Question

What are X and P in the following reaction sequence?

• A. Isomer combination (a)
• B. Isomer combination (b)
• C. Isomer combination (c)
• D. Isomer combination (d)

Hint:

Remember:
1. Cannizzaro reaction of a dialdehyde yields a hydroxy-acid salt.
2. Treatment of a 1,2-hydroxy-acid with acid ($\text{HCl}$) always drives intramolecular esterification to yield a cyclic ester, known as a lactone.
3. A lactone contains a carbonyl ($C=\text{O}$) and a ring oxygen, immediately eliminating hemiacetal structures like those in (b) and (d).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The reaction sequence involves an intramolecular Cannizzaro reaction of a phthalaldehyde derivative followed by an acid-catalyzed lactonization.

Step 2: Detailed Explanation:
$\bullet$ Step 1 (NaOH, Heat): The starting material has one -CHO and one -CDO group. In a Cannizzaro reaction, a hydride/deuteride is transferred.
$\bullet$ Because the C-H bond is weaker than the C-D bond, the transfer of hydride (H$^{-}$) from the -CHO group is kinetically preferred.
$\bullet$ Thus, the -CHO is oxidized to -COONa, and the -CDO group is reduced to a deuterated alcohol group: -CH(OH)D. This is intermediate X.
$\bullet$ Step 2 (HCl gas): The acid protonates the carboxylate. Since the -COOH and the -CH(OH)D alcohol are in ortho positions, they undergo spontaneous dehydration to form a cyclic ester (lactone).
$\bullet$ The resulting product P is a phthalide containing a CHD group in the ring.

Step 3: Final Answer:
Structure X has -COONa and -CH(OH)D, and structure P is the corresponding lactone.
This matches Option (A).