Page 38 - Mechanism and Theory in Organic Chemistry

Delocalized rr Bonding 27
But the important point for most purposes is that the highest-energy bonding MO
and the lowest-energy antibonding MO in ethylene will be the T and T* levels,
with the o's lower than the n and the a*'s higher than the n*.
1.4 DELOCALIZED 7C BONDING
In the allyl system (23), each carbon is trigonal, and each uses sp2 hybrids to
make bonds to its neighbors. The procedure outlined in the previous section is
23
therefore adequate for constructing the a MO's. The system of a orbitals obtained
is called the o framework. After constructing the cr framework, a p orbital remains
on each carbon. Thesep orbitals are the basis orbitals for the n system of molecular
orbitals.
Formation of rr Systems
In allyl, the three basisp orbitals can be symbolized as shown in 24. Now there is
no way to avoid the problem of the central p orbital interacting with more than
one other orbital. One approach is to go to the quantum theory rules and work
through the prescribed procedures to find how the three orbitals will combine.
The method, at an approximate level, is the Hiickel theory. It is described in detail
in the references cited earlier (see footnote 2, p. lo), and a brief derivation and
an example are given in Appendix 2. Herc, we illustrate the results for some
simple systems; later, in Chapter 10, we shall develop a method of obtaining the
same results qualitatively in a simple way.
The first rktcuxmember in making n system orbitals is that the number
of MO's is going to be the same as the number of basis p orbitals used. Th_us,for
a141 weshall get-three n MO's. The lowest-energy one will be the combination
It will have the shape shown in 25, and it wiIl be bonding.