Which energy diagram best represents the given reaction ?

Options

A.
B.
C.
D.

Answer

D

Explanation

It is possible to interconvert cis and trans alkenes, but the it-bond must be broken first. This requires a considerable amount of energy - around `260 "kJ mol"^(-1)` . One way to break the `pi` -bond would be to promote an electron from the 1r -orbital to the `pi^(**)` -orbital. If this were to happen, there would be one electron in the bonding it -orbital and one in the antibonding `pi^(**)` -orbital and hence no overall bonding. Electromagnetic radiation of the correct energy could promote the electron from HOMO to LUMO. The correct energy actually corresponds to light in the ultraviolet (UV) region of the spectrum. Thus, shining UV light on an alkene would promote an electron from its bonding `pi` -molecular orbital to its antibonding `pi^(**)` -molecular orbital, thereby breaking the it `pi` -bond (but not the o-bond) and allowing rotation to occur.