The presence of different functional groups can significantly affect the mechanism of photochemical reactions in organic compounds. Functional groups are specific atoms or groups of atoms within a molecule that determine its chemical properties and reactivity. In photochemical reactions, the absorption of light energy leads to the formation of excited states, which can then undergo various chemical transformations. The presence of different functional groups can influence the absorption properties, excited state lifetimes, and the pathways of these transformations. Here are some specific examples:1. Carbonyl group C=O : The presence of a carbonyl group in a molecule can lead to the formation of excited states with n-* and -* transitions. These excited states can undergo various reactions, such as Norrish Type I and Type II reactions. In Norrish Type I reactions, the excited carbonyl group undergoes homolytic cleavage of the C-H bond, generating radicals that can further react. In Norrish Type II reactions, the carbonyl group interacts with a nearby hydrogen atom, leading to hydrogen abstraction and the formation of a biradical intermediate.Example: Acetone CH3-CO-CH3 can undergo Norrish Type I reaction upon photoexcitation, leading to the formation of methyl and acetyl radicals.2. Aromatic compounds: Aromatic compounds, such as benzene and its derivatives, can undergo photochemical reactions due to the presence of electrons. Upon absorption of light, these compounds can form excited singlet and triplet states, which can then undergo various reactions, such as electrophilic aromatic substitution, cycloaddition, or rearrangement reactions.Example: Benzophenone C6H5-CO-C6H5 can undergo a [2+2] cycloaddition reaction upon photoexcitation, leading to the formation of a cyclobutane derivative.3. Alkenes C=C : Alkenes can undergo photochemical reactions due to the presence of electrons. Upon absorption of light, alkenes can form excited singlet and triplet states, which can then undergo various reactions, such as cis-trans isomerization, dimerization, or addition reactions.Example: Maleic acid HOOC-CH=CH-COOH can undergo a photochemical cis-trans isomerization reaction, leading to the formation of fumaric acid HOOC-CH=CH-COOH .4. Nitro group NO2 : The presence of a nitro group in a molecule can lead to the formation of excited states with n-* and -* transitions. These excited states can undergo various reactions, such as reduction, fragmentation, or rearrangement reactions.Example: Nitrobenzene C6H5-NO2 can undergo a photochemical reduction reaction upon photoexcitation, leading to the formation of nitrosobenzene C6H5-NO and eventually aniline C6H5-NH2 .In summary, the presence of different functional groups in organic compounds can significantly affect the mechanism of photochemical reactions by influencing the absorption properties, excited state lifetimes, and the pathways of chemical transformations.