Page 1033 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Acyl radicals can fragment by loss of carbon monoxide. Decarbonylation is slower 1017
than decarboxylation, but the rate also depends on the stability of the radical that is
formed. 126 For example, rates for decarbonylations giving tertiary benzylic radicals are SECTION 11.2
8
−1
on the order of 10 s , whereas the benzoyl radical decarbonylates to phenyl radical Characteristics of
Reactions Involving
with a rate on the order of 1s −1 (see also Table 11.3, Entries 45 to 48). When reaction Radical Intermediates
of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both
isopropyl chloride and isobutyroyl chloride are formed, indicating that decarbonylation
is competitive with the chlorine atom transfer.
O
CCl 4
(CH ) CHCCl + CCl
O O 3 2 3
(CH ) CHCH + In (CH ) CHC
3 2
3 2
CCl 4
- CO (CH ) CH (CH ) CHCl + CCl 3
3 2
3 2
Radicals derived from ethers and acetals by hydrogen abstraction are subject to
-scission, with formation of a ketone or ester, respectively.
R COR' R C O + R'
2
2
O
RC(OR') 2 RCOR' + R'

These fragmentations are sufficiently slow that the initial radicals can undergo reactions
such as addition to alkenes at rates that are competitive with fragmentation.
A special case of fragmentation is that of 1,4-diradicals, where it can lead to two
alkene molecules. In the case of 1,4-diradicals without functional group stabilization,
reclosure to cyclobutanes is competitive with fragmentation to two molecules of alkene.
The most recent of many detailed computational studies indicates that there is no
barrier between the diradical and either the cyclization or fragmentation products. 127

CH CH CH CH 2
2
2
2
2H C CH 2
2
A study of the lifetimes of the triplet biradicals A, B, and C, which were
generated from the corresponding photoexcited azo compounds, gave the order of
lifetime A > B > C. The lifetime of A is about 2 6×10 −7 s, which is quite long for
a 1,4-diradical. 128

A B C

126
D. E. Applequist and L. Kaplan, J. Am. Chem. Soc., 87, 2194 (1965); W. H. Urry, D. J. Trecker, and
H. D. Hartzler, J. Org. Chem., 29, 1663 (1964); H. Fischer and H. Paul, Acc. Chem. Res., 20, 200
(1987).
127 E. Ventura, M. Dallos, and H. Lischka, J. Chem. Phys., 118, 10963 (2003).
128
W. Adam, K. Hannemann, and R. M. Wilson, J. Am. Chem. Soc., 106, 7646 (1984); W. Adam,
K. Hannemann, and R. M. Wilson, Angew. Chem. Int. Ed. Engl., 24, 1071 (1985); W. Adam, H. Platsch,
J. Sendelbach, and J. Wirz, J. Org. Chem., 58, 1477 (1993).