Holfmann

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  • Holfmann

    1. 1. The HofmannRearrangement Dr. Khalid Hussain
    2. 2. Introduction In previous lectures we have discussed the movement of alkyl or aryl groups etc. that involves the migration to carbon atom, either cation or electron deficient (carbene). Like carbon, nitrogen can easily be madeelectron deficient, R2N** or RN ** * ** Hence, it is expected to facilitate migration to itself as it is observed in R3C+ of R2C**
    3. 3. Example is Hofmann rearrangementConversion of an amide to an aminecontaining 1 carbon less by the action ofalkaline hypobromite (BrO-) O BrO- RCNH2 RNH2 + CO2
    4. 4. The Hofmann RearrangementTreatment of amides with bromine in basic solutiongives an amine with loss of the carbonyl carbon in the form of carbon dioxide. O Br2/NaOH RCNH2 RNH2 + CO2
    5. 5. The Hofmann RearrangementAmides on reacting with bromine and sodium hydroxide form isocyanate which upon hydrolysis and decarboxylation produce amine
    6. 6. Examples O Br2, NaOH(CH3)3CCH2CNH2 (CH3)3CCH2NH2 H2O3,3 dimethyl butylamide 2,2 dimethyl propylamine O Br2, KOH CNH2 NH2 H2OBr Br
    7. 7. Mechanism of rearrangement1- Removal of proton from nitrogen of the amide2- Formation of N-bromo amide3- Removal of another proton from nitrogen4- migration of alkyl group5- Formation of isocynate6-Hydrolysis of isocyanate
    8. 8. Mechanism of rearrangement R C NH2 R C NH-Br O O Br2/NaOH OH-Isocyanate R C N-Br C N R + O O ** R C N ** Br- H2O O - C N R O ** R-NH-COOH + Carbamic acidCO2 + R-NH2
    9. 9. Curtius, degradation of acid azides, and Lossen,decomposition of hydroximic acid, also involvesHofmann type rearrangement O O O NH2-OH NH2-NH2 hydrazide R-C-NH-OH R-C-O-Et R-C-NH-NH2 OH- NaNO2 HCl O O O -OH- ** Heat + ** R-C-N-OH R-C-N R-C-N-N N ** -N2 ** Acyl azide O==C==N--RHydroxamic acid isocyanate
    10. 10. Br2OH- OH- -Br

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