Differences between Photochemical and Thermal Reactions

When a molecule is irradiated with a frequency that matches the energy difference between the ground and excited state, a photochemical reaction may occur. Photochemical reactions differ significantly from thermal reactions. 

Excited-state molecules are more reactive than the corresponding ground-state molecules because  

Thermodynamically-favourable reactions involve a decrease in Gibbs free energy, AG , for the reaction. Thus, potentially more products (Pi, P2 and P3) are available via the photochemical route than via the thermal route (Pi). 

Whereas a thermal reaction from R to P2 or P3 involves an increase in Gibbs free energy and so will not occur spontaneously, the photochemical reactions from R involve a decrease in free energy and so are more likely to occur spontaneously. This allows the photochemical production of so-called energy-rich or strained compounds such as P2 and P3 to be carried out at low temperatures, as these products may undergo decomposition at higher temperatures. 

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Throughout this section on the differences between photochemical and thermal reactions, mention has been made of the electronically excited states that are key species in photoprocesses. We need now to look in more detail at the production ol such excited states by absorption of light, and at the nature of the excited states of organic molecules. 

The main difference between photochemical and thermal reaction is the presence of a radiation-activated step. The rate of reaction of this step is proportional to the local volumetric rate of energy absorption (LVREA). For any emission model, the LVREA is a function of the spatial variables, of the physical properties and geometrical characteristics of the lamp-reactor system, and some physicochemical properties of the reacting mixture. The most important design parameter that is pertinent in photochemical and photocatalytic reactions is the effective attenuation coefficient. 

Relate the principal differences between photochemical reactions and thermal reactions and explain these differences in terms of excited-state and ground-state species. 

A volume profile for the reactions in Eq. (35) was constructed [119] with the aid of partial molar volume measurements on both the isomers, and is presented in Figure 27. The profile clearly demonstrates the dissociative nature of both the photochemical and thermal isomerization reactions, the large difference in the partial molar volume of the transition states being the effect of electrostriction and the difference in partial molar volume between Co11 and Co111. 

The differences between thermally and photochemically induced reactions of 2 are illustrated by the photolyses of cyclotrisilane 2 in the presence of disparate isocyanides. While in the thermally induced process ring-enlarged molecules can be isolated, the photolytic process results in the formation of 2,4-disila- and 3,4-disilacyclobutanediimines. A plausible explanation for the formation of these... 

Understand the differences between the reaction profiles of a photochemical reaction and a thermal reaction. 

For ligand substitution and substitution-related (isomerisation and racemisation) reactions of complexes in solution, the difference between the thermal and photochemical reactions may be explained as ... 

Photochemical cycloaddition reactions between sydnones (1) and 1,3-dipolarophiles take place to give products which are different from, but isomeric with, the thermal 1,3-dipolar cycloaddition products. These results are directly interpreted in terms of reactions between the 1,3-dipolarophiles and Ae nit mine (316). The photochemical reactions between sydnones and the following 1,3-dipolarophiles have been reported dicyclopentadiene, dimethyl acetylene dicarboxylate, dimethyl maleate, dimethyl fumarate, indene, carbon dioxide, and carbon disulfide. ... 

In addition to these differences between excited-state and ground-state properties that influence chemical behaviour, there are some practical considerations that give photochemistry its distinctive features. In a thermal reaction, heat energy is normally supplied in an indiscriminate way to all the species in the reaction mixture— substrates, solvent and products—and this makes it difficult, for example, to prepare heat-sensitive compounds. In a photochemical reaction light can. in principle, be supplied selectively to just one... 

Many thermal reactions are effectively irreversible under the conditions employed, but some are reversible and an equilibrium position is reached between substrates and products. The position of equilibrium depends on the standard free energy difference between the two (AC - = - RT In K and on reagent concentrations, and A varies with temperature. Such considerations rarely apply to photochemical reactions, the overwhelming maiority of which are effectively irreversible (1.3), and the products are not in thermodynamic... 

Such reactions may occur thermally or photochemically, and the differences between the two normally show up in two ways. First, in a thermal reaction the direction of change will be towards the equilibrium position, favouring the more thermodynamically stable compound. whereas in a photochemical reaction the direction of change will be towards a photostationary state that favours the compound with the lower absorption coefficient at the wavelength of irradiation. It is therefore normal for conjugated dienes to be converted efficiently into cyclobutenes using wavelengths that are absorbed bv the diene but not by the cydoalkene 12.12). 

The second difference between thermal and photochemical elec-trocyclic processes is seen in the stereochemical course of reaction both types of process are stereospecific, but for a given system of electrons the thermal and photochemical specificities are in opposite senses. The relevant stereochemical feature is the relationship... 

Brown and Edwards have studied the photochemical reaction of ethyl azidoformate with dihydropyran and have isolated in good yield the very reactive aziridine 115. The thermal reaction between the two compounds, however, takes a completely different course via triazoline to imino lactone. 

This expression is different from that first proposed by Bodenstein.28) Alyea and Lind2 found the same rate law for the reaction induced by a-particle bombardment, except that the specific ionization (number of ion pairs produced) replaced 70 in eq. (3-j). Bodenstein, Lenher, and Wagner28 further examined the photochemical reaction between 200 and 300°C and the thermal reaction at temperatures over 400°C. They proposed the mechanism... 

In the general discussion of the differences between thermal and photochemical reactions (section 4.1), it was mentioned that the primary product of the latter is often a high-energy product which retains a large part of the electronic excitation energy of the reactant. This is precisely the case when a biradical is formed in an unconcerted isomerization reaction, and such a biradical is of course a genuine chemical intermediate. 

There is an advantage to these photodissociation reactions that produce a 16e intermediate because thermal reactions of these complexes proceed by associative pathways (19,22). The low energy associative thermal path is attributed (19,22) to the ability of the heterodiene ligands to accept an electron pair in the Sjj2 transition state. Mechanistic differences between thermal and photochemical CO replacements can lead to reactivity differences because of different steric requirements of the intermediates, e.g., Equation 11. 

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