Page 65 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

44 CH 2 CH 2 CH 2 O

CHAPTER 1
π∗ LUMO 0.2426 LUMO π∗ 0.2467
Chemical Bonding
and Molecular Structure
π HOMO – 0.3709 HOMO π – 0.4697

Fig. 1.19. Relative energy of the and orbitals
∗
in ethene and formaldehyde. Energies values in
au are from W. L. Jorgensen and L. Salem, The
Organic Chemists Book of Orbitals, Academic
Press, New York, 1973.

oxygen provides two additional electrons, so that in place of the CH group of ethene,
2
the oxygen of formaldehyde has two pairs of nonbonding electrons. This introduces
an additional aspect to the reactivity of formaldehyde. The oxygen atom can form a
bond with a proton or a Lewis acid, which increases the effective electronegativity of
the oxygen.
Another key change has to do with the frontier orbitals, the (HOMO) and
(LUMO) orbitals. These are illustrated in Figure 1.19. One significant difference
∗
between the two molecules is the lower energy of the and ∗ orbitals in
formaldehyde. These are lower in energy than the corresponding ethene orbitals
because they are derived in part from the lower-lying (more electronegative) 2p z
orbital of oxygen. Because of its lower energy, the orbital is a better acceptor
∗
of electrons from the HOMO of any attacking nucleophile than is the LUMO of
ethene. We also see why ethene is more reactive to electrophiles than formaldehyde.
In electrophilic attack, the HOMO acts as an electron donor to the approaching
electrophile. In this case, because the HOMO of ethene lies higher in energy than the
HOMO of formaldehyde, the electrons are more easily attracted by the approaching
electrophile. The unequal electronegativities of the oxygen and carbon atoms also
distort electron distribution in the molecular orbital. In contrast to the symmet-
rical distribution in ethene, the formaldehyde MO has a higher atomic coeffi-
cient at oxygen. This results in a net positive charge on the carbon atom, which
is favorable for an approach by a nucleophile. One method of charge assignment
(see Section 1.4.1) estimates that the orbital has about 1.2 electrons associated
with oxygen and 0.8 electrons associated with carbon, placing a positive charge
of +0 2e on carbon. This is balanced by a greater density of the LUMO on the
carbon atom.
One principle of PMO theory is that the degree of perturbation is a function
of the degree of overlap of the orbitals. Thus in the qualitative application of MO
theory, it is important to consider the shape of the orbitals (as indicated quantitatively
by their atomic coefficients) and the proximity that can be achieved by the orbitals
within the limits of the geometry of the reacting molecules. Secondly, the strength of
an interaction depends on the relative energy of the orbitals. The closer in energy, the
greater the mutual perturbation of the orbitals. This principle, if used in conjunction
with reliable estimates of relative orbital energies, can be of value in predicting the
relative importance of various possible interactions.
Let us illustrate these ideas by returning to the comparisons of the reactivity of
ethene and formaldehyde toward a nucleophilic species and an electrophilic species.