Page 133 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

112 1.14. In the Hückel MO treatment, orbitals on nonadjacent atoms are assumed to have
no interaction. The concept of homoconjugation suggests that such orbitals may
CHAPTER 1 interact, especially in rigid structures in which the orbitals are directed toward
Chemical Bonding one another. Consider, e.g, norbornadiene, (bicyclo[2.2.1]hepta-2,5-diene).
and Molecular Structure

a. Construct an MO diagram according to HMO theory and assign orbital
energies.
b. Construct the qualitative MO diagram that would result from significant
overlap between the C(3) and C(5) and C(2) and C(6) orbitals.
c. The ionization potentials (IP) of some 2-substituted norbornadienes are given
below. The two IP values pertain to the system. Use PMO theory to
analyze the effect these substituents have on the IP. Use a qualitative MO
diagram to show how the substituents interact with the two double bonds
and how this affects the IP. Discuss the effect the substituents have on
the IP.

X IP 1 IP 2
H 8 69 9.55
CH 3 O 8 05 9.27
CN 9 26 10.12

1.15. a. Sketch the nodal properties of the Hückel HOMO orbital of the pentadienyl
cation.
b. The orbital coefficients of two of the MOs of pentadienyl are given
below. Specify which is of lower energy; classify each orbital as bonding,
nonbonding, or antibonding; and specify whether each orbital is S or A with
respect to a plane bisecting the molecule perpendicular to the plane of the
structure.

= 0 50 +0 50 −0 50 −0 50+
x 1 2 4 5
= 0 58 −0 58 +0 58
y 1 3 5
1.16. There has been discussion as to whether unsaturated EWGs such as formyl or
cyano stabilize or destabilize carbocations.

O O + N N +
H + H H + C H + C
H H