Cope rearrangement: practice and mechanism
The σ bond between C3 and C4 of the 1,5-diene breaks, a new one forms between C1 and C6, and both double bonds move along by one position ([3,3]-sigmatropic).
Example
A 1,5-diene rearranges on heating through a chair-like six-membered transition state ([3,3]-sigmatropic): the σ bond between C3 and C4 breaks, a new one forms between C1 and C6 and both double bonds move. The more stable, more substituted 1,5-diene forms.
Mechanism step by step
- 1
On heating the 1,5-diene adopts a chair-like conformation in which C1 and C6 are close together.
- 2
In one concerted step the σ bond C3–C4 breaks and the new σ bond C1–C6 forms.
- 3
The π bonds shift to C2=C3 and C4=C5: again a 1,5-diene, usually the more substituted one.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Not just the double bonds move. In the Cope rearrangement a C–C bond breaks and a new one forms; the carbon skeleton changes.
- No ring forms. The new C1–C6 bond forms while the C3–C4 bond breaks; the product is again an open-chain 1,5-diene.
Often confused with: Claisen rearrangement · Oxy-Cope rearrangement · Diels–Alder reaction
Frequently asked questions
What does [3,3]-sigmatropic mean?
A σ bond "moves": it breaks between atoms 1 and 1′ and re-forms between atoms 3 and 3′, counting from the old bond in both directions.
Cope or Claisen?
Same mechanism. Cope: carbon only (1,5-diene). Claisen: an O in the chain (allyl vinyl ether), and a C=O group forms.