Write a second resonance contributor for each of the following. Is the charge or unpaired...

Write a second resonance contributor for each of the following. Is the charge or unpaired electron shared equally by both allylic carbons? If not, which one bears more of the charge or unpaired electron?

(a)

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(b)

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(c)

Sample Solution

(a) First, identify the allylic unit by picking out the sequence. Of the two double bonds in this structure, only the one at the left is part of . The double bond at the right is separated from the positively charged carbon by a CH₂ group, so is not conjugated to it. Move electrons in pairs from the double bond toward the positively charged carbon to generate a second resonance structure.

The two contributing structures are not equivalent; therefore, the positive charge is not shared equally between C-1 and C-3. C-1 is a primary carbon, C-3 is secondary. More of the positive charge resides on C-3 than on C-1. The original structure (left) contributes more to the resonance hybrid than the other (right).