Question

When a dilute solution of acetaldehyde in D₂O containing NaOD is shaken, explain why the methyl hydrogens are exchanged with deuterium but the hydrogen attached to the carbonyl carbon is not.

Short answer

The protons which are directly attached to the carbonyl group are not acidic and cannot be replaced by deuterium.

Step-by-step solution

Alpha hydrogens

The hydrogens that are attached to prior to the carbonyl carbon are acidic. These protons are more readily abstract by the base than the protons that are directly attached to the carbonyl carbon.

Formation of the desired product

A base removes the proton from the $\alpha$carbon of the carbonyl group and forms an enolate ion. The enolate ion then reacts with an electrophile because they are negatively charged, enolate ions are better nucleophiles than enols.The protons which are directly attached to the carbonyl group are not acidic and can not be replaced by deuterium.