Hofmann Elimination

Hofmann Elimination Definition:


Tetra-alkyl ammonium salts undergo elimination to form the least substituted alkene. This is called the Hofmann elimination.



Hofmann Elimination Explained:


Amines, like alcohols, contain a poor leaving group which must be converted into the better leaving group. This must be accomplished by treating the amine with an excess of methyl iodide. This method is called Hofmann elimination which converts an amine into a quaternary ammonium salt prior to β elimination.




The ammonium salt converts into another one with a different anion. The silver(I) oxide (Ag2O) replaces the iodide anion with hydroxide anion (a strong base).



When this salt is heated, hydroxide anion removes a proton from the β carbon atom by the mechanism of E2 forming the new π bond of the alkene.



As we can see, product (alkene) is opposite of the Zaytsev product (more substituted alkene). In some cases, when more than one regioisomer is possible, Hoffmann elimination, in contrast to most E2 processes, tends to give less substituted alkenes as the major product.