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Arrange the following alkoxide ions in decreasing order of stability: (A) CH3-CH(NO2)-O- (an alkoxide with a NO2 on the same carbon as O-), (B) -O-C(=O)-CH(NO2)- (a carboxylate-type oxygen with NO2 on the adjacent carbon), (C) O2N-CH2-CH2-O- (an alkoxide with NO2 on the carbon beta to O-).

1. (C) > (B) > (A)
2. (C) > (A) > (B)
3. (B) > (C) > (A)
4. (B) > (A) > (C)

Correct answer: (B) > (A) > (C)

Solution

Stability of an alkoxide/anion increases with delocalization of the negative charge. (B) is a carboxylate-type oxygen where the charge is resonance-delocalized over two oxygens AND further stabilized by the adjacent NO2 (-I), making it most stable. (A) and (C) are simple alkoxides with no resonance delocalization of the O- charge, stabilized only by the electron-withdrawing inductive effect of NO2. Since the inductive effect falls off with distance, NO2 on the same (alpha) carbon in (A) stabilizes more than NO2 on the beta carbon in (C). Order: (B) > (A) > (C).

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