Page 329 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg
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310 hybridization, the question arises as to whether the stabilization is through delocal-
ization, polar, or polarization effects. The substituents increase acidity in the order
CHAPTER 3 Se > S > O and Br > Cl > F, which indicates that some factor apart from electroneg-
Structural Effects on ativity (bond polarity) must contribute to the stabilization. The stabilization has been
Stability and Reactivity
described both in terms of polarization and d orbital participation for the larger
elements. The latter effect, as expressed in resonance terminology, implies shortening
of the C−X bond. G2 calculations show very slight shortening for the C–PH and
2
82
C−SH bonds, but the halogens do not show such a trend. Polarization appears to be
the main mechanism for carbanion stabilization by the heavier elements. 83
Another computational approach to assessing carbanion stabilization by
substiuents entails calculation of proton affinity. Table 3.16 gives the results of G2
and MP4/6-31G computations. The energy given is the energy required to remove
∗
a proton from the methyl group. The strong stabilization of the -electron acceptors,
such as BH CH=O NO , and CN, is evident. The second-row elements are in the
2 2
order of electronegativity F > OH > NH , but the effects are comparatively small.
2
The stabilization by third- and fourth-row elements (S, P, Se) are reproduced, and the
halogen order, F < Cl < Br, also suggests that polarization is more important than
dipolar stabilization.
These computational studies provide a description of carbanion stabilization
effects that is consistent with that developed from a range of experimental observations.
The strongest effects come from conjugating EWG substituents that can delocalize the
negative charge. Carbon atom hybridization is also a very strong effect. The effect of
saturated oxygen and nitrogen substituents is relatively small and seems to be O > N,
suggesting a polar effect. This may be opposed by electron-electron repulsion arising
from the unshared electrons on nitrogen and oxygen.
Table 3.16. Gas Phase Proton Affinity of
Substituted Methanes (in kcal/mol)
Compound G2 a MP4/6-31G ∗ b
418 8
CH 3 NH 2
CH 3 OH 414 6 417 7 b
412 8
CH 3 OCH 3
393 9
CH 3 PH 2
CH 3 SH 397 6 403 1 b
CH 3 SeH 399 3 b
CH 3 F 410 4 412 6 b
CH 3 Cl 398 2 404 5 b
CH 3 Br 393 5 400 3 b
363 0
CH 3 BH 2
CH 3 CH=O 368 1 367 1 c
392 5 c
CH 3 CH=CH 2
358 4
CH 3 NO 2
CH 3 CN 375 0 375 9 c
a. P. M. Mayer and L. Radom, J. Phys. Chem. A, 102, 4918
(1998).
b. J. E. Van Verth and W. H. Saunders, Jr., J. Org. Chem., 62,
5743 (1997).
c. W. H. Saunders, Jr., and J. E. Van Verth, J. Org. Chem., 60,
3452 (1995).
82 P. M. Mayer and L. A. Radom, J. Phys. Chem. A., 102, 4918 (1998).
83
P. Speers, K. E. Laidig, and A. Streitwieser, J. Am. Chem. Soc., 116, 9257 (1994).