Question

cis-3,4-Dimethylcyclobutene undergoes thermal ring opening to form the two products shown. One of the products is formed in 99% yield, the other in 1% yield. Which is which?

Short answer

Under thermal conditions, the conrotation ring closing is the major product. The conrotation ring-closing can take place when the substituents are cis to each other, that means are in the same direction as shown below:

Step-by-step solution

Conrotatory ring closure

Under thermal conditions, the conrotation ring closing is the major product. The conrotation ring-closing can take place when the substituents are cis to each other, that means are in the same direction as shown below:

Disrotatory ring closure

The disrotatory ring-closing under thermal conditions is minor product. In disrotatory ring-closing, the substituents are trans to each other as shown below: