The photochemical reaction that occurs when a simple alkene is exposed to ultraviolet UV radiation is known as the photochemical cis-trans isomerization. This reaction involves the conversion of a cis-alkene to a trans-alkene or vice versa. The mechanism of this reaction can be explained through a step-by-step process involving photochemical intermediates.1. Absorption of UV radiation: When a simple alkene is exposed to UV radiation, it absorbs the energy and enters an excited state. The energy absorbed promotes an electron from the bonding orbital to the antibonding * orbital, creating an excited state to * transition.2. Formation of the diradical intermediate: The excited state alkene undergoes a change in its electronic configuration, leading to the formation of a diradical intermediate. This diradical intermediate has two unpaired electrons, one on each carbon atom of the double bond. The diradical intermediate is in a planar sp2 hybridized configuration.3. Rotation around the single bond: In the diradical intermediate, the bond between the two carbon atoms becomes a single bond, allowing for free rotation around it. This rotation enables the molecule to convert from the cis to the trans configuration or vice versa.4. Recombination of the diradical intermediate: After the rotation, the two unpaired electrons in the diradical intermediate can recombine to form a new bond, resulting in the formation of the isomerized alkene. The molecule returns to its ground state, releasing the excess energy as heat or light fluorescence or phosphorescence .5. Overall reaction: The overall photochemical cis-trans isomerization involves the conversion of a cis-alkene to a trans-alkene or vice versa, with the help of UV radiation and the formation of a diradical intermediate.In summary, the photochemical reaction of a simple alkene exposed to ultraviolet radiation involves the absorption of UV light, formation of a diradical intermediate, rotation around the single bond, recombination of the diradical intermediate, and the formation of the isomerized alkene. This mechanism allows for the interconversion of cis and trans isomers in alkenes.