Page 340 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

(a) Charge density 321
–1.17 –1.08 –1.21 –1.21
O O O O SECTION 3.4
+0.19 – 0.66 +0.14 – 0.14 +0.07 – 0.56 +0.07 – 0.43
H 3 C C F H 3 C C CF 3 H 3 C C OH H 3 C C NH 2 Electronic Substituent
Effects on Reaction
+1.64 +1.080 +1.63 +1.57
Intermediates
(b) π-Bond order
O 0.56 O 0.61 O 0.64 O 0.63 O 0.53 O 0.52
H 3 C C F H 3 C C Cl H 3 C C CN H 3 C C CH 3 H 3 C C OH H 3 C C NH 2
0.13 0.14 0.04 0.04 0.21 0.28

Fig. 3.20. Charge density (a) and bond orders (b) for carbonyl derivatives.

The bond energies of the O=C−X bonds increase sharply with electronegativity.
For NH and OH there are different values for perpendicular and planar structures,
2
reflecting the resonance contribution to the planar form.
O O O O

CH 3 CH 3 CH 3 NH 2 CH 3 OH CH 3 F
BE (exp) 83.5 83.9 97.9 123.5
(97.8) planar (109.5) planar

The increasing C−X bond strength (apart from resonance) is attributed to the electro-
static attraction associated with the difference in electronegativity. This is accentuated
by the already existing C=O charge separation and leads to a favorable juxtaposition
of positive and negative charge. 108

δ −
O
δ −
δ +C X

There have been several attempts 109 to assign relative importance to the polar
(including electrostatic) and resonance components of the stabilization carboxylate

Table 3.22. Separation of the Resonance Component
of the Stabilization Energy
Total SE H rot H
F 16 7 – 16 7
OH 27 7 11 5 16 2
19 3 13 9 5 4
NH 2
Cl 6 8 – 6 8
SH 6 1 8 1 −2 0
−3 9 0 −3 9
PH 2
−12 7 0 −12 7
SiH 3
108 J. B. Levy, Struct. Chem., 10, 121 (1999).
109
P. C. Hiberty and C. P. Byrman, J. Am. Chem. Soc., 117, 9875 (1995); R. W. Taft, I. A. Koppel,
R. D. Topsom, and F. Anvia, J. Am. Chem. Soc., 112, 2047 (1990); M. R. F. Siggel, A. Streitwieser,
Jr., and T. D. Thomas, J. Am. Chem. Soc., 110, 8022 (1988).