Chapter 11.1, Problem 2P

Interpretation Introduction

Interpretation:

Second resonance contributor for each of the given structures is to be written. Whether the charge or unpaired electron is shared equally by both allylic carbons or not is to be determined. If the charge or unpaired electron is not shared equally, the carbon atom which bears more charge or unpaired electron is to be identified.

Concept Introduction:

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The carbon atom which is adjacent to either of the double bonded carbon atoms is the allylic carbon atom. An allylic carbon atom is ${\text{sp}}^3$ hybridized.

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The allylic unit can be positively charged carbocation ${\text{(C=C-C}}^{+})$, a neutral free radical ${\text{C=C-CH}}_2{}^{•}$, or a negatively charged carbanion ${\text{(C=C-CH}}_2{:}^{-})$.

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Each of the above stabilizes by delocalization involving pi electrons in the double bond.

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The positive charge, negative charge, or unpaired electron is shared by the two carbons at the opposite ends of the allyl group.

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In allylic species that are not symmetrically substituted, the two resonance structures are not equivalent and do not contribute equally to the resonance hybrid.

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The most stable resonance structure contributes more to the resonance hybrid.