1. Mechanism

2. Variations

3. Applications

4. See also

5. References

6. Bibliography

The Hofmann rearrangement is the organic reaction of a primary amide to a primary amine with one fewer carbon atom.^[1][2][3]

The reaction is named after its discoverer – August Wilhelm von Hofmann. This reaction is also sometimes called the Hofmann degradation, and should not be confused with the Hofmann elimination.

Mechanism

The reaction of bromine with sodium hydroxide forms sodium hypobromite in situ, which transforms the primary amide into an intermediate isocyanate. The formation of an intermediate nitrene is not possible because it implies also the formation of a hydroxamic acid as a byproduct, which has never been observed. The intermediate isocyanate is hydrolyzed to a primary amine, giving off carbon dioxide.^[2]

1. Base abstracts an acidic N-H proton, yielding an anion.
2. The anion reacts with bromine in an α-substitution reaction to give an N-bromoamide.
3. Base abstraction of the remaining amide proton gives a bromoamide anion.
4. The bromoamide anion rearranges as the R group attached to the carbonyl carbon migrates to nitrogen at the same time the bromide ion leaves, giving an isocyanate.
5. The isocyanate adds water in a nucleophilic addition step to yield a carbamic acid (aka urethane).
6. The carbamic acid spontaneously loses CO₂, yielding the amine product.

Variations

Several reagents can substitute for bromine. sodium hypochlorite,^[4] lead tetraacetate,^[5] N-bromosuccinimide, (bis(trifluoroacetoxy)iodo)benzene,^[6] and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) can affect a Hofmann rearrangement. In the following example, the intermediate isocyanate is trapped by methanol, forming a carbamate.^[7]

In a similar fashion, the intermediate isocyanate can be trapped by tert-butyl alcohol, yielding the tert-butoxycarbonyl (Boc)-protected amine.

The Hofmann Rearrangement also can be used to yield carbamates from α,β-unsaturated or α-hydroxy amides^[2][8] or nitriles from α,β-Acetylenic amides^[2][9] in good yields (≈70%).

For amiloride, hypobromous acid was used to effect Hofmann rearrangement.

Applications

• Aliphatic & aromatic amides are converted into aliphatic and aromatic amines, respectively
• In the preparations of anthranilic acid from phthalimide
• Nicotinic acid is converted into 3-Aminopyridine
• The symmetrical structure of α-phenyl propanamide does not change after Hofmann reaction.
• Gabapentin from mono-amidation 1,1-cyclohexane diacetic acid anhydride with ammonia to 1,1-cyclohexane diacetic acid mono-amide; followed by ‘Hoffmann’ rearrangement: {{US patent|20080103334}}

See also

• Beckmann rearrangement
• Curtius rearrangement
• Iodoform reaction
• Lossen rearrangement
• Schmidt reaction
• Weerman degradation

References

Bibliography

• {{cite book |last=Clayden | first=Jonathan|title=Organic Chemistry |publisher=Oxford University Press Inc.|year=2007 |pages=1073 |isbn=978-0-19-850346-0}}
• {{cite book |last=Fieser | first=Louis F.|title=Advanced Organic Chemistry |publisher=Reinhold Publishing Corporation, Chapman & Hall, Ltd. |year=1962 |pages=499–501}}

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