Problems and exercisesHofmann Degradation
1. Hofmann degradation in the decalins
Decalin (C10H18) is a bicycloalkane made of two fused cyclohexane rings that share a bond. It is the compound obtained by complete hydrogenation of naphthalene, which is why it is also called decahydronaphthalene. Consider trans-decalin bearing a trimethylammonium group (NMe3+) at position 2. One of the two isomers, 2-(R) or 2-(S), undergoes Hofmann elimination, whereas the other does not react at all. Explain this fact. Why does the same not happen in cis-decalin, where both the 2-(R) and 2-(S) isomers undergo Hofmann elimination?
2. Halides and ammonium salts
The cis isomer of 1-bromo-4-tert-butylcyclohexane undergoes elimination (E2) about 106 times faster than the trans isomer. However, neither cis- nor trans-N,N-dimethyl-4-tert-butylcyclohexylamine undergoes Hofmann degradation when heated with a base. Explain these facts.
3. Which alkene is formed?
Give the main alkene obtained in the Hofmann degradation of 2-pentylamine (exhaustive methylation, Ag2O and heat). Is it the same as the one given by dehydration of 2-pentanol?
4. Double degradation
What final product is obtained after two successive Hofmann degradations of N-methylpiperidine?