Page 1088 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

12

Photochemistry

Introduction

The photochemical reactions of organic compounds attracted great interest beginning
in the 1960s. As a result, many useful and fascinating reactions were uncovered and
photochemistry is now an important synthetic tool in organic chemistry. A firm basis
for mechanistic descriptions of many photochemical reactions has been developed.
Some of the more general types of photochemical reactions are discussed in the
present chapter. In Section 12.2, the photochemistry of alkenes, dienes, and polyenes is
considered, including the relationship of photochemical reactions to orbital symmetry
principles. Important reaction types include cis-trans isomerization, electrocyclic
reactions, photocycloadditions, and rearrangements. In subsequent sections, character-
istic photochemical reactions of carbonyl compounds and aromatic rings are introduced.

12.1. General Principles

We begin by summarizing the basic elements of photochemical reactions. The
first condition that must be met is that the reactant absorb light emitted by the source.
For light to be absorbed, the compound must have an energy level that corresponds to
the energy of the radiation. Organic photochemical reactions usually involve excited
electronic states. Depending on functionality, organic compounds can have electronic
absorption bands in the ultraviolet and/or the visible region of the spectrum. Most of
the photochemistry we discuss involves unsaturated groups, mainly alkenes, carbonyl
∗
compounds, and arenes, in which an electron is promoted to an antibonding orbital.
These excited states involve promotion of electrons in valence shell orbitals. The
excited states can be singlets or triplets. In a singlet excited state the excited electrons
retain opposite spins, whereas in triplet excited states they have parallel spins. The
photoexcitation of organic molecules can also involve Rydberg states, which involve
excitation of an electron from the valence level to a higher shell, typically 3s and
3p for organic molecules. The Rydberg states are similar to a radical cation in the
1073