Cope elimination is a reaction which is performed to identify the unknown tertiary amine. It involves treatment of a tertiary amine with hydrogen peroxide to obtain amine oxide which on heating gives N,N-diakylhydroxyl amine and an alkene. In other words a new variant of the Hofmann's elimination involves the pyrolysis of an amine oxide prepared by the action of H2O2 on a tertiary amine. This elimination which is called Cope elimination which occurs at rather mild temperature.
In general the cope elimination reaction is written as
MechanismBack to Top
Cope reaction is believed to take place through a cyclic transition state as depicted below.
Cope elimination occurs under milder conditions than Hofmann elimination. It is particularly useful when a snesitive or reactive alkene must be synthesized by the elimination of an amine.
It is to be noted that cope elimination is cis and needs lower temperatures than the pyrolysis of quatenary ammonium hydroxides.
ApplicationsBack to Top
The synthetic utility of the cope elimination is comparable to the Hofmann elimination of quatenary ammonium hydroxides, but takes place at lower temperatures. The Cope elimination is almost free of side reactions due to the intramolecular nature of the elimination. However, in certain cases, the product alkene may isomerize to the more stable conjugated system, and allyl-or benzyl migration is sometimes observed to give O-allyl or benzyl substituted hydroxyamines. Cyclic amine oxides can also be pyrolysed but with 6-membered rings the reaction is usually low-yielding or does not occur.
Amine oxides are well known as surfactants that can be used in a wide variety of applications. In order to illustrate the large number of uses for amine oxides. A key use of amine oxide surfactants is in such applications as laundery detergents, sanitizers, clearners, emulsifiers and personal care compositions.
The direction of the cope elimination is governed almost entirely by the number of hydrogen atoms at the various $\beta$-positions, and therefore there is no preference for the formation of the least substituted alkene unlike in the Hofmann elimination reaction. Upon pyrolysis N-cyclohexyl derivatives however form predominantly exocyclic olefins, since the formation of the endocyclic double bond would require the cyclohexane ring to be almost planar in the transition state.