Page 333 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

314

CHAPTER 3
Structural Effects on
Stability and Reactivity

C C C O C O

. .
CHCH O C CH . ROCH
CH 2 2 2 2
R
Fig. 3.19. PMO representation of p-orbital interactions with (a) C=C,
(b) C=O, and (c) OR substituents. The form of the resulting SOMO is shown
at the bottom.

orbital occupied by the single electron. The net effect is stabilizing, since there are
two electrons in the stabilized orbital and only one in the destabilized one. The radical
center is now more nucleophilic because the energy of the SOMO is raised.
Substituent effects on radicals can be expressed as radical stabilization energies
(RSE). Table 3.17 gives some RSEs determined by one such approach developed by
Leroy, 99 100 which can be defined as the difference between the observed enthalpy of
atomization H and the sum of standard bond energies.
a
RSE = H − BE
a
The H can be obtained from thermodynamic data or calculated theoretically. 101
a
These RSE values are generally consistent with chemical experience. For example, the
low stability attributed to aryl and vinyl radicals, as opposed to the high stability of
benzyl and allyl radicals, is consistent with their respective reactivities. The somewhat
less familiar stability of aminoalkyl and acyl radicals is reproduced. The need for
care is illustrated by the value for Cl C·, which implies considerable destabilization.
3
Chemical experience (see Section 11.4.2) shows that CCl · is a quite accessible species,
3
although it appears in Table 3.17 as one of the most destabilized radicals. The C−H
BDE for H−CCl is intermediate between those for pri and tert C−H bonds. The
3
apparent “destabilization” of CCl · is due to a “destabilization” that is also present in
3
the reactant Cl C−H. Similarly, the acetyl radical is listed as destabilized, but it, too,
3
is an accessible species. We return to this issue in Topic 11.1. Schemes for assignment
of RSE that depend on the definition of standard bond energy terms are subject to the
particular definitions that are used. These systems, however, are valuable in providing a
basis for comparing substituent effects on a qualitative basis. These approaches can be
99
G. Leroy, D. Peeters, and C. Wilante, Theochem, 5, 217 (1982); G. Leroy, Theochem, 5, 77 (1988).
100 Other approaches to radical stabilization energies: C. Ruchardt and H. D. Beckhaus, Top. Curr. Chem.,
130, 1 (1985); F. M. Welle, H.-D. Beckhaus, and C. Ruchardt, J. Org. Chem., 62, 552 (1997);
F. G. Bordwell and X.-M. Zhang, Acc. Chem. Res., 26, 510 (1993); F. G. Bordwell, X.-M. Zhang, and
R. Filler, J. Org. Chem., 58, 6067 (1993).
101
J. Espinosa-Garcia and G. Leroy, Recent Devel. Phys. Chem., 2, 787 (1998).