Questions & Answers

Write structure of compounds A and B in each of the following reaction:

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Hint: Identify the set of reagents used in the above series of chemical reactions. Bromine in the presence of sodium hydroxide is the characteristic reagent for Hoffmann bromamide reaction. Acetic anhydride in the presence of pyridine is used to produce the acyl ion which acts as an incoming electrophile.

Complete step-by-step answer:
Hoffmann rearrangement is an organic reaction of a primary amide leading to the formation of a primary amine with one fewer carbon atom. Based on this we will write the chemical reaction for benzamide with bromine and sodium hydroxide.

The structure of compound A is given above. The name of the compound is aniline. Aniline i.e. compound A is then exposed to acetic anhydride in the presence of pyridine, we get phenyl ethanamide. This is because the acyl ion acts as an electrophile and attacks the lone pair of electrons present on the nitrogen atom and thus giving compound B. The reaction is given below and compound B as well.

Note: Do not confuse Hoffmann's rule with Hofmann rearrangement. Hoffmann's rule is applied when a peroxide initiator is used. It is widely used in organic reactions. However, Hoffmann bromamide reaction is used specifically for amides. Along with this exists one more reaction i.e. the Hofmann elimination. This elimination reaction is for an amine where the least stable alkene is formed. The Hoffmann reaction series has a reaction mechanism opposite to the series of reactions based on Saytzeff rule. All Hoffmann reactions involve the use of peroxide as an initiator.