Page 278 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

CH CH −H+X → CH CH −X +H–X 259
2
3
2
3
2
X Break Form SECTION 3.1
Thermodynamic Stability
CH 3 CH 2 −H X 2 CH 3 CH 2 −X H−X H
F 100.5 38 113.1 109 −83 6
Cl 100.5 57 84.2 102 −28 7
Br 100.5 45 70.0 87 −11 5
I 100.5 36 55.8 71 +9 7

While bonds of similar type, e.g., C−C, C−O, C−Cl, are of approximately the
same strength, the precise value depends on both hybridization and the degree of
substitution. For instance, as can be seen in Table 3.2, there is a range from 105.0
to 95.7 kcal/mol for the C−H bonds in methane, ethane, propane (C(2)−H), and
2
3
isobutane (C(2)−H). The differences between C−H bonds for sp , sp , and sp carbon
is even greater, as can be seen from the significantly different C−H BDE values
2
for ethane, ethene, ethyne, and benzene. Similarly, C−C bonds between sp carbons
3
are considerably stronger than those between two sp carbons, as is indicated by
the C(2)−C(3) BDE of 116.9 kcal/mol for 1,3-butadiene. For estimation of reaction
enthalpy using Equation (3.4), the most appropriate BDE must be chosen.
3.1.2.2. Relationships between Bond Energies and Electronegativity and Hardness.
In his efforts to correlate important chemical properties, Pauling recognized that the
difference in electronegativity between two bonded atoms contributes to bond strength.
He proposed the empirical relationship

BDE AB = 1/2 BDE AA +BDE +23 2 (3.5)
BB
where is the difference in electronegativity of the two atoms. A related expression is

BDE AB = BDE AA ×BDE 1/2 +30 2 (3.6)
BB
Both relationships propose that BDE is a function of the strength of the two homonu-
3
clear bonds and an increment for electronegativity differences. Although the quanti-
tative reliability of the relationships in Equations (3.5) and (3.6) is limited, the equations
do indicate that there is an increment to bond strength that is related to electronegativity
differences. Subsequently, many investigators have probed the accuracy, scope, and
theoretical foundation of these relationships and have suggested other formulations
4
that improve the accuracy. A refinement of the empirical relationship that includes a
term for polarizability gives the equation
BDE AB = BDE AA ×BDE 1/2 + 2 883 A + B (3.7)
BB
5
where are polarizability parameters for each element. The polarizability parameters
particularly improve the relationship for third-row atoms and other highly polarizable
groups.

3 L. Pauling, J. Am. Chem. Soc., 54, 3570 (1932); L. Pauling, The Nature of the Chemical Bond, 3rd
Edition, Cornell University Press, Ithaca, NY, 1960, Chap. 3.
4 R. R. Reddy, T. V. R. Rao, and R. Viswanath, J. Am. Chem. Soc., 111, 2914 (1989).
5
J. W. Ochterski, G. A. Petersson, and K. B. Wiberg, J. Am. Chem. Soc., 117, 11299 (1995).