Page 1133 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1118 electron density from oxygen to carbon. The excited state can be represented as a
hybrid with dipolar and diradical structures.
CHAPTER 12
Photochemistry •
O O+ O O 1.345
hν 1.384
1.393
– •

Although Lewis structures of this type are not entirely adequate descriptions of the
excited states, they do correspond to the MO picture by indicating polarization of
charge and the presence of polar radical-like centers. In addition to the increased
energy content, the high reactivity of the excited states is associated with the presence
of half-filled orbitals. The two SOMO orbitals in the excited states have enhanced
radical, cationic, or anionic character.
For aromatic carbonyl compounds, as for styrene (see p. 1084), there are excited
states associated with the aromatic ring. The absorption spectrum of benzaldehyde, for
example, is believed to include two triplet levels and at least five singlet bands. 105

6.23
∗
5.98 1(π - π )
4.89
4.33
1 ∗
3.71 (n - π )
∗
3.49 3 (π - π )
3 ∗
3.40 (n - π )
Computed absorption
levels in eV for benzaldehyde
For acetophenone, the S state, like acetaldehyde adopts a H−C−C−O staggered
1
conformation, whereas T retains the eclipsed conformation found in the ground
1
state. 106

12.3.1. Hydrogen Abstraction and Fragmentation Reactions

One of the most common reactions of photoexcited carbonyl groups is hydrogen
atom abstraction from solvent or some other hydrogen donor. A second common
reaction is cleavage of the carbon-carbon bond adjacent to the carbonyl group, which
is called -cleavage.

.
X–H .
hν R C OH + X
2
R C O [R C O] ∗ or
2
2
.
RC . O + R
The hydrogen atom abstraction can be either intramolecular or intermolecular. The
intermediates that are generated are free radicals. If these radicals come to thermal
equilibrium, they have the same structure and reactivity as radicals generated by
105 V. Molina and M. Merchan, J. Phys. Chem. A, 105, 3745 (2001).
106
J. L. Tomer, L. H. Spanler, and D. W. Pratt, J. Am. Chem. Soc., 110, 1615 (1988).