Question

Which compound in each pair forms the higher percentage of gem-diol at equilibrium?

a.

or

b.

or

Short answer

a.

b.

Step-by-step solution

Hydrate formation

Carbonyl compounds would undergo hydration to form diols. The thermodynamic stability of the hydrate would depend on the nature of substituents on the carbonyl carbon.

 Comparison of given compounds  

Consider the given pair of organic compounds,

a. Among the given compounds, butanal would form the higher percentage of gem-diols at equilibrium than but-2-one.

This occurs because alkyl group being electron-releasing group (ERG) would stabilize the carbonyl compound shifting the equilibrium backward.

Given pair a.

b. Among the given compounds, 2,2-difluropropanal would form the higher percentage of gem-diols at equilibrium than propan-1-al.

This occurs because halides being an electron-withdrawing group (EWG) would destabilize the carbonyl compound shifting the equilibrium forward.

The presence of fluoride ions would develop a partial positive charge on the alpha-carbon, which would destabilize the carbonyl carbon.

Given pair b.

Stable carbonyl compound

The carbonyl compound that would give a higher percentage of gem-diol is as follows:

a.

Compound that forms the higher percentage of gem-diol at equilibrium in pair a.

Compound that forms the higher percentage of gem-diol at equilibrium in pair b.