Question

the correct order of stability of the following carbanions is?
i.
ii.
iii.

1. i<ii<iii
2. i>ii>iii
3. ii<i<iii
4. iii<i<ii

Explanation:

Step1: Aromaticity and Stability

Aromatic compounds are highly stable. Antiaromatic compounds are highly unstable. Non - aromatic compounds have stability in between.

Step2: Hückel's Rule for Aromaticity

Hückel's rule: For a planar, cyclic, conjugated compound, if it has \(4n + 2\) (\(n=0,1,2,\cdots\)) \(\pi\) - electrons, it is aromatic. If it has \(4n\) \(\pi\) - electrons (\(n = 1,2,\cdots\)), it is anti - aromatic.

• For structure I: It is a three - membered ring with a negative charge. The number of \(\pi\) - electrons \(=4\) (\(n = 1\) in \(4n\)), so it is anti - aromatic (unstable).
• For structure II: It is a five - membered ring with a negative charge. The number of \(\pi\) - electrons \(=6\) (\(n = 1\) in \(4n+2\)), so it is aromatic (stable).
• For structure III: It is a six - membered ring with a negative charge. The number of \(\pi\) - electrons \(=8\) (\(n = 2\) in \(4n\)), so it is anti - aromatic (unstable). But the six - membered ring has more resonance compared to the three - membered ring.

Answer:

1. III < I < II