Page 281 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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262 Example. Calculation of H for Isooctane (2,2,4-trimethylpentane)
f
CHAPTER 3
Structural Effects on 5C- C H 3 a −10 00 −50 00
Stability and Reactivity 1C C 2 H 2 b −5 00 −5 00
−2 40 −2 40
1C C 3 H c
−0 10 −0 10
13C C 4
3 gauche correction −0 80 +2 40
Calculated value −55 10
Group equivalent methods can be extended to functionalized compounds by assigning
the enthalpy components of the substituent groups. 14
Various other methods for making calculations based on bond dissociation
energies more precise have been developed. A method developed by G. Leroy and
co-workers is based on extensive thermochemical data, as well as energies calculated
by MO methods. 15 The heat of atomization of unstrained hydrocarbons is equated to
the number of C−C and primary, secondary and tertiary C−H bonds. The heat of
atomization is given by

H = N E +N E +N E +N E (3.9)
atom cc cc pri C−H pri sec C−H sec tert CH tert
For stable compounds with known H the stabilization (or destabilization) is then
atom
the difference between H and the calculated sum of standard bond energies:
atom
SE = H atom − BE standard (3.10)

Leroy and co-workers developed an extensive series of standard bond energy terms.
Terms for specific substituent effects were also assigned, e.g., the C −C and
d
O
C−H terms are adjustments for bonds between double bonds and having oxygen
substituents, respectively. Comparison of the sum of the standard BE with the actual
H atom gives the extra stabilization present in the compound. Calculations for butadiene
stabilization (conjugation) and dimethyoxymethane (anomeric effect) are given below.

Butadiene Dimethoxymethane
Bond energies: 2E C=C 2(137.23) Bond energies 4E C−O 4(91.66)
4(100.30) 2(97.53)
4E C d −H 2 2E C−H s
2(99.78) 4 C−H O 4 −2 05
2E C d −H 1 s
E(C−C) 85.44 6E C−H O p 6(95.87)
c
2 C −C 2(3.88)
d
968 42kcal/mol −1 1128 72kcal/mol −1
N AB B AB= N AB B AB=
971.5 1133.06
H atom H atom
SE 3.08 kcal/mol SE 4.34 kcal/mol
The enthalpy change of a homolytic bond dissociation is expressed as the
difference in stabilization of the products and reactants. A virtue of this approach is

14 J. B. Pedley, R. D. Naylor, and S. P. Kirby, Thermochemical Data of Organic Compounds, 2nd Edition,
Chapman and Hall, London, 1986, Chap. 2.
15
G. Leroy, M. Sana, and C. Wilante, J. Mol. Structure: Theochem., 234, 303 (1991).