[Chemistry Class Notes] Hoffmann Bromamide Reaction Mechanism? Pdf for Exam

In this article, we will learn about the following concepts – 

• Hoffmann Bromamide Reaction Mechanism- An introduction

• What is Hoffmann Bromamide Reaction Mechanism?

• Example of the  Mechanism

• Steps of Hoffmann Bromamide Reaction Mechanism

• Application of the Mechanism 

• Limitations of the Mechanism

• Key learning from the chapter 

• Frequently asked questions 

Hoffmann’s bromamide degradation reaction

As the name of the reaction suggests, Hoffmann’s bromamide degradation reaction was given by August Wilhelm Von Hoffmann. It is also known as the 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. 

In Hoffmann bromamide degradation reaction, an amide reacts with bromine and an aqueous solution of sodium hydroxide which produces primary amine. This is a degradation reaction as the 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 + 2H2O 

(General amide)     (Bromine)    (Sodium Hydroxide)    

In other words, Hoffmann reaction can be written as follows – 

Primary amide Br₂+NaOH→ Primary amine 

Secondary and tertiary amides don’t show Hoffmann bromamide reactions. 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 the following steps – 

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

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 the nitrogen atom of O=C=NBr and isocyanate is formed. Now alkyl or aryl groups are attached directly to nitrogen atoms.              

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

Applications of Hoffmann bromamide degradation reaction

Some of the applications of the mechanism are discussed below- 

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 the symmetrical structure of -phenyl propanamide. 

Limitations of Hoffmann bromamide reaction 

No theory is perfect and comes with its own set of limitations. The essential step is to identify and learn about the limitations for further rework. The limitation of the Hoffmann Bromamide Reaction Mechanism is are as follows- 

Conclusion:

This article focuses on Hoffmann Bromamide Reaction, its mechanism, uses/applications, and limitations to help you grasp the concept easily. The proper understanding of concepts is a crucial part of learning.