The concerted photochemical reaction between two olefins leading to a cyclobutane ring is:
(a)$\pi 2_s + \pi 2_a$ cycloaddition
(b)$\pi 2_s + \pi 2_s$ cycloaddition
(c)$\sigma 2_s + \sigma 2_s$ cycloaddition
(d)$\pi 2_s + \sigma 2_a$ cycloaddition
Answer
Answer (as printed): B
Explanation
Consider the π molecular orbitals (MO) of the olefins, HOMO (π) – the bonding MO and the LUMO (π*) anti-bonding molecular orbital (MO). The electronic transition takes place between HOMO of one alkene and LUMO of other alkene under photochemical conditions only. The molecular orbital diagram given below gives a better understanding.
Explanation as extracted from the printed page; notation may be imperfect.