\b[A to B is a photochemical six-electron electrocyclic ring opening; B to C is a thermal six-electron electrocyclic ring closure.] The broken or formed sigma bond is between the two methyl-bearing saturated carbons of the cyclohexadiene. Both transformations conserve the <carbon> skeleton. The light-induced and thermal processes differ in their orbital occupation and therefore in their stereochemical selection rule.

Solution

Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 44 / b / iii / Solution

 Solution (source code)