Questions: Electrocyclic and Sigmatropic Reactions
4 questions to test your understanding
Score: 0 / 4
Question 1 Multiple Choice
Under thermal conditions, how does a conjugated triene (6 π electrons) close to a cyclohexadiene?
AConrotatory
BDisrotatory
CEither, with equal ease
DIt cannot close thermally
For 4n + 2 π electrons, thermal electrocyclic reactions are disrotatory. The HOMO of a hexatriene has its terminal lobes in phase on the same face, so turning both ends in opposite directions brings matching lobes together.
Question 2 True / False
A photochemical electrocyclic ring closure follows the same rotational mode as the corresponding thermal reaction.
TTrue
FFalse
Answer: False
Light promotes an electron into the next orbital, which becomes the new HOMO and has the opposite terminal symmetry. The allowed rotation therefore switches between conrotatory and disrotatory.
Question 3 Multiple Choice
Heating an allyl vinyl ether gives a γ,δ-unsaturated carbonyl compound. What is this reaction?
AA [1,5] hydrogen shift
BA Diels–Alder cycloaddition
CA [3,3] Claisen rearrangement
DAn electrocyclic ring opening
The Claisen rearrangement is a [3,3] sigmatropic shift. The C–O σ bond breaks, a new C–C σ bond forms three atoms along each fragment, and the vinyl ether becomes a carbonyl.
Question 4 Short Answer
Explain why the thermal ring closure of butadiene to cyclobutene is conrotatory.
Think about your answer, then reveal below.
Model answer: In the ground state, the HOMO of butadiene is ψ₂, whose terminal p lobes have opposite phase on the same face. Only turning both ends in the same direction (conrotation) brings same-phase lobes together to form the new σ bond.
The stereochemistry follows from the symmetry of the HOMO's end lobes. This is the frontier-orbital view of the Woodward–Hoffmann rules.