The mechanism of photochemical reactions in organic compounds with conjugated systems differs from those without due to the nature of their electronic structure and the way they interact with light. Let's first understand the key differences between conjugated and non-conjugated systems.Conjugated systems are characterized by alternating single and double bonds between carbon atoms, where the p-orbitals of the carbon atoms overlap, creating a delocalized -electron cloud. This delocalization of electrons leads to the formation of molecular orbitals that extend over the entire conjugated system. In contrast, non-conjugated systems have isolated double bonds or functional groups that do not share this delocalized electron cloud.Now, let's discuss how these differences affect the photochemical reactions:1. Absorption of light: Conjugated systems generally have lower energy gaps between their highest occupied molecular orbital HOMO and lowest unoccupied molecular orbital LUMO compared to non-conjugated systems. This is because the delocalized -electron cloud allows for better energy distribution. As a result, conjugated systems can absorb lower energy longer wavelength light, while non-conjugated systems require higher energy shorter wavelength light for excitation.2. Excited states: Upon absorption of light, electrons in both conjugated and non-conjugated systems can be promoted from the HOMO to the LUMO, creating an excited state. However, the nature of these excited states is different. In conjugated systems, the excited states are more delocalized and can involve the entire conjugated system, while in non-conjugated systems, the excited states are more localized around the specific functional group or double bond.3. Reaction pathways: The delocalized nature of excited states in conjugated systems allows for various reaction pathways, such as electron transfer, energy transfer, or bond rearrangement. In non-conjugated systems, the reaction pathways are more limited due to the localized nature of the excited states. Additionally, the lower energy gap in conjugated systems can lead to faster reaction rates, as the activation energy required for the reaction is lower.4. Stability of intermediates: Photochemical reactions often involve the formation of reactive intermediates, such as radicals, ions, or excited states. In conjugated systems, these intermediates can be stabilized by the delocalized -electron cloud, which can distribute the charge or unpaired electron over the entire conjugated system. This stabilization can lead to longer lifetimes for these intermediates and can affect the overall reaction mechanism. In non-conjugated systems, the intermediates are less stable and may have shorter lifetimes, leading to different reaction outcomes.In summary, the mechanism of photochemical reactions in organic compounds with conjugated systems differs from those without due to the delocalized nature of the -electron cloud, which affects the absorption of light, the nature of excited states, the available reaction pathways, and the stability of reactive intermediates.