Page 276 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

(2,2,3,3-tetramethylbutane) of the C isomers. For alkenes, substitution on the double 257
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bond is stabilizing. There is a range of nearly 7 kcal/mol for the C H 12 isomers. The
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data for C alkenes, for example show: SECTION 3.1
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Thermodynamic Stability

1-hexene E-3-hexene 2-methyl-2-pentene 2,3-dimethyl-2-butene
ΔH f ° = –17.7 kcal/mol ΔH f ° = –20.6 kcal/mol ΔH f ° = –23.5 kcal/mol ΔH f ° = –24.3 kcal/mol

These relationships are a result of the stabilizing effect of branching and double-bond
substitution and hold quite generally, except when branching or substitution results in
van der Waals repulsions (see Section 2.3), which have a destabilizing effect.

3.1.2. Calculation of Enthalpy of Formation and Enthalpy of Reaction
In Chapter 1, we introduced various concepts of structure and the idea that the
properties of molecules are derived from the combination of the properties of the atoms.
One of the qualitative conclusions from these considerations is that the properties
of CH ,CH , CH, and C groups in hydrocarbons are expected to be similar from
3
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molecule to molecule, as long as they are not perturbed by a nearby functional group.
Several methods for the calculation of thermodynamic data based on summation of
group properties have been developed and are discussed in the next two sections.

3.1.2.1. Calculations of Enthalpy of Reaction Based on Summation of Bond Energies.
The computation of molecular energy by MO or DFT methods gives the total binding
energy of a molecule. This is a very large number, since it includes all the electron-
nuclei forces in the atoms, not just the additional attractive forces of the bonding
electrons. The total energy can be converted to an energy representing all bonding
between atoms by subtracting the energy of the individual atoms. This difference in
energy is called the energy of atomization. This quantity still represents an energy that
is far larger then the change involved in chemical reactions, which is of primary interest
to chemists. The focus of chemical reactivity is on the bonds that are being formed
and broken in the reaction. Useful relationships between structure and reactivity can
be developed by focusing on bond dissociation energies (BDE). The most completely
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developed information pertains to homolytic bond dissociation, which is the energy
required to break a specific bond in a molecule with one electron going to each of the
atoms. From the general bond energies in Part A of Table 3.2 we can discern several
trends. One is that C−C bonds are considerably stronger than the other homonuclear
bonds for the second-row elements (compare with O−O and N−N bonds). We can also
note that the bonds in the dihalogens are relatively weak, with a somewhat irregular
trend with respect to position in the periodic table: F < Cl > Br > I . The bonds to
2 2 2 2
hydrogen are also slightly irregular: N < C < O < F. For the hydrogen halides, there
is a sharp drop going down the periodic table.
It is known that the immediate molecular environment significantly affects the
bond energy, as is illustrated by the data in Part B of Table 3.2. For hydrocarbons the
C−H bond dissociation energy depends on the degree of substitution and hybridization

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For a discussion of the measurement and application of bond dissociation energies, sec S. J. Blanksby
and G. B. Ellison, Acc. Chem. Res., 36, 255 (2003).