Chapter D, Problem D.11P

Interpretation Introduction

Interpretation:

It is to be determined on the basis of FMO theory whether the carbanion rearrangement analogous to the $\text{1, 2-hydride shift}$ in Equation $\text{D-10}$ is allowed or forbidden.

Concept introduction:

A reaction must go through a high energy transition state for the reactants to be converted to products. A high difference between the reactants and the transition state, called energy of activation, leads to a very low rate of reaction. For the rate of this step to be reasonable, the activation energy must be relatively low. For this to happen, the transition state must be stabilized substantially.

One way in which a transition state may be stabilized is an overlap between molecular orbitals of the reactants. If the highest energy occupied MO (HOMO) of one reactant can overlap substantially with the lowest energy unoccupied MO (LUMO) of the other reactant in the transition state, the transition state is stabilized. The reaction is then said to be an allowed reaction. For this to happen, the symmetry (sign) of the HOMO and LUMO must be the same in the overlapping region. The interacting MOs of the two reactants are called frontier molecular orbitals (FMO).

If the symmetries of the FMOs are different, there is no constructive interference and no net gain in energy. The reaction then becomes a forbidden reaction.