Page 324 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 3.12. Computed Substituent (De)Stabilization of Carbocation (in kcal/mol) a 305
X Ethyl Vinyl X Ethyl Vinyl SECTION 3.4
H 0 00 0 00 C≡N −16 02 −11 71 Electronic Substituent
Effects on Reaction
18 63 25 89 CH=O −10 25 −4 51
CH 3
Intermediates
CH 2 OH 15 30 21 69 CO 2 H −8 41 −5 92
CH 2 CN −2 10 4 76 H 2 N 64 96 53 69
CH 2 Cl 5 59 12 90 HO 37 43 25 92
3 62 9 63 O 2 N −23 09 −25 44
CH 2 CF 3
CH 2 F 4 98 10 89 F 6 95 −9 25
−23 56 −16 41 Cl 9 83 11 17
CF 3
CH 2 =CH 31 98 25 64 Br 9 52 12 70
HC≡C 18 17 25 64 I 13 32 20 53
36 77 54 10
C 6 H 5
42 97 47 06
c-C 3 H 5
a. K. van Alem, G. Lodder, and H. Zuilhof, J. Phys. Chem., 106, 10681 (2002).
Netherlands. 71 This study compared MP2/6-311++G(d,p), CBS-Q, and B3LYP/6-
311+G(d,p) calculations and found that the CBS-Q method gave the best correlation
with experimental data. There was also good correlation with MP2 and B3LYP results,
which could be used to obtain “CBS-Q-like” results when the CBS-Q method was not
applicable. Table 3.12 gives some of the computed stabilization + and destabilization
− relative to ethyl and vinyl cations found using isodesmic reactions:

H C CH 3 + H 3 C CHX H C CH X + H CCH 2 +
3
3
2
3
+
H C CH 2 + H 2 C CX H 2 C CHX + H C CH
2
2
+ +
The results show substantial destabilization for CF , CN, CH=O, CO H, and NO 2
2
3
groups.
The relative stability, charge distribution, and bond orders obtained by the
calculation identified three main factors in carbocation stabilization as -donation,
-hyperconjugation, and the -polar effects.
+ Y + +
X C
X
π-donor β-hyperconjugation α-polar
The -donor and -hyperconjugation effects are present for both alkyl and vinyl
carbocations. The vinyl carbocations exhibit somewhat larger stabilizations because
they are inherently less stable. The vinyl cations also appear to be more sensitive to
the -polar effect. Note that the F, OH, and NH substituents are less stabilizing of
2
vinyl cations than of alkyl cations. This is believed to be due to the greater sensitivity
to the polar effect of the more electronegative sp carbon in the vinyl cations. The NPA
and bond order characterizations of the carbocations are shown in Table 3.13. The
values give the charge associated with the cationic carbon and the bond order between
the cationic carbon and the substituent X.

71
K. van Alem, G. Lodder, and H. Zuilhof, J. Phys. Chem. A., 106, 10681 (2002).