Page 1089 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1074 valence shell but with an additional diffuse distribution of the excited electron. In many
molecules, there is mixing of character of valence shell excited states with Rydberg
CHAPTER 12 states, and the latter can decay to valence shell excited states.
Photochemistry Table 12.1 lists the general regions of absorption for the classes of organic
molecules that are discussed in this chapter. A number of light sources can be used,
the most common for preparative scale work being mercury vapor lamps, which emit
mainly at 214, 254, 313, and 366 nm. The composition of the radiation reaching the
sample can be controlled by filters. For example, if the system is constructed so that
light passes through standard glass, only wavelengths longer than 300–310 nm reach
the sample because the glass absorbs below this wavelength. Pure fused quartz, which
transmits down to 200 nm, must be used if the 254-nm radiation is desired. Other
glasses have cutoff points between those of quartz and standard glass. Filter solutions
that absorb in specific wavelength ranges can also be used to control the energy of
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the light reaching the sample. Mechanistic studies are frequently done using lasers,
which permits intense radiation at specific wavelengths and for brief periods of time.
The energy supplied by a particular wavelength of light can be calculated from
the fundamental equation
E = h (12.1)

The energy in kcal/mole is
4
E = 2 86×10 /
where is wavelength in nm. Thus, light of = 254nm equals 112.6 kcal/mol, an
energy sufficient to rupture most single bonds. The energy is also often expressed in
terms of the frequency of the light:
−1
E cm = 349 8cm −1 /kcal/mol
Energy is also some times expressed in eV, where 1eV = 23 14kcal/mol.
When a quantum of light is absorbed, the electronic configuration changes to
correspond to an excited state. Three general points about this process should be
emphasized:

Table 12.1. Approximate Wavelength Ranges for
Lowest-Energy Absorption Band of Representative
Organic Compounds

Reactant Absorption maxima (nm)
Monoalkenes 190–200
Acyclic dienes 220–250
Cyclic dienes 250–270
Aryl-substituted alkenes 270–300
Saturated ketones 270–280

-Unsaturated ketones 310–330
Benzene derivatives 250–280
Aromatic ketones and aldehydes 280–300

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Detailed information on the emission characteristics of various sources and the transmission properties
of glasses and filter solutions can be found in A. J. Gordon and R. A. Ford, The Chemist’s Companion,
Wiley-Interscience, New York, 1972, pp. 348–368 and in S. L. Murov, I. Carmichael, and G. L. Hug,
Handbook of Photochemistry, 2nd Edition, Marcel Dekker, New York, 1993.