Page 103 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

82 Y Y –

CHAPTER 1 C C
X +
Chemical Bonding X
and Molecular Structure
Heteroatom hyperconjugation can also be expressed in MO terms. The n, , and ∗
orbitals are involved, as depicted below. If the A atom is the donor and C the acceptor,
the MO perturbation indicates a stabilization of the A n orbital by partial population
∗
of the B–C orbital.
σ * stabilized σ *

n
σ n σ
B
A C

This stereoelectronic interaction has a preference for an anti relationship between
the donor electron pair and the acceptor orbital. Such interactions were first recog-
∗
nized in carbohydrate chemistry, where the term anomeric effect originated. We use the
more general term heteroatom hyperconjugation in the discussion here. The n → ∗
interaction should be quite general, applying to all carbon atoms having two heteroatom
substituents. Such compounds are generally found to be stabilized, as indicated by the
results from an HF/3-21G level calculations given in Table 1.23.
A study of the extent of hyperconjugation in disubstituted methanes using
B3LYP/6-31G ∗∗ calculations and NPA analysis found that ∗ acceptor capacity
increases with electronegativity, i.e., in the order C < N < O < F for the second row. 110
However, acceptor capacity also increases going down the periodic table for the
∗
halogens, F < Cl < Br < I. The electronegativity trend is readily understood, but the
trend with size had not been widely recognized. The effect is attributed to the lower
∗
energy of the orbitals with the heavier elements. Donor ability also appears to
increase going down the periodic table. This trend indicates that softness (polariz-
ability) is a factor in hyperconjugation. The stabilizations for substituted methylamines
according to these B3LYP/6-31G ∗∗ calculations are as follows:

Table 1.23. Calculated Stabilization in kcal/mol for
Disubstituted Methanes a

Y(acceptor)
X(donor) NH 2 OH F
10.6 12 7 17 6
NH 2
OH 17 4 16 2
F 13 9
a. P. v. R. Schleyer, E. D. Jemmis, and G. W. Spitznagel, J.Am. Chem. Soc.,
107, 6393 (1985).
110
I. V. Alabugin and T. A. Zeidan, J. Am. Chem. Soc., 124, 3175 (2002).