Page 992 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

976 One aspect of both EPR and CIDNP studies that should be kept in mind is that
either is capable of detecting very small amounts of radical intermediates. Although
CHAPTER 11
this sensitivity makes both techniques very useful, it can also present a pitfall. The most
Free Radical Reactions prominent features of either ESR or CIDNP spectra may actually be due to radicals
that account for only minor amounts of the total reaction process. An example of this
was found in a study of the decomposition of trichloroacetyl peroxide in alkenes.
O O
Cl CCOOCCCl 3 2 Cl C· + 2 CO 2
3
3
Cl C· + CH 2 C(CH ) Cl 3 CCH C(CH )
3 2
3
3 2
2
·
CH 3
Cl C· + Cl CCH C(CH ) Cl CH + Cl CCH C CH 2
2
3 2
3
3
3
2
3
·
In addition to the emission signals of CHCl and Cl CCH C CH
=CH , which are
3
3
2
3
2
the major products, a strong emission signal for Cl CCHCl was identified. However,
2
3
this compound is a very minor product of the reaction and when the signals have
returned to their normal intensity, Cl CCHCl is present in such a small amount that
3 2
it cannot be detected. 18
11.1.4. Generation of Free Radicals
There are several reactions that are used frequently to generate free radicals, both
to study radical structure and reactivity and in synthetic processes. Some of the most
general methods are outlined here. These methods will be encountered again when
we discuss specific examples of free radical reactions. For the most part, we defer
discussion of the reactions of the radicals until that point.
Peroxides are a common source of radical intermediates. Commonly used
initiators include benzoyl peroxide, t-butyl peroxybenzoate, di-t-butyl peroxide, and

t-butyl hydroperoxide. Reaction generally occurs at relatively low temperature (80 −
100 C). The oxygen-oxygen bond in peroxides is weak (∼ 30kcal/mol) and activation

energies for radical formation are low. Dialkyl peroxides decompose thermally to give
two alkoxy radicals. 19
140°C
(CH ) COOC(CH ) 2(CH 3) CO·
3 3
3 3
3
Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are
initially formed lose CO very rapidly. 20 In the case of aroyl peroxides, products can
2
be derived from either the carboxyl radical or the radical formed by decarboxylation. 21
The decomposition of peroxides can also be accomplished by photochemical excitation.
18 H. Y. Loken, R. G. Lawler, and H. R. Ward, J. Org. Chem., 38, 106 (1973).
19
W. A. Pryor, D. M. Huston, T. R. Fiske, T. L. Pickering, and E. Ciuffarin, J. Am. Chem. Soc., 86, 4237
(1964).
20 J. C. Martin, J. W. Taylor, and E. H. Drew, J. Am. Chem. Soc., 89, 129 (1967); F. D. Greene, H. P. Stein,
C.-C. Chu, and F. M. Vane, J. Am. Chem. Soc., 86, 2080 (1964).
21
D. F. DeTar, R. A. J. Long, J. Rendleman, J. Bradley, and P. Duncan, J. Am. Chem. Soc., 89, 4051
(1967).