(a) Draw a resonance structure for the allyl anion that...

(a) Draw a resonance structure for the allyl anion that shows, along with the following structure, that the two $\mathrm{CH}_{2}$ carbons are equivalent and indistinguishable. $$ \left[\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\ddot{\mathrm{C}} \mathrm{H}_{2} \leadsto\right] $$ (b) According to the resonance structures, how much negative charge is on each of the $\mathrm{CH}_{2}$ carbons? (c) Draw a single hybrid structure for the allyl anion that shows shared bonds as dashed lines and charges as partial charges.

(a) Draw a resonance structure for the allyl anion that shows, along with the following structure, that the two $\mathrm{CH}_{2}$ carbons are equivalent and indistinguishable.
$$
\left[\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\ddot{\mathrm{C}} \mathrm{H}_{2} \leadsto\right]
$$
(b) According to the resonance structures, how much negative charge is on each of the $\mathrm{CH}_{2}$ carbons?
(c) Draw a single hybrid structure for the allyl anion that shows shared bonds as dashed lines and charges as partial charges.

Key Concepts

Resonance Structures

Resonance structures are different Lewis representations of the same molecule that depict the delocalization of electrons within the molecule. They help illustrate how electrons, such as the negative charge or pi electrons, are shared among multiple atoms, contributing to the overall stability of the molecule.

Charge Delocalization

Charge delocalization refers to the spreading out of an electron's negative charge over several atoms rather than being localized on one. This distribution reduces the energy of the molecule, providing additional stability by allowing the negative charge to be shared, as seen in the allyl anion where the charge is equally divided between the terminal CH2 groups.

Equivalence of Carbon Atoms

The equivalence of atoms in resonance structures indicates that certain atoms have the same electronic environment and properties. In the allyl anion, the terminal CH2 carbons are equivalent because the delocalized negative charge is shared equally between them, making their chemical reactivity and bonding properties identical.

Resonance Hybrid

The resonance hybrid is a conceptual model that represents the true structure of a molecule as an average of all its resonance forms. Instead of switching between distinct resonance structures, the actual molecule has bonds and charges that are a blend of what is depicted in the individual resonance forms, often illustrated with dashed lines and partial charges.

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