Page 280 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

K = 30 261
5 5
2
CF 3 CO 2 CH 2 CHCH 3 C H SCH CHCH 3
SC H O CCF 3 SECTION 3.1
6 5
2
Isomer with the more electronegative substituent on the Thermodynamic Stability
more highly substituted site is the more stable compound.
These treatments of bond energies illustrate that there are fundamental relation-
ships between bond strengths and atomic properties such as electronegativity and
polarizability. The crucial point is that bond strength is increased by electronegativity
differences in a given row of the periodic table as a result of an increment that is due
to electrostatic attraction.

3.1.2.3. Calculation of H Using Transferable Group Equivalents. The idea that
f
the properties of molecules are the sum of its component atoms and groups has led
to the development of schemes by which thermodynamic properties can be calculated
as the sum of contributions from all structural units. 11 The most highly developed is
that of S. W. Benson and co-workers. 12 The molecule is divided into its component
groups. For example, isooctane (which incidentally is the standard for 100 in octane
ratings) consists of five C–(C)(H) (a), one C–(C) (H) (b), one C–(C) (H) (c), and
3 2 2 3
one C–(C) (d), as labeled on the structure.
4
(CH )
3 a
(CH ) (C) d (CH ) (CH) c (CH )
3 a
3 a
2 b
(CH ) (CH )
3 a
3 a
The four groups designated above are sufficient to describe all alkanes. Finer
distinctions can be made if these groups are subdivided further, depending on the
number of hydrogens on the adjacent carbon. For example, a CH group might be
3
found in four different environments:

CH −CH CH −CH CH CH −CH CH C
3 3 3 2 3 3 3 3 4
n = 3 n = 2 n = 1 n = 0

Modified increments are also assigned to carbons adjacent to double or triple bonds or
benzene rings. The enthalpy of formation of a molecule can then be calculated as the
sum of the contributions of the component groups. The main limitation of this method
is that it does not explicitly consider long-range nonbonded interactions. The group
equivalents refer to strain-free molecules. Further refinement can be incorporated by
taking account of nonbonded interactions. For example, gauche interactions can be
counted and applied as a correction to the sum of group equivalents. 13

11
For a discussion of the pioneering efforts in this field, see J. D. Cox and G. Pilcher, Thermochemistry
of Organic and Organometallic Compounds, Academic Press, New York, 1970, Chap. 7.
12 N. Cohen and S. W. Benson, Chem. Rev., 93, 2419 (1993).
13
N. Cohen and S. W. Benson, The Chemistry of Alkanes and Cycloalkanes, S Patai and Z. Rappoport, eds.,
Wiley, 1992, Chap. 6; S. W. Benson and N. Cohen, in Computational Thermodynamics, K. K. Irikura
and D. J. Frurip, eds., ACS Symposium Series, 677, 20 (1996).