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Which of the following carbanions is the most stable?

1. CH3(-)
2. CH2=CH-CH2(-)
3. C6H5-CH2(-)
4. p-NO2-C6H4-CH2(-)

Correct answer: p-NO2-C6H4-CH2(-)

Solution

Carbanion stability increases as the negative charge is delocalized. The methyl carbanion has no delocalization. Allyl and benzyl carbanions are stabilized by resonance with the pi system. In the p-nitrobenzyl carbanion, the charge is delocalized over the ring AND further onto the strongly electron-withdrawing -NO2 group (its oxygen atoms), giving the greatest stabilization.

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