Hoffmann Bromamide Reaction Mechanism

As the name of the reaction suggests, Hoffmann bromamide degradation reaction was given by August Wilhelm Von Hoffmann. It is also known as Hoffmann rearrangement reaction. This reaction is used to form primary amines. Let’s understand the name of the reaction first, so that it will be easy for you to remember the reaction. 

Hoffmann – Named after the German Chemist August Wilhelm Von Hoffmann.

Bromamide – Bromine molecule and amide are used in the reaction. 

Degradation – One less Carbon atom you get in the product than reactant after the reaction. Thus, degradation of carbon takes place. 

Hoffmann bromamide degradation reaction

In Hoffmann bromamide degradation reaction, an amide reacts with bromine and aqueous solution of sodium hydroxide which produces primary amine. This a degradation reaction as primary amine in the product has one carbon lesser than primary amide (in the reactant). 

Hoffmann bromamide degradation reaction can be written as follows – 

R-CO-NH2      +     Br2      + 4NaOH    🡪    R-NH2 + Na2CO3 + 2NaBr + 2H2

General amide   Bromine Sodium Hydroxide     

In words Hoffmann reaction can be written as follows – 

Primary amide Br2+NaOH Primary amine 

Secondary and tertiary amides don’t show Hoffmann bromamide reaction. Only one unit or molecule of bromine is used in the reaction. 

Example of Hoffmann bromamide degradation reaction

 1. Preparation of aniline 

2. Preparation of methylamine 

Hoffmann Bromamide Degradation Reaction Mechanism 

Hoffmann bromamide reaction mechanism can be explained in following steps – 

Step 1: In this step primary amide reacts with sodium hydroxide. Hydroxide ion(anion) of NaOH attacks on a group of primary amide which results in deprotonation of primary amide and forms water and negatively charged primary amide ion. 

Step 2: In this step while reacting with primary amide ion, one atom of bromine molecule develops partially positive charge while another atom develops partially negative charge due to negatively charged primary amine. It results in the formation of R-CO-NHBr (bromamide) and elimination of Br-

Step 3: In this step, another molecule of sodium hydroxide reacts with R-CO-NHBr and another water molecule is eliminated and R-CO-NBr is left behind.                            

Step 4: In this step, R (alkyl or aryl group) gets detached from bromamide anion as R- which results in OC-NBr. Its rate determines the step.

Step 5:In this step R- attacks on nitrogen atom of O=C=NBr and isocyanate is formed. Now alkyl or aryl groups are attached directly to nitrogen atoms.              

Step 6: In isocynate, a carbon atom is attached with two highly electronegative elements (oxygen and nitrogen), so it develops partially positive charge. In this step isocynate reacts with water molecules and removal of carbon dioxide molecules takes place. Which finally results in the formation of primary amine.  

Applications of Hoffmann bromamide degradation reaction

  1. It is used to produce primary aromatic and aliphatic amines. 

  2. It is used in the preparation of aniline. 

  3. It is used in the preparation of anthranilic acid and phthalimide. 

  4. 3-Aminopyridine is produced by this reaction from nicotinic acid. 

  5. Hoffmann reaction does not change symmetrical structure of -phenyl propanamide. 

Limitations of Hoffmann bromamide reaction 

Secondary and tertiary amides can’t be used in the Hoffmann bromamide reaction to produce primary amines.