Page 453 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

+ 89
434 An NPA analysis has been performed on F C and Cl C . According to this analysis,
+
3
3
chlorine is a slightly better donor than fluorine, and fluorine’s polar effect is, of
CHAPTER 4 +
course, stronger. The net result is that the carbon in F C is much more positive than
3
Nucleophilic Substitution the carbon in Cl C . Note that even the Cl electrons are slightly shifted to the sp 2
+
3
+
carbon. According to this analysis, the positive charge in Cl C is carried entirely by
3
+
the chlorines, whereas in F C the positive charge resides entirely on carbon.
3
0.38 σ
σ – 0.18 0.09 +0.39
F F Cl Cl
C+ C+
+1.54 – 0.17
0.2 0.3
F π Cl π
Electron-withdrawing groups that are substituted directly on the cationic site are
destabilizing. Table 4.18 gives an indication of the relative retardation of the rate of
ionization and the calculated destabilization for several substituents.
The trifluoromethyl group, which exerts a powerful polar effect, is strongly
destabilizing both on the basis of the kinetic data and the MO calculations. The cyano
and formyl groups are less so. In fact, the destabilization of these groups is considerably
less than would be predicted on the basis of their polar substituent constants. Both the
cyano and formyl groups can act as donors, even though the effect is to place partial
positive charge and electron deficiency on nitrogen and oxygen atoms, respectively.
+
C + C N : C C N: + C + C O: C + C O:
H H
These resonance structures are the nitrogen and oxygen analogs of the allyl cation.
The effect of this delocalization is to attenuate the polar destabilization by these
substituents. 90 These interactions are reflected in MO energies, bond lengths, and
charge distributions calculated for such cations 91 (review Section 3.4.1).

Table 4.18. Destabilization of 2-Substituted i-Propyl
Cation by EWG Substituents
Z Solvolysis rate Destabilization
relative to Z = H HF/4-31G
(kcal/mol)
CN ∼ 10 −3a 9.9 b
∼ 10 −3c 37.3 b
CF 3
CH=O – 6.1 b
a. P. G. Gassman and J. J. Talley, J. Am. Chem. Soc., 102, 1214 (1980).
b. M. N. Paddon-Row, C. Santiago, and K. N. Houk, J. Am. Chem. Soc.,
102, 6561 (1980).
c. K. M. Koshy and T. T. Tidwell, J. Am. Chem. Soc., 102, 1216 (1980).

89
G. Frenking, S. Fau, C. M. Marchand, and H. Gruetzmacher, J. Am. Chem. Soc., 119, 6648 (2000).
90 T. T. Tidwell, Angew. Chem. Int. Ed. Engl., 23, 20 (1984); P. G. Gassman and T. T. Tidwell, Acc. Chem.
Res., 16, 279 (1983); J. L. Holmes and P. M. Mayer, J. Phys. Chem., 99, 1366 (1995); J. L. Holmes,
F. P. Lossing, and P. M. Mayer, Chem. Phys. Lett., 212, 134 (1993).
91
D. A. Dixon, P. A. Charlier, and P. G. Gassman, J. Am. Chem. Soc., 102, 3957 (1980); M. N. Paddon-
Row, C. Santiago, and K. N. Houk, J. Am. Chem. Soc., 102, 6561 (1980); D. A. Dixon, R. A. Eades,
R. Frey, P. G. Gassman, M. L. Hendewerk, M. N. Paddon-Row, and K. N. Houk, J. Am. Chem. Soc.,
106, 3885 (1984); X. Creary, Y.-X. Wang, and Z. Jiang, J. Am. Chem. Soc., 117, 3044 (1995).