The mechanism of a photochemical reaction in organic compounds differs from a thermal reaction in several ways. 1. Energy source: Photochemical reactions are initiated by the absorption of light energy photons , while thermal reactions are driven by heat energy.2. Excited states: In photochemical reactions, the absorption of light promotes the reactant molecules to an excited electronic state. This excited state has a different energy distribution and reactivity compared to the ground state, which is involved in thermal reactions.3. Reaction pathways: Due to the involvement of excited states, photochemical reactions can follow different reaction pathways compared to thermal reactions. These pathways may involve intermediates or transition states that are not accessible in thermal reactions.4. Selectivity: Photochemical reactions can be more selective than thermal reactions because they can be controlled by the wavelength of light used. This allows for the selective excitation of specific chromophores within a molecule, leading to specific reaction pathways.5. Quantum yield: The efficiency of a photochemical reaction is described by its quantum yield, which is the ratio of the number of molecules that undergo the reaction to the number of photons absorbed. This is different from the rate of a thermal reaction, which depends on the temperature and the activation energy of the reaction.Several factors influence the efficiency of a photochemical reaction:1. Absorption of light: The efficiency of a photochemical reaction depends on the ability of the reactant molecules to absorb light. The absorption depends on the wavelength of light and the presence of chromophores within the molecule.2. Quantum yield: A high quantum yield indicates that a large proportion of the absorbed photons lead to a productive reaction. Factors affecting quantum yield include the nature of the excited state, the presence of competing non-radiative decay pathways, and the stability of the reaction intermediates.3. Concentration of reactants: The efficiency of a photochemical reaction can be influenced by the concentration of the reactants. High concentrations can lead to increased light absorption and higher reaction rates, but they can also cause side reactions or quenching of the excited state.4. Solvent effects: The choice of solvent can have a significant impact on the efficiency of a photochemical reaction. Solvents can affect the absorption of light, the stability of the excited state, and the rate of the reaction.5. Temperature: Although photochemical reactions are primarily driven by light, temperature can still play a role in the efficiency of the reaction. Higher temperatures can increase the rate of the reaction, but they can also lead to increased non-radiative decay pathways and decreased quantum yield.6. Presence of catalysts or sensitizers: Catalysts or sensitizers can be used to increase the efficiency of a photochemical reaction by facilitating the transfer of energy between the excited state and the reactant molecules or by stabilizing the reaction intermediates.