Page 984 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

968 11.1.2. Long-Lived Free Radicals

CHAPTER 11 Radicals that have long lifetimes and are resistant to dimerization, dispropor-
Free Radical Reactions tionation, and other routes to self-annihilation are called persistent free radicals.
Scheme 11.1 gives some examples of long-lived free radicals. A few free radicals
are indefinitely stable, such as Entries 1, 3, and 6, and are just as stable to ordinary
conditions of temperature and atmosphere as typical closed-shell molecules. Entry 2
is somewhat less stable to oxygen, although it can exist indefinitely in the absence of
oxygen. The structures shown in Entries 1, 2, and 3 all permit extensive delocalization
of the unpaired electron into aromatic rings. These highly delocalized radicals show
little tendency toward dimerization or disproportionation. The radical shown in Entry

3 is unreactive under ordinary conditions and is thermally stable even at 300 C. 2
The bis-(t-butyl)methyl radical shown in Entry 4 has only alkyl substituents and
yet has a significant lifetime in the absence of oxygen. The tris-(t-butyl)methyl radical
3

has an even longer lifetime with a half-life of about 20 min at 25 C. The steric
hindrance provided by the t-butyl substituents greatly retards the rates of dimerization
of these radicals. Moreover, they lack -hydrogens, precluding the normal dispropor-
tionation reaction. They remain highly reactive toward oxygen, however. The extended
4
lifetimes have more to do with kinetic factors than with inherent stability. Entry 5 is
a sterically hindered perfluorinated radical that is even more long-lived than similar
alkyl radicals.
Certain radicals are stabilized by synergistic conjugation involving two or more
functional groups. Entries 6 and 7 are examples. Galvinoxyl, the compound shown in
Entry 6 benefits not only from delocalization over the two aromatic rings, but also from
the equivalence of the two oxygens, which is illustrated by the resonance structures.
The hindered nature of the oxygens also contributes to persistence.
(CH ) 3 C C(CH ) 3 (CH ) C C(CH )
3 3
3 3
3
3
. O CH O O CH O .
) C
) C
3 3
3 3
(CH 3 3 C(CH ) (CH 3 3 C(CH )
Entry 7 also benefits from interaction between the ester and amino groups, as is
discussed in Section 11.1.6.
There are only a few functional groups that contain an unpaired electron and yet
are stable in a wide range of structural environments. The best example is the nitroxide
group illustrated in Entry 8. There are numerous specific nitroxide radicals that have
been prepared and characterized. The unpaired electron is delocalized between nitrogen
and oxygen in a structure with a N−O bond order of 1.5.
R R R
: – : . :
N . + O: N O . or δ+ . N O . δ–
R : R : : R :
Many nitroxides are stable under normal conditions, and heterolytic reactions can be
carried out on other functional groups in the molecule without affecting the nitroxide
2
M. Ballester, Acc. Chem. Res., 18, 380 (1985).
3 G. D. Mendenahall, D. Griller, D. Lindsay, T. T. Tidwell, and K. U. Ingold, J. Am. Chem. Soc., 96,
2441 (1974).
4
For a review of various types of persistent radicals, see D. Griller and K. U. Ingold, Acc. Chem. Res.,
9, 13 (1976).