Concept:When a carbonyl compound dissolves in water, it undergoes a reversible hydration reaction to form a gem-diol (hydrate). The equilibrium highly favors the hydrate only if there is minimal steric hindrance and maximum electrophilicity at the carbonyl carbon.
Formula:$$ >C=O + H_2O \rightleftharpoons >C(OH)_2 $$
Solution:- Formaldehyde (\( HCHO \)) has two very small hydrogen atoms attached to the carbonyl carbon.
- This creates zero steric hindrance for the incoming water molecule.
- Furthermore, lacking any electron-donating alkyl groups, its carbonyl carbon is exceedingly electrophilic and highly reactive.
- As a result, in an aqueous solution, formaldehyde exists almost entirely (>99.9%) as its hydrate, methanediol (formalin).
Why other options are incorrect:As you add bulky, electron-donating alkyl groups (Acetaldehyde, Propanal, Acetone), the electrophilicity of the carbon decreases and steric hindrance increases, pushing the equilibrium strongly back toward the unhydrated carbonyl form.
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