Question

A tertiary amine reacts with hydrogen peroxide to form a tertiary amine oxide.

Tertiary amine oxides undergo a reaction similar to the Hofmann elimination reaction (Section 10.10), called a Cope elimination. In this reaction, a tertiary amine oxide, rather than a quaternary ammonium ion, undergoes elimination. A strong base is not needed for a Cope elimination because the amine oxide acts as its own base.

Does the Cope elimination have an alkene-like transition state or a carbanion-like transition state?

Accepted Answer

Understand the Cope elimination reaction: In this reaction, a tertiary amine oxide undergoes elimination to form an alkene and a hydroxylamine. The reaction is intramolecular and does not require an external base because the amine oxide acts as its own base. Recall the nature of the transition state in elimination reactions: In an E2 elimination, the transition state typically involves the simultaneous breaking of the C-H bond and the C-X bond (where X is the leaving group). This results in the formation of a double bond (alkene). Analyze the role of the amine oxide: The oxygen in the amine oxide is highly electronegative and can abstract a proton from the β-carbon. This leads to the formation of a double bond between the α- and β-carbons, while the leaving group is the hydroxylamine. Determine the type of transition state: Since the reaction involves the concerted breaking of bonds and the formation of a double bond, the transition state resembles an alkene-like structure. This is because the electrons are delocalized in a planar arrangement, similar to the π-bond in an alkene. Conclude: The Cope elimination has an alkene-like transition state, not a carbanion-like transition state, because the reaction proceeds through a concerted mechanism where bond-breaking and bond-forming occur simultaneously, avoiding the formation of a carbanion intermediate.

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Key Concepts

Cope Elimination

Transition States

Amine Oxides