Energy diagram illustrating the relationship between thermal and photochemical reactions

Mention should be made of several pitfalls that can render the interpretation of photochemical reactions subject to some uncertainties  

2) Is the reaction concerted As was emphasized in the preceding chapter, orbital symmetry considerations apply only to concerted reactions. The possible involvement of triplet excited states and, as a result, a nonconcerted process is much more common in photochemical reactions than in thermal processes. A concerted mechanism must be established before the orbital symmetry rules can be applied. 

The photochemistry of carbonyl compounds has been extensively studied both in solution and in the gas phase. It is not surprising that there are major differences between the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collisions, whereas in the liquid phase, the energy is rapidly transferred to the solvent or to other components of the solutions. Solution photochemistry will be emphasized here, since most organic chemists interested in either mechanistic studies or preparative photochemistry have studied this aspect of the problem. 

An alternative excited state available to carbonyl compounds involves promotion of a bonding n--electron to the antibonding ir -orbital. This is called a tt-tt transition, and is most likely to occur when the ketone group is conjugated with an extensive 7r-bonding system. 

It is not possible to draw unambiguous Lewis structures of excited states of the sort that are so useful for depicting ground-state chemistry. Instead, it is common to asterisk the normal carbonyl structure and provide information about the multiplicity of the excited state if it is available  

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Fig. 11.5. Energy diagram illustrating the relationship between thermal and photochemical reactions.

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