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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3260

The mesylate shown carries a carbon-14 label (14C) at the carbon bearing the leaving group. It undergoes acetolysis entirely through a symmetrical bridged phenonium ion (neighbouring aryl participation), which is opened by acetate at either carbon with equal probability. What percentage of the 14C label is found in the CH2 group attached to oxygen in the product acetate?

  1. A0 percent
  2. B50 percent
  3. C25 percent
  4. D100 percent

Answer: B

The compound is 2-(4-methoxyphenyl)ethyl mesylate labelled at C1. The aryl ring displaces mesylate to form a symmetrical spiro phenonium ion in which both CH2 carbons are equivalent. Acetate opens it at either carbon with equal probability, so half of the product has the label on the CH2-OAc carbon and half on the benzylic carbon. The answer is 50 percent.
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