Hofmann Elimination - Rule, Mechanism & Example

Hofmann elimination reactions are the elimination reactions of quaternary ammonium salts producing tertiary amines and alkenes. It is also known as Hofmann exhaustive methylation and Hofmann degradation. The products of this reaction tertiary amines and alkenes are known as Hofmann products. Silver hydroxide and heat is used in the reaction to get the product. This reaction was given by August Wilhelm Von Hofmann in 1851.

Hofmann Elimination Reaction - 

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Hofmann Elimination of Cyclic Amines – 

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Hofmann Rule

According to Hofmann rule the less stable alkene will be the main product in Hofmann Elimination reactions or other such kinds of elimination reactions. This rule is used in the prediction of regioselectivity of elimination reactions. 

So, when you have a bulky leaving group in the elimination reaction, the least substituted alkene will be the main product. 

Examples of reactions which follow Hofmann rule - 

Cope elimination also follows Hofmann rule. 

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Reaction - 

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Hofmann Elimination Mechanism 

Hofmann Elimination Reaction Mechanism involves E2 elimination mechanism. It involves 3 steps. The Hofmann exhaustive methylation mechanism starts with formation of the ammonium iodide salt then ammonium iodide salt reacts with silver oxide and gives silver iodide as precipitate. Deprotonation of water also takes place by silver oxide. It results in the formation of hydroxide ions. Now, this mixture is heated. This facilitates the elimination reaction and gives rise to alkene. 

Here we have explained all three steps in detail – 

Step 1. 

Ammonium iodide salt is formed in this step. Tertiary amine reacts with methyl iodide and forms ammonium iodide salt. 

Reaction – 

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Step 2. 

In this step substitution of iodide anion with hydroxide formed by deprotonation of water molecules. 

Reaction – 

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Step 3. 

In this step reaction mixture is heated to start elimination reaction and to get the required product alkenes. 

Reaction - 

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In the above step the tertiary amine product is also formed. 

E2 transition state has been shown in the diagram below – 

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Three steps involved in the Hofmann elimination mechanism have been explained above can be explained in the following way as well – 

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Uses of Hofmann Elimination Reaction –

  • The primary use of Hofmann Elimination reaction is to synthesize alkenes. 

  • Physiological importance of Hofmann elimination reaction – the design of skeletal muscle relaxant is superior to tubocurarine because of its less cardiac side effects and self-destruction mechanism into blood by Hofmann Elimination reaction. 

  • It has uses in the pharmaceutical field as well. 

  • It is used in the synthesis of anthranilic acid. 

  • It is used in the formation of primary reactants involved in the production of benzene and its derivatives. 

  • It is used in the production of precursors of tryptophan. 

  • It is used in the production of sweetening agents.

Hofmann Elimination reaction is an important name reaction for CBSE Class 12 Board examination perspective. Generally, direct questions are asked related to name reactions in class 12 CBSE board exams. Although, this reaction is an important name in competitive exams point of view as well. Generally, questions based on Hofmann reactions are asked in JEE and NEET exams as well. So, you should understand this reaction nicely and should practice its mechanism many times. Still if you have any doubts refer free PDFs of study material and NCERT Solutions of class 12 Chemistry available on Vedantu website.