Deuterium Exchange in Carbonyl Compounds

The mechanism of the base-catalyzed exchange of the a-hydrogens (eq. 9.45) involves two steps. 

The base (methoxide ion) removes an a-proton to form the enolate anion. Reprotonation, but with CH3OD, replaces the a-hydrogen with deuterium. With excess CH3OD, all four a-hydrogens are eventually exchanged. 

The mechanism of the acid-catalyzed exchange of the a-hydrogens (eq. 9.46) also involves several steps. The keto form is first protonated and, by loss of an a-hydrogen, converted to its enol. 

---