What is the structural characteristic of the intermediate species involved in the rate-determining rearrangement step of the Hoffmann bromamide degradation?

Answer and explanation

Correct answer: A. An alkyl isocyanate (R-N=C=O) formed by rearrangement of an acyl nitrene intermediate

  1. Step 1: In the Hoffmann bromamide reaction, the primary amide reacts with and to form N-bromoamide ().
  2. Step 2: Deprotonation by base gives the bromamide anion (), which loses bromide ion to form a transient acyl nitrene ().
  3. Step 3: The acyl nitrene rapidly undergoes 1,2-migration: the alkyl/aryl group () migrates with its bonding electron pair from the carbonyl carbon to the electron-deficient nitrogen atom.
  4. Step 4: This rearrangement forms an alkyl isocyanate (), which is subsequently hydrolyzed by alkaline water to generate the primary amine () and carbonate.
Answer
An alkyl isocyanate (R-N=C=O) formed by rearrangement of an acyl nitrene intermediate
Why not the others: 'A planar carbocation...' is not formed in this base-promoted concerted rearrangement; 'A free radical formed by homolytic cleavage...' is incorrect as the mechanism is strictly polar/heterolytic; 'An aromatic carbanion...' misstates the intermediate structure.

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