Page 325 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

306 Table 3.13. NPA and Bond Order Characterizations of Substi-
tuted Carbocations
CHAPTER 3
+
X C(+) C -X Bond Order
Structural Effects on
Stability and Reactivity
Alkyl Vinyl Alkyl Vinyl
0 647 0 585 0 19 0 20
CH 3
CH 2 OH 0 702 0 568 0 15 0 17
CH 2 CN 0 633 0 598 0 15 0 18
CH 2 Cl 0 719 0 585 0 18 0 16
CH 2 F 0 659 0 577 0 14 0 15
0 612 0 601 0 13 0 18
CH 2 CF 3
0 666 0 618 −0 07 −0 06
CF 3
CH 2 =CH 0 492 0 440 0 34 0 27
HC≡C 0 542 0 704 0 28 0 29
0 420 0 383 0 40 0 29
C 6 H 5
0 558 0 525 0 46 0 42
c-C 3 H 5
C≡N 0 602 0 572 0 13 0 15
HC=O 0 664 0 656 0 02 −0 03
HO 2 C 0 650 0 587 −0 02 −0 02
H 2 N 0 531 0 394 0 55 0 47
HO 0 242 0 125 0 76 0 67
O 2 N 0 499 0 424 0 02 0 04
F 0 592 0 524 0 37 0 39
Cl 0 363 0 274 0 61 0 60
Br 0 284 0 205 0 67 0 65
I 0 193 0 123 0 74 0 68
a. K. van Alem, G. Lodder, and H. Zuilhof, J. Phys. Chem. A, 106, 10681 (2002).

Except for -donor substituents, most of the positive charge resides on the cationic
carbon. The strongest effect is seen for the halogens, where there is a sharp drop in
the positive charge in the order F > Cl > Br > I. Note from the NPA charge at the
cationic carbon that F is more effective than CH in delocalizing the positive charge.
3
However, the net stabilization is much less because the polar effect of the C−F
bond is strongly destabilizing. 72 The resonance effect is also indicated by increased
bond order to the substituent (shown in bold).

0.243 0.647 0.492 0.592 0.363
H H H H
+ + + H + +
CH3 C –0.04 CH3 C 0.19 CH 3 0.34 CH 3 C 0.37 CH3 C 0.61
H CH 3 CH CH 2 F Cl
Charge on trivalent carbon (upper) and bond order (bold) on carbocation
One feature of the charge distributions in carbocations that deserves attention is
the greater positive charge calculated for the trigonal carbon in 2-propyl cation than
for ethyl cation. This tendency for more substituted alkyl carbocations to be more
positive has been found in other calculations 73 and is consistent with the relative 13 C
chemical shifts observed in NMR. 74 How does this relationship accord with the very
strong stabilizing influence on alkyl groups on carbocations? Levy 75 has argued that

72
K. van Alem, E. J. R. Sudhölter, and H. Zuilhof, J. Phys. Chem. A, 102, 10860 (1998).
73 R. Hoffmann, J. Chem. Phys., 40, 2480 (1964).
74 G. A. Olah and A. M. White, J. Am. Chem. Soc., 91, 5801 (1969).
75
J. B. Levy, Struct. Chem., 10, 121 (1999).