The mechanism of photochemical reactions in organic compounds differs from thermal reactions in several ways. Photochemical reactions involve the absorption of light energy by molecules, while thermal reactions are driven by heat energy. Here are some key differences between the two types of reactions:1. Initiation: Photochemical reactions are initiated by the absorption of a photon by a molecule, which promotes an electron to a higher energy level, creating an excited state. In thermal reactions, the energy required for the reaction is provided by heat, which increases the kinetic energy of the molecules, allowing them to overcome the activation energy barrier.2. Reaction pathways: Photochemical reactions often follow different reaction pathways compared to their thermal counterparts. This is because the excited state of a molecule can have different properties and reactivity than the ground state. As a result, photochemical reactions can lead to the formation of products that are not typically observed in thermal reactions.3. Selectivity: Photochemical reactions can be more selective than thermal reactions, as they can be controlled by the wavelength of light used to initiate the reaction. By choosing a specific wavelength, chemists can selectively excite certain molecules or functional groups, while leaving others unaffected.4. Quantum yield: The efficiency of a photochemical reaction is often 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 can be influenced by several factors, such as the absorption spectrum of the reactants, the efficiency of the energy transfer, and the rate of competing processes e.g., fluorescence, phosphorescence, or non-radiative decay .Factors that influence the efficiency of photochemical reactions include:1. Absorption of light: The efficiency of a photochemical reaction depends on the ability of the reactants to absorb light at the wavelength used for the reaction. Molecules with strong absorption bands at the chosen wavelength will be more likely to undergo the reaction.2. Concentration of reactants: The efficiency of a photochemical reaction can be influenced by the concentration of the reactants. Higher concentrations can lead to increased probability of photon absorption and reaction, but can also result in more significant side reactions or product inhibition.3. Solvent: The choice of solvent can have a significant impact on the efficiency of a photochemical reaction. Solvents can affect the absorption spectrum of the reactants, the rate of energy transfer, and the stability of the excited state.4. Temperature: Although photochemical reactions are not driven by heat, temperature can still play a role in their efficiency. Higher temperatures can increase the rate of competing thermal reactions or non-radiative decay processes, which can decrease the overall efficiency of the photochemical reaction.5. Presence of catalysts or sensitizers: Catalysts or sensitizers can be used to increase the efficiency of photochemical reactions by facilitating energy transfer or stabilizing the excited state of the reactants.