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20.17 The Hofmann Rearrangement

20.17 The Hofmann Rearrangement. O. RC N H 2. The Hofmann Rearrangement. Treatment of amides with bromine in basic solution gives an amine with loss of the carbonyl carbon. Br 2. +. R N H 2. CO 3 2–. HO –. O. O. CNH 2. NH 2. Br. Br. Examples. Br 2 , NaOH. (CH 3 ) 3 CCH 2 CNH 2.

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20.17 The Hofmann Rearrangement

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  1. 20.17The Hofmann Rearrangement

  2. O RCNH2 The Hofmann Rearrangement Treatment of amides with bromine in basic solutiongives an amine with loss of the carbonyl carbon. Br2 + RNH2 CO32– HO–

  3. O O CNH2 NH2 Br Br Examples Br2, NaOH (CH3)3CCH2CNH2 (CH3)3CCH2NH2 H2O (94%) Br2, KOH H2O (87%)

  4. O O RCNH2 RCNHBr Mechanism of the Hofmann Rearrangement • The Hofmann rearrangement involves 6 steps in 3 stages. • 1. formation of an N-bromo amide (2 steps) • 2. conversion of the N-bromo amide to an isocyanate (2 steps) • 3. hydrolysis of the isocyanate (2 steps) RNCO RNH2

  5. •• O •• – •• O RC H •• •• N H •• H Step 1

  6. – •• •• O O •• •• •• – •• •• O O RC H H RC •• •• •• N N H H •• •• H H Step 1

  7. – •• O •• •• RC N •• •• •• Br Br •• •• •• •• H Step 2

  8. – •• •• O O •• •• •• RC RC •• – N N Br •• •• •• •• •• •• Br Br Br •• •• •• •• •• •• •• •• H H Step 2

  9. •• O •• RC •• N Br •• •• •• H – •• O H •• •• Step 3

  10. – •• •• O O •• •• •• RC C R •• •• N Br N Br •• •• •• •• •• •• H H – •• O O H H •• •• •• •• Step 3

  11. – •• •• O O •• •• •• C C R •• N R N Br •• •• •• •• – •• Br •• •• •• Step 4

  12. •• •• O O •• •• – •• C C O •• •• N N R R •• •• H •• NH2 R Steps 5 and 6 H2O HO– + CO32–

  13. H O C6H5CH2 C CNH2 H3C H C6H5CH2 C NH2 H3C Stereochemistry • alkyl group migrates with retention of configuration Br2, NaOH, H2O

  14. O O CH3(CH2)14CNH2 CH3(CH2)14NHCOCH3 Isocyanates are intermediates • When the reaction is carried out in methanol instead of water, the product shown is isolated. Br2 NaOCH3CH3OH

  15. O O CH3(CH2)14CNH2 CH3(CH2)14NHCOCH3 CH3(CH2)14N via: C O Isocyanates are intermediates • When the reaction is carried out in methanol instead of water, the product shown is isolated. Br2 NaOCH3CH3OH

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