Page 395 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

376 in C−H acidity. 184 Acidity was estimated by using AM1 computations, calibrated to
experimental deprotonation energies. For small-ring, bicyclic, and cage compounds,
CHAPTER 3 1 13
a correlation was found for the E for deprotonation and the H- C coupling constant:
Structural Effects on
Stability and Reactivity
E deprot = 1870 kJ −0 93J H−C
A similar correlation pertains to cyclic alkenes and dienes in which the deprotonation
2
is from an sp carbon:
E deprot = 1867 kJ −1 05J H−C

These correlations suggest that the dominant factor for these compounds is the
hybridization of the C−H undergoing deprotonation. For hydrocarbons for which
delocalization is expected to be the major factor, e.g., toluene and diphenylmethane, a
different kind of correlation was found:

E = 1640 kJ −0 70 E
deprot relax
where E relax is the stabilization found when the carbanion constrained to the geometry
of the original structure is allowed to relax to the minimum energy structure. These
results support the idea that C−H acidity depends primarily on hybridization and anion
stabilization. In addition to the hydrocarbons that show correlations with one of the two
factors, some hydrocarbons are correlated by equations that contain terms both terms.

General References

C. F. Bernasconi, ed., Investigation of Rates and Mechanisms: Techniques of Chemistry, 4th ed. Vol. VI,
Part 1, John Wiley & Sons, New York, 1986.
B. K. Carpenter, Determination of Organic Reaction Mechanisms, Wiley-Interscience, New York, 1984.
K. A. Connors, Chemical Kinetics, VCH Publishers, New York, 1990.
J. D. Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Academic Press,
London, 1970.
G. G. Hammes, Principles of Chemical Kinetics, Academic Press, New York, 1978.
J. Hine, Structural Effects on Equilibria in Organic Chemistry, Wiley-Interscience, New York, 1984.
C. D. Johnson, The Hammett Equation, Cambridge University Press, Cambridge, 1973.
L. Melander and W. H. Saunders, Jr., Reaction Rates of Isotopic Molecules, New York, 1980.
M. J. Pilling and P. Seakins, Reaction Kinetics, 2nd ed. Oxford University Press, Oxford, 1995.
C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, Wiley-VCH, Weinhem, 2003.

Problems

(References for these problems will be found on page 1158.)
3.1. Use thermochemical relationships to obtain the requested information.

a. The H of cyclohexane, cyclohexene, and benzene are, respectively, −29 5,
f
−1 1, and +18 9kcal/mol. Use this information to estimate the resonance
stabilization of benzene.
184
I. F. Tupitsyn, A. S. Popov, and N. N. Satsepina, Russian J. Gen. Chem., 67, 379 (1997).