyl carbon stabilize the carbonyl group and therefore decrease the amount of hydrate at the equilibrium.

Electron-withdrawing groups near the carbonyl carbon destabilize the carbonyl group and therefore increase the amount of hydrate at the equilibrium.

Compound at the higher concentration of hydrate at equilibrium 

2-oxo-2-phenylacetaldehyde exists at a higher concentration of hydrate at equilibrium because two carbonyl groups are present adjacent to each other, which destabilize each other.

Structure of 2-oxo-2-phenylacetaldehyde

In 2-phenylacetaldehyde, the carbonyl group is relatively stabilized due to the +I effect of the group. Hence the amount of hydrate at the equilibrium is less.

Structure of 2-phenylacetaldehyde