Review and Practice Questions
Review:
Electrocyclic reactions are a type of pericyclic reaction in which a conjugated π-system undergoes a ring closure or ring opening through a cyclic transition state. These reactions are highly stereospecific and follow the Woodward-Hoffmann rules, which predict whether the reaction will proceed via conrotatory or disrotatory motion based on the electron count and reaction conditions (thermal vs. photochemical). The Frontier Molecular Orbital (FMO) theory and Walsh diagrams help explain how orbital symmetry dictates the stereochemical outcome. The transition state can also be analyzed using Hückel vs. Möbius aromaticity models, where aromatic transition states favor allowed pathways.
Several experimental methods can be used to analyze electrocyclic reactions. UV-Vis spectroscopy helps track conjugation changes in photochemical electrocyclizations, while NMR spectroscopy (¹H and NOESY) determines stereochemistry by identifying spatial proton interactions. Kinetic and thermodynamic studies, such as activation energy (Ea) and entropy changes (ΔS), provide insights into the feasibility and mechanistic nature of these reactions.
Questions:
1) What type of motion (conrotatory or disrotatory) occurs in the photochemical ring opening of cyclobutene?
Solution:
Electron count: 4 π-electrons in product (1,3-butadiene)
Photochemical condition → 4n system
Rule: 4n under photochemical → disrotatory
Answer: Disrotatory
2) Draw the thermal ring closure product of the following substituted 1,4-pentadiene. Label stereochemistry if relevant.
Solution:
Thermal closure of a 4π system → conrotatory
Terminal groups rotate same direction
Methyl group ends up cis to hydrogen on ring
Answer: Product is cis-3-methylcyclobutene
3) Will the thermal ring opening of cis-3,4-dimethylcyclobutene result in an (E,E), (Z,Z), or (E,Z) diene?
Solution:
Starting material: 4 π-electron system in ring → thermal → conrotatory
In conrotatory motion, cis substituents rotate in the same direction
This leads to one substituent pointing up and the other down → (E,Z) stereochemistry
Answer: (E,Z)-2,4-hexadiene
4) A compound with a 6 π-electron triene system undergoes ring closure to a cyclohexadiene under UV light.
Will the reaction proceed via conrotatory or disrotatory motion?
Solution:
6 π-electrons → 4n+2 system
Answer: Conrotatory
5) Is the thermal ring closure of 1,3,5,7-octatetraene symmetry-allowed or forbidden?
Solution:
Electron count: 8 π → 4n (n = 2)
Thermal condition → conrotatory motion allowed for 4n
Answer: Allowed, proceeds via conrotatory mechanism