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JEE Advanced levelSingle correct optionDifficulty 3 of 3o1144

Ethyl 4-oxopentanoate, , is treated in sequence with (i) ethylene glycol, dry (ketone converted to ketal); (ii) excess , dry ether; (iii) dilute . The final product is

  1. A2,5-dimethylhexane-2,5-diol
  2. Bhexane-2,5-dione
  3. C5-methylhexane-2,5-dione
  4. D5-hydroxy-5-methylhexan-2-one

Answer: D

The ketone is masked as a ketal, which is inert to Grignard reagents. Excess converts the ester into the tertiary alcohol , losing the ethoxy group. Aqueous acid in step (iii) removes the ketal and restores the ketone, giving . Without protection both groups would react to give the diol.
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