Page 327 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

308 Because the nonbonding orbital is occupied, stability increases with s character, the
3
converse of the situation for carbocations. The order of stability of carbanions is sp <
CHAPTER 3 sp <sp. The relative stability of gas phase carbanions can be assessed by the energy
2
Structural Effects on of their reaction with a proton, which is called proton affinity. The proton affinities
Stability and Reactivity
of the prototypical hydrocarbons methane, ethene, and ethyne have been calculated
∗
at the MP4/6-31+G level. 78 The order is consistent with the electronegativity trends
2
discussed in Section 1.1.5, and the larger gap between sp and sp , as compared to sp 2
3
and sp , is also evident. The relative acidity of the hydrogen in terminal alkynes is
one of the most characteristic features of this group of compounds.
Proton affinity (kcal/mol)
R +H −→ R −H
+
−
CH 4 H C=CH 2 HC≡CH
2
419.4 408.7 375.1
The order of gas phase carbanion stability has been calculated as Et < Me ∼
iso-Pr < tert-Bu by both MP2/6-31 + G d p and B3LYP/6-31 + G d p calcula-
80
79
tions. Experimental values in the same order have been recorded. The pri < sec <
tert order reflects the ability of the larger groups to disperse the negative charge
(polarizability). The Et < Me order indicates that electron donation by CH (relative to
3
H) slightly destabilizes the ethyl carbanion. However, a larger tert-butyl substituent (in
the neopentyl anion) has a net stabilizing effect that is evidently due to polarizability.
Note that the effect of substitution is very much smaller than for carbocations. The
range between primary and tertiary carbanions is only about 10 kcal/mol, compared to
about 75 kcal/mol for carbocations. The alkyl anions are calculated to be pyramidal, in
3
agreement with the sp hybridization model. The bond angles at the trivalent carbon
−
−
−

are as follows: CH 109 4 CH CH 111 0 CH CH 109 1 , and CH C
−

3 2
3 3
3
2
3
81
109 6 . AIM charges were also calculated B3LYP/6-311+G , as shown below,
∗∗
and indicate some -delocalization to the -hydrogens. Note in particular that in the
ethyl and 2-propyl anions the anti hydrogens are more negative than the gauche. Note
also the considerable reduction in the charge at carbon as the anion becomes larger.
These trends are in accord with dispersal of the negative charge by both hypercon-
jugation and polarization. Table 3.14 compares proton affinities calculated by several
methods with experimental values.
– 0.072
– 0.081 H
H – 0.434
C – 0.559 H C C C C – 0.311
H
H H C H – 0.147
H H H – 0.153
– 0.146 – 0.147 H
– 0.147
methyl anion ethyl anion 2-propyl anion
78 W. H. Saunders, Jr., and J. E. Van Verth, J. Org. Chem., 60, 3452 (1995).
79
R. R. Sauers, Tetrahedron, 55, 10013 (1999).
80 C. H. DePuy, S. Gronert, S. E. Barlow, V. M. Bierbaum, and R. Damrauer, J. Am. Chem. Soc., 111,
1968 (1989).
81
P. Burk and K. Sillar, Theochem, 535, 49 (2001).