Chapter 14, Problem 14.1P

Interpretation Introduction

Interpretation:

A VB picture of the $\text{π}$ orbitals and the resonance hybrid of the allyl anion (${\text{CH}}_{\text{2}}{\text{ = CH - CH}}_{\text{2}}^{\text{-}}$) are to be drawn. Whether the two pictures agree or disagree is to be explained.

Concept introduction:

Some chemical and structural features of molecules are unaccounted for by VB theory, most notably electron delocalization, or resonance, over three or more nuclei. The VB theory picture of a species that has resonance is inaccurate because it represents a single resonance structure, not the hybrid.

Answer to Problem 14.1P

The VB picture of the $\text{π}$ orbitals of allyl anion is

The structure of the resonance hybrid of allyl anion is

Explanation of Solution

All three carbon atoms in the allyl anion are ${\text{sp}}^{\text{2}}$ hybridized. Each atom has one pure p AO, with all three parallel to each other. Therefore, VB picture of the allyl anion can be drawn as

This corresponds to a double bond between C2 and C3, with a $\text{-1}$ formal charge on C1. This does not agree with the resonance hybrid because the hybrid shows the charge delocalized on C1 and C2. Also, the two bonds in the hybrid, $\text{C1}-\text{C2}$ and $\text{C2}-\text{C3}$, are equivalent, with a partial double bond character,

Conclusion

The VB theory picture of a given species represents a single resonance structure and not the hybrid.