Page 323 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

304 stabilizing by delocalization (but we discuss the opposing polar effect of the halogens
below).
CHAPTER 3
Structural Effects on H R H H
Stability and Reactivity +
C N+ C O+ C F+ R C O R C O +
H R H R H

The stabilizing effect of delocalization can be seen even with certain functional groups
that are normally considered to be electron withdrawing. For example, computations
indicate that cyano and carbonyl groups have a stabilizing resonance effect. This
is opposed by a polar effect, so the net effect is destabilizing, but the resonance
component is stabilizing.

R O R O + R R
C + C C C C + C N: C C N: +
R R R R R R
Resonance stabilization of carbocations by electron pair donation

Table 3.11 shows some gas phase stabilization energies computed by various
methods. Columns A and B refer to stabilization of substituted methyl (methylium)
carbocations, whereas columns C and D are referenced to ethylium ions. Because these
values refer to the gas phase, where there is no solvation, the absolute stabilization
energies are substantially larger than those found in solution studies. Nevertheless,
the results provide a good indication of the relative effect of the various substituents
on carbocation stability. Note that of all the substituents included, only NO is
2
strongly destabilizing. The data for F, Cl, CN, and CH=O indicate small stabilization,
reflecting the compensating polar (destabilizing) and delocalization (stabilizing) effects
mentioned earlier.
The most wide-ranging and internally consistent computational comparison
of substituent effects on carbocation was reported by a group working in The

Table 3.11. Calculated Substituent Stabilization on Carbocations (in kcal/mol)
+ +
CH 3 CH 3 CH 2
Substituent PM3 a MP2/6-31G ∗∗b MP2/6-311+G(d,p) c B3LYP/6-311+G(d,p) c
80 2
NH 2
CH 3 O 57 6
OH 51 4
Ph 56 3
CH 2 =CH 43 3 66 1 30 9 37 3
29 0 41 5 17 1 23 4
CH 3
F 5 5 6 0 7 1
Cl 9 7 10 6
CN 5 0 4 3
CH=O 1 7 0 2
−30 8 −22 3
NO 2
a. A. M. El-Nahas and T. Clark, J. Org.Chem., 60, 8023 (1995).
b. X. Creary, X.Y. Wang, and Z. Jiang, J. Am. Chem. Soc., 117, 3044 (1995).
c. K. van Alem, E. J. R. Sudholter, and H. Zuilhof, J. Phys. Chem. A, 102, 10860 (1998).