Page 337 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

318 was then applied to a wider range of radicals. G3(MP2), AUMPQ, and B3LYP/6-31+G
methods are also satisfactory. Some of these data are included in Table 3.19.
CHAPTER 3
Table 3.20 provides some other comparisons. The first column gives experimental
Structural Effects on BDEs derived from thermodynamic data. The other columns give H calculated for
Stability and Reactivity
the dissociative reaction, using various computational methods:
. .
X −CH −H → X −CH +H H = BDE
2 2
The computations that do not include electron correlation (HF) lead to large errors,
but the other current methods that are applicable to molecules of this size perform
satisfactorily.
Comparison of the thermochemical, kinetic, and computational evaluation of
radical substituent effects provides a consistent picture. Delocalization, as in allylic
and benzylic systems, provides ∼ 15kcal/mol of stabilization. Conjugated EWGs
such as acyl and cyano provide significant stabilization, usually in the range of 5–
10 kcal/mol. Oxygen and especially nitrogen groups provide stabilization as well. In
2
contrast, the sp and sp C−H bonds directly on vinyl, aryl, and alkynyl carbons are
difficult to break, and the corresponding radicals are considered to be destabilized.
An interesting contrast to these are acyl radicals, which are relatively easily formed
from aldehydes by hydrogen atom abstraction (see Section 11.4.3). Stabilization results
from conjugation with the oxygen electrons. These radicals are isoelectronic with NO
and like NO, a Linnett-type structure can be drawn for acyl radicals. Showing a bond
order of 2.5.

o o
RC O RC – O + R C O N O
x x x x

Table 3.20. Comparison of Experimental and Calculated C−H Bond Dissociation Energies
for Hydrocarbons and Representative Derivatives (in kcal/mol)

Compound Experimental Theoretical BDE
BDE (av.) a HF/6-31G(d,p) MP2/6-31G(d,p) CBS-4 b CBS b G2(MP2) b G2 b B3LYP/6-31G(d,p)
H−H 104 2 79 5 95 8 104 0 106 4
CH 3 −H 104 8 79 4 100 3 103 6 103 3 104 0 104 0 105 8
CH 3 CH 2 −H 101 1 76 8 98 9 99 9 100 0 100 0 100 9 101 1
CH 3 2 CH−H 97 6 74 5 95 3 97 0 97 4 98 4 98 5 97 1
CH 3 3 C−H 96 5 72 7 93 7 94 0
CH 2 =CHCH 2 −H 84 9 85 0 87 5 87 3
FCH 2 −H 101 7 77 8 96 0 99 1
ClCH 2 −H 101 5 76 6 95 0 99 7
HOCH 2 −H 96 2 74 1 92 3 95 6 95 3 96 2 96 2 95 2
H 2 NCH 2 −H 92 2 67 5 85 0 91 8 91 9 93 1 93 1 87 7
HSCH 2 −H 94 1 74 9 93 1 96 3
NCCH 2 −H 93 4 69 8 96 2 93 8
O=CHCH 2 −H 94 2 67 3 93 5 93 4
CH 3 COCH 2 −H 95 1 69 4 94 4 94 0
HO 2 CCH 2 −H 96 0 73 7 95 2 97 2
CH 3 SO 2 CH 2 −H 99 0 80 8 101 2 103 4
Cl 3 C−H 95 6 71 8 89 4 92 0
a. H.-G. Korth and W. Sicking, J. Chem. Soc., Perkin Trans., 2, 715 (1997),
b. J. W. Ochterski, G.A. Petersson, and K. B. Wiberg, J. Am. Chem. Soc., 117, 11299 (1995).