Electrocyclic reaction (6π, thermal): practice and mechanism
The triene closes the ring between its termini. A thermal 6π ring closure is disrotatory, so the two methyl groups from (2E,4Z,6E)-octatriene end up cis.
Example
On heating a conjugated triene closes to a cyclohexadiene: the termini form a σ bond and the double bonds shift. Thermally a 6π system rotates disrotatorily (Woodward–Hoffmann), so (2E,4Z,6E)-octatriene gives cis-5,6-dimethylcyclohexa-1,3-diene.
Mechanism step by step
- 1
The triene adopts the s-cis,s-cis conformation, so that C1 and C6 are close together.
- 2
The termini rotate: thermally in opposite directions (disrotatory), so that lobes of the same phase overlap (HOMO of the 6π system).
- 3
The new σ bond C1–C6 forms and three π bonds become two: cyclohexa-1,3-diene. The stereochemistry follows from the direction of rotation.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- trans would come from conrotatory ring closure, i.e. photochemically (light). Thermally a 6π system rotates disrotatorily: cis.
- A cyclobutene would come from a diene (4π). In the triene all six π electrons take part and a six-membered ring forms.
- Ring closure does not give a benzene ring: only one new σ bond forms while one π bond disappears. An aromatic ring would also need loss of H₂ (oxidation).
- In the product the double bonds are conjugated (1,3-diene) between the former inner carbons. A 1,4-diene would need an extra shift.
Often confused with: Diels–Alder reaction · Catalytic hydrogenation
Frequently asked questions
How do I remember the Woodward–Hoffmann rules?
4n π electrons (e.g. 4π): thermal conrotatory, photochemical disrotatory. 4n+2 (e.g. 6π): the reverse, thermal disrotatory, photochemical conrotatory.
What happens to (2E,4Z,6E)-octatriene under light?
Photochemically it rotates conrotatorily: trans-5,6-dimethylcyclohexa-1,3-diene forms.
When is an electrocyclic reaction disrotatory?
Thermally, 6 π electrons (4n+2) close disrotatorily and 4 π electrons conrotatorily; photochemically it is the other way round.