Chapter D, Problem D.9P

Interpretation Introduction

Interpretation:

The $\text{1, 2-}$ methyl shift involving ${{\text{(CH}}_{\text{3}}\text{)}}_{\text{3}}{\text{CC}}^{+}{\text{HCH}}_{\text{3}}$ is to be discussed.

Concept introduction:

In a $\text{1, 2-}$ alkyl shift, a $\text{C}-\text{H}$ or $\text{C}-\text{C}$ bond adjacent to a carbocation is broken, and the positive charge is rearranged. It moves to the C atom whose bond is broken.

The carbocation rearrangement involves single species, so the carbocation must have both HOMO and LUMO for orbital interaction. The HOMO is the $\text{σ}$ orbital that connects to the migrating group, and the LUMO is the empty $\text{p}$ orbital of the positively charged C atom. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) must have axial overlap or parallel overlap. The axial overlap between HOMO and LUMO has substantial net overlap, so this reaction is allowed. If the HOMO and LUMO do not have a substantial overlap, this overlap is forbidden.