Page 393 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

374 H
:
CHAPTER 3
Structural Effects on
Stability and Reactivity

As we discussed in Section 3.4.2, measurements in the gas phase, which eliminate
the effect of solvation, show structural trends that parallel measurements in solution
but with larger absolute energy differences. Table 3.38 gives H of gas phase proton
dissociation data for some key hydrocarbons. These data show a correspondence
with the hybridization and delocalization effects observed in solution. The very large
heterolytic dissociation energies reflect both the inherent instability of the carbanions
and also the electrostatic attraction between the oppositely charged carbanion and
proton that separate.
There have been several studies aimed at using computations to examine hydro-
carbon acidity. The proton affinity values for a number of hydrocarbons were calculated
by both ab initio and DFT methods. 179 Some of the results are shown in Table 3.39.
Alkorta and Elguero found that there is good correlation between PAs calcu-
lated by B3LYP/6-311 ++G ∗∗ computations and available gas phase experimental
measurements. 182 There was also good correlation with solution pK values. Based
on these correlations, they made the interesting prediction that the as yet unknown
hydrocarbon tetrahedrane would be comparable to water in acidity.
Knowledge of the structure of carbanions is important to understanding the stereo-
chemistry, stability, and reactivity. Theoretical calculations at the ab initio level (HF/4-
31G) indicate a pyramidal geometry at carbon in the methyl and ethyl anions. The

optimum H−C−H angle in these two carbanions was calculated to be 97 –100 .An
interesting effect is observed in that the PA (basicity) of methyl anion decreases in
a regular manner as the H−C−H angle is decreased. 183 This increase in acidity with
decreasing internuclear angle has a parallel in small-ring compounds, in which the
acidity of hydrogens is substantially greater than in compounds having tetrahedral
geometry at carbon. Pyramidal geometry at carbanions can also be predicted on the

Table 3.38. Enthalpy of Proton
Dissociation for Some Hydrocarbons
(Gas Phase)
Hydrocarbon Enthalpy (kcal/mol)
Methane 418 8
Ethene 407 5
Cyclopropane 411 5
Benzene 400 8
Toluene 381 0
a. S. T. Graul and R. R. Squires, J. Am. Chem.
Soc., 112, 2517 (1990).

182 I. Alkorta and J. Elguero, Tetrahedron, 53, 9741 (1997).
183
A. Streitwieser, Jr., and P. H. Owens, Tetrahedron Lett., 5221 (1973); A. Steitwieser, Jr., P. H. Owens,
R. A. Wolf, and J. E. Williams, Jr., J. Am. Chem. Soc., 96, 5448 (1974); E. D. Jemmis, V. Buss,
P. v. R. Schleyer, and L. C. Allen, J. Am. Chem. Soc., 98, 6483 (1976).