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: B ✓ checked by 4AB · confidence high
Explanation
A photochemical [2+2] cycloaddition is suprafacial on both components, [π2s + π2s]: the excited-state HOMO of one alkene matches the LUMO of the other in phase.