Page 282 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

that it recognizes that there may be special stabilization, e.g., conjugation and anomeric 263
effects, in the reactants as well as in the dissociated radicals.
SECTION 3.1

BDE = ·BE − BE − BE +SE −SE −SE (3.11) Thermodynamic Stability
standard standard R-Z R· Z·
R-Z R· Z·
Table 3.4 gives some representative results.
According to this analysis, the weakening of the C−H bonds in isobutane and
toluene is largely due to the stabilization of the resulting radicals. However, even
though trichloromethyl radicals are quite stable, there is also considerable stabilization
in the starting material chloroform, and the C−H bond in chloroform is not weakened
as much as that in isobutane. More is said about separating structural effects in reactant
and intermediate radicals in Topic 11.1.
o
The calculation of H is now usually done by computational chemistry, but the
f
success of the group equivalent approaches makes an important point. The properties of
groups are very similar from molecule to molecule, similar enough to make additivity
schemes workable. However, specific interactions, e.g., nonbonded interactions, that
depend on the detailed structure of the molecule are not accounted for. Whenever
interactions that are not accounted for by the group equivalents exist, there will be a
discrepancy between the calculated and actual properties of the molecule. Analyses
such as that of Leroy can provide valuable insights and concepts. In particular, they
provide a means for recognizing stabilization effects present in reactants, as demon-
strated by the calculations for 1,3-butadiene and dimethoxymethane.

3.1.2.4. Calculation of Enthalpy of Formation by Molecular Mechanics. Molecular
mechanics (MM) is a systematic approach to the calculation of molecular energy based
on the summation of bond properties and nonbonding (e.g., van der Waals) interac-
tions (review Section 2.4). MM provides a means for analyzing the energy differences
between molecules and among various geometries of a particular molecule. 16 Several
systems of parameters and equations for carrying out the calculations have been
developed. The method most frequently used in organic chemistry is the one developed
by N. L. Allinger and co-workers. 17 In the most recent version of MM calculations

Table 3.4. Calculation of the C−X Bond Dissociation Energies
(in kcal/mol; 298.15 K)
R−X N AB E AB SE 0 R −X SE 0 R· BDE(R−X)
Et−H 99 8 0 0 100 3
t-Bu−H 96 8 0 8 3 7 93 9
Et−Cl 83 3 0 −0 5 83 8
t-Bu−Cl 85 6 0 3 7 81 9
C 6 H 5 CH 2 −H 99 8 −1 0 11 6 87 1
t-Bu 2 CH−H 98 2 −6 0 −5 9 98 2
Cl 3 C−H 93 9 −16 3 −18 0 95 7

16 F. H. Westheimer, in Steric Effects in Organic Chemistry, M. S. Newman, ed., Wiley, New York, 1956,
Chap. 12; J. E. Williams, P. J. Stang, and P. v. R. Schleyer, Annu. Rev. Phys. Chem., 19, 531 (1968);
D. B. Boyd and K. P. Lipkowitz, J. Chem. Educ., 59, 269 (1982); P. J. Cox, J. Chem. Ed., 59, 275
(1982); N. L. Allinger, Adv. Phys. Org. Chem., 13, 1 (1976); E. Osawa and H. Musso, Top. Stereochem.,
13, 117 (1982); U. Burkett and N. L. Allinger, Molecular Mechanics, ACS Monograph 177, American
Chemical Society, Washington, DC, 1982.
17
N. L. Allinger, Y. H. Yuh, and J. -H. Li, J. Am. Chem. Soc., 111, 8551 (1989).