Page 993 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

O O O 977
80–100°C ·
CH COOCCH 3 2CH 3 CO 2CH · + 2CO 2 SECTION 11.1
3
3
O O O Generation and
Characterization of Free
80–100°C 2Ph· + 2CO Radicals
PhCOOCPh 2PhCO· 2
Peroxyesters are also sources of radicals. The acyloxy portion normally loses carbon
dioxide, so peroxyesters yield an alkyl (or aryl) and alkoxy radical. 22

O
·
RCOOC(CH ) R · + CO 2 + OC(CH )
3 3
3 3
The thermal decompositions described above are unimolecular reactions that
should exhibit first-order kinetics. Peroxides often decompose at rates faster than
expected for unimolecular thermal decomposition and with more complicated kinetics.
This behavior is known as induced decomposition and occurs when part of the peroxide
decomposition is the result of bimolecular reactions with radicals present in solution,
as illustrated specifically for diethyl peroxide.

X · + CH CH OOCH CH 3 CH CHOOCH CH + HX
·
2
3
3
3
2
2
·
H
CH CHOOCH CH 3 CH CH O + OC 2 5
3
3
2
·
The amount of induced decomposition that occurs depends on the concentration and
reactivity of the radical intermediates and the susceptibility of the reactant to radical
.
attack. The radical X may be formed from the peroxide, but it can also be derived
from subsequent reactions with the solvent. For this reason, both the structure of the
peroxide and the nature of the reaction medium are important in determining the extent
of induced decomposition relative to unimolecular homolysis. All of the peroxides are
used in relatively dilute solution. Many peroxides are explosive, and due precautions
must be taken.
Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. t-Butyl
hydroperoxide is often used as a radical source. Detailed studies on the mechanism
of the decomposition indicate that it is a more complicated process than simple
unimolecular decomposition. 23 The alkyl hydroperoxides are sometimes used in
conjunction with a transition metal salt. Under these conditions, an alkoxy radical
is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-
electron reduction by the metal ion. 24
–
(CH ) COOH + M 2+ (CH ) CO · + OH + M 3+
3 3
3 3
A technique that provides a convenient source of radicals for study by ESR
involves photolysis of a mixture of di-t-butyl peroxide, triethylsilane, and the alkyl
bromide corresponding to the radical to be studied. 25 Photolysis of the peroxide gives
22
P. D. Bartlett and R. R. Hiatt, J. Am. Chem. Soc., 80, 1398 (1958).
23 R. Hiatt, T. Mill, and F. R. Mayo, J. Org. Chem., 33, 1416 (1968), and accompanying papers.
24 W. H. Richardson, J. Am. Chem. Soc., 87, 247 (1965).
25
A. Hudson and R. A. Jackson, Chem. Commun., 1323 (1969); D. J. Edge and J. K. Kochi, J. Am. Chem.
Soc., 94, 7695 (1972).