3. Larger Molecules

The group orbitals introduced in Chap. 2 and used to interpret structures using Walsh diagrams also form an ideal basis for building up and understanding the MOs of large molecules. This will be demonstrated for several examples below, but before discussing the examples in detail, we should consider the two alternative approaches to understanding molecular structures and reactivity, molecular orbital- and valence bond- (VB) theory.

The MO- and VB- approaches are alternative ways to look at the same problem - chemical bonding. This is unfortunately not always made completely clear in organic or inorganic textbooks, but rather the two approaches are used side by side without any consideration of their relationship to each other. Often, chemists use the treatment that is best suited for the problem in hand. It is, for instance, very difficult to find a molecular orbital description of an aldol condensation in current textbooks, whereas the stereoselectivity of electrocyclic reactions is usually discussed using MO-theory. This is because an aldol condensation, which can be treated using MO-theory, can be described much more succinctly using typical "arrow pushing" (i.e. VB) arguments:

Figure 3.1 Schematic representation of the aldol condensation.

An electrocyclic reaction, on the other hand, can be depicted using a VB-picture, but its stereochemistry cannot be deduced from such a treatment:

Figure 3.2 Electrocyclic interconversion of 1,3,5-hexatriene to 1,3-cyclohexadiene.

Generally, a VB-treatment is used to teach chemistry (because it is more succinct and less complicated than MO-theory) until cases (like electrocyclic reactions) are treated that cannot be understood properly within simple VB-theory. This need not be the case, but is usually a good compromise for teaching chemical bonding and reactivity effectively.

There is, however, one quantity, atomic charge, that has very different meanings within the two theories. Simple VB-theory uses Lewis structures to describe the bonding in molecules. Formal Lewis charges are assigned in order to complete the octet of a given atom, but do not indicate that the atom in question actually has the formal charge (atomic charges are not physically measurable quantities, but can be assigned by a variety of analyses within MO-theory). Take, for instance, the pyridinium and tetramethylammonium cations:

Figure 3.3 Pyridinium and tetramethylammonium cations.

The nitrogen is the most electronegative atom in these ions, so that it is the last place that we would expect to find a positive charge. In fact, whichever method we use to calculate the atomic charges, the nitrogens turn out to be slightly negative. The positive charge in the tetramethylammonium ions is almost completely situated on the 12 hydrogens.

We will now treat some common molecules using qualitative MO-theory. It is, however, important to remember that this treatment is an alternative to the common Lewis (VB) picture. We will point out some of the differences for the individual molecules.

Next:
Ethylene
Cyclopropane
p-Systems
Hyperconjugation