Photochemical reactions involve the absorption of light energy by molecules, which leads to the formation of excited states and subsequent chemical reactions. The mechanisms of photochemical reactions differ for alkenes, alkanes, and aromatic compounds due to their distinct molecular structures and electronic configurations.1. Alkenes: Alkenes contain a carbon-carbon double bond, which consists of a sigma bond and a pi bond. When alkenes absorb light energy, the electrons can be excited to a higher energy level, forming a * pi-star excited state. This excited state can undergo various reactions, such as: a. Cis-trans isomerization: The excited state can undergo rotation around the double bond, leading to the formation of geometric isomers. b. [2+2] Cycloaddition: Two alkenes can react with each other to form a four-membered ring cyclobutane through a concerted mechanism. c. Intersystem crossing: The excited state can undergo intersystem crossing to a triplet state, which can then react with other molecules via radical mechanisms.2. Alkanes: Alkanes contain only carbon-carbon single bonds. When alkanes absorb light energy, they can undergo homolytic bond cleavage, leading to the formation of carbon-centered radicals. These radicals can then react with other molecules via radical mechanisms, such as: a. Hydrogen abstraction: The carbon-centered radical can abstract a hydrogen atom from another molecule, forming a new radical and a new bond. b. Radical substitution: The carbon-centered radical can react with a halogen molecule, leading to the formation of a new carbon-halogen bond and a halogen radical.3. Aromatic compounds: Aromatic compounds, such as benzene, contain a conjugated system of alternating single and double bonds. The electrons in these systems are delocalized, forming a stable, lower-energy configuration. When aromatic compounds absorb light energy, the electrons can be excited to a higher energy level, forming a * excited state. This excited state can undergo various reactions, such as: a. Electrophilic aromatic substitution: The excited state can react with an electrophile, leading to the formation of a new carbon-electrophile bond and a new aromatic compound. b. [4+2] Cycloaddition Diels-Alder reaction : The excited state can react with a diene, forming a six-membered ring through a concerted mechanism. c. Energy transfer: The excited state can transfer its energy to another molecule, leading to the formation of an excited state in the other molecule, which can then undergo further reactions.In summary, the mechanisms of photochemical reactions in alkenes, alkanes, and aromatic compounds differ due to their distinct molecular structures and electronic configurations. Alkenes undergo reactions involving * excited states, alkanes undergo radical reactions, and aromatic compounds undergo reactions involving * excited states and energy transfer.