Reactions of Quaternary Ammonium Salts Hofmann Elimination

REACTIONS OF QUATERNARY AMMONIUM SALTS HOFMANN ELIMINATION 

Section 21-5 described how complete alkylation of amines leads to the corresponding quaternary alkylammoninm salts. These species are nnstable in the presence of strong base, because of a bimolecular elimination reaction that famishes alkenes (Section 7-7). The base attacks the hydrogen in the j8-position with respect to the nitrogen, and a trialkylamine departs as a nentral leaving gronp. 

In the procedure of Hofmann elimination, the amine is first completely methylated with excess iodomethane (exhaustive methylation) and then treated with wet silver oxide (a source of HO ) to produce the ammonium hydroxide. Heating degrades this salt to the alkene. When more than one regioisomer is possible, Hofmann elimination, in contrast to most E2 processes, tends to give less substituted alkenes as the major products. Recall that this result adheres to Hofmann s rule (Section 11-6) and appears to be caused by the bulk of the ammonium group, which directs the base to the less hindered protons in the molecule. 

Give the structures of the possible alkene products of the Hofmann elimination of (a) Af-ethylpropanamine (ethylpropylamine) and (b) 2-butanamine. (Caution In the Hofmann elimination of 2-butanamine, you need to consider several products. Hint Review Section 11-6). 

The Hofmann elimination of amines has been used to elucidate the structure of nitrogen-containing natural products, such as alkaloids (Section 25-8). Each sequence of exhaustive methylation and Hofmann elimination cleaves one C-N bond. Repeated cycles allow the heteroatom to be precisely located, particularly if it is part of a ring. In this case, the first carbon-nitrogen bond cleavage opens the ring. 

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