Orbital Symmetry Considerations Related to Photochemical Reactions

Orbital Symmetry Considerations Related to Photochemical Reactions 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types considered in the preceding chapter and examining the application of orbital symmetry considerations to the photochemical mode of reaction. 

The case of 2+2 cycloaddition of two alkene units can serve as an example. This reaction was classified as a forbidden thermal reaction on the basis of an orbital correlation diagram (Fig. 11.2) that showed that the ground-state molecules would lead to an excited cyclobutane, and would therefore involve a prohibitive energy requirement. 

How does the situation change when a photochemical reaction involving one ground-state alkene and one excited alkene is to be considered We can assume the same symmetrical approach as in the thermal reaction, so the same array of orbitals is involved. The occupation of the orbitals is different however the tti, (SS) orbital is 

Consideration of the HOMO-LUMO interactions also indicates that the 2-1-2 additions would be allowed photochemically. The HOMO in this case is the excited alkene 7r -orbital. The LUMO is the ir of the ground-state alkene, and a bonding interaction is present between the carbons where new bonds must be formed  

A Striking illustration of the relationship between orbital symmetry considerations and the outcome of photochemical reactions can be found in the stereochemistry of electrocyclic reactions. In Chapter 11, the distinction between the conrotatory and the disrotatory mode of reaction as a function of the number of electrons in the system was described. Orbital symmetry considerations predict, and it has been verified experimentally, that photochemical electrocyclic reactions show a reversal of stereochemistry  

How does the situation change when a photochemical reaction involving one ground state alkene and one excited alkene is to be considered We can assume the 

SECTION 13.2. ORBITAL SYMMETRY CONSIDERATIONS RELATED TO PHOTOCHEMICAL REACTIONS 

A plot of 1 versus quencher concentrations, [Q], then gives a line with the slope k /k. It is usually possible to assume that quenching is diffusion-controlled, permitting assignment of a value to k. The rate of photoreaction, k, for the excited intermediate can then be calculated. 

In this chapter, the discussion will center on the reactions of excited states, rather than on the other routes available for dissipation of excess energy. The chemical reactions of photoexcited molecules are of interest primarily for three reasons  

The population of an antibonding orbital in the excited state allows the occurrence of chemical transformations that are electronically not available to ground-state species. 

Either the singlet or the triplet state may be involved in a photochemical reaction, whereas only singlet species are involved in most thermal processes. This permits the formation of intermediates that are unavailable under thermal conditions. 

---

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

---