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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).
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