Photochemical reactions differences between thermal

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. 

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. 

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... 

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. 

The alternative mechanism to CO dissociation, proposed by Stufkens (23) for the DAB complexes, is not consistent with the difference between thermal and photochemical reaction products, Equation 11. In solution Kokkes et al. propose that one end of the DAB chelate dissociates on photolysis. If this were the case it would be difficult to understand why the photochemical reaction (where the DAB ligand is half attached) leads only to CO displacement, while the associative thermal reaction leads only to DAB displacement. Consider the mechanism, Equation 13, established (19) for thermal loss of DAB. The key to DAB loss is formation of the mono-dentate species D of Equation 13. This intermediate is identical to that proposed by Kokkes et al. (23) for photochemical CO replacement. According to their mechanism, Equation 14, the same species D, forms in a two step process and would therefore be thermally equilibrated. Thus the alternative mechanism is not consistent with thermal chemistry of these systems. 

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

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... 

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. 

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). 

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... 

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. 

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