Ch_322b_20.10

Ch_322b_20.10 - The Hofmann Rearrangement Amines from...

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The Hofmann Rearrangement: Amines from Primary Amides Primary amides are converted into amines with loss of the carbonyl carbon atom in aqueous basic solutions of Br 2 or Cl 2 . R C NH 2 O = + X 2 + 4 NaOH RNH 2 + 2NaBr + Na 2 C O 3 + 2 H 2 O amide amine The Hofmann rearrangement of primary amides provides primary amines exclusively with no contamination from secondary or tertiary amines.
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A Mechanism for the Hofmann Rearrangement in Br 2 /KOH Solution The proposed mechanism begins with base-promoted N-bromination, analogous to ! -bromination of enolizable ketones. The N-bromoamide is more acidic than the unsubstituted amide and deprotonation readily occurs: RCNH O = - + Br-Br RCNHBr O - + Br nucleophilic attack: N-bromination step RCN H 2 O pK a ~ 22 + HO - RCNH O - + H 2 O acid-base reaction low conc. RC-N-Br O H : pK a << 22 + HO - RC-N-Br O _ + H 2 O anion is unstable
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The Rearrangement Step molecular rearrangement of unstable anion to isocyanate O = : _ C N R Br The R group (alkyl or aryl) shifts from the carbonyl
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Ch_322b_20.10 - The Hofmann Rearrangement Amines from...

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