Question

Number of deutorium (D) present in final product obtained on prolong treatment with NaOD/D20

• A. 6
• B. 9
• C. 10
• D. 5

Answer 1

Answer: (B)

9

Answer 2

The compound 2,6-dimethylcyclohex-2-en-1-one undergoes isotopic exchange with deuterium under prolonged treatment with NaOD/D$_2$O. Exchange occurs at positions activated by the carbonyl group or conjugation.

1. Alpha-hydrogens at C6: C6 is alpha to the carbonyl. It has one hydrogen on the ring that is readily exchanged. (1 D)
2. Methyl hydrogens at C6: The methyl group attached to C6 also has hydrogens that are considered alpha to the carbonyl and are exchanged. (3 D)
3. Methyl hydrogens at C2: These hydrogens are allylic to the C2=C3 double bond, which is conjugated with the carbonyl group. These are labile and undergo exchange. (3 D)
4. Methylene hydrogens at C4: Under prolonged treatment with a strong base, hydrogens at the C4 position (beta to the carbonyl) can also exchange, especially since C4 is adjacent to C3, part of the conjugated system. (2 D)

Total deuterium atoms incorporated = 1 (C6 ring) + 3 (C6 methyl) + 3 (C2 methyl) + 2 (C4 methylene) = 9 D.