The mechanism of photochemical reactions in aromatic compounds differs from that in aliphatic compounds due to their distinct molecular structures and electronic configurations.1. Electronic transitions: Aromatic compounds, such as benzene, have a conjugated system of alternating single and double bonds, which allows for the delocalization of electrons. This delocalization results in the formation of molecular orbitals that can undergo electronic transitions upon absorption of light. In contrast, aliphatic compounds lack this conjugated system, and their electronic transitions are mainly limited to n-* and n-* transitions.2. Excited states: Upon absorption of light, aromatic compounds can form singlet and triplet excited states. The singlet excited state can undergo intersystem crossing to form a more stable triplet excited state. In aliphatic compounds, the excited states are generally less stable and have shorter lifetimes.3. Reaction pathways: Aromatic compounds can undergo various photochemical reactions, such as electrophilic aromatic substitution, energy transfer, and electron transfer processes. These reactions can lead to the formation of new chemical bonds or the rearrangement of existing ones. In contrast, aliphatic compounds primarily undergo reactions involving the cleavage of chemical bonds, such as homolytic or heterolytic bond cleavage, leading to the formation of radicals or ions.4. Photostability: Aromatic compounds are generally more photostable than aliphatic compounds due to their extended conjugation and resonance stabilization. This means that aromatic compounds are less likely to undergo photochemical degradation compared to aliphatic compounds.In summary, the mechanism of photochemical reactions in aromatic compounds differs from aliphatic compounds due to their unique electronic transitions, excited state properties, reaction pathways, and photostability. These differences are mainly attributed to the presence of a conjugated system in aromatic compounds, which allows for the delocalization of electrons and influences their photochemical behavior.