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JEE Advanced levelSingle correct optionDifficulty 3 of 3ro015

Assertion (A): DIBAL-H at converts ethyl pentanoate into pentanal after aqueous work-up, whereas gives pentan-1-ol. Reason (R): With DIBAL-H at low temperature the aluminium hemiacetal intermediate is stable and collapses to the aldehyde only on addition of water.

  1. ABoth A and R are true but R is not the correct explanation of A
  2. BBoth A and R are true and R is the correct explanation of A
  3. CA is true but R is false
  4. DA is false but R is true

Answer: B

DIBAL-H (one hydride, bulky) adds once to the ester carbonyl and the tetrahedral aluminium hemiacetal persists at , so no second hydride adds. Water then releases the aldehyde. is more reactive and expels ethoxide to give an aldehyde that is reduced further to the alcohol. Both statements are true and R explains A.
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