Exams › JEE Advanced › Chemistry

Among the following ions, which is the most stable?

1. Ph-CH2⁻ (benzyl carbanion)
2. H2C=CH-CH2⁻ (allyl carbanion)
3. OH⁻ (hydroxide ion)
4. ^-O-C(=O)-CH3 (acetate ion)

Correct answer: ^-O-C(=O)-CH3 (acetate ion)

Solution

The most stable anion has the most stabilized negative charge. Acetate (CH3COO⁻) has the charge delocalized over two equivalent oxygen atoms via resonance, and the conjugate acid (acetic acid, pKa approx 4.75) is much stronger than water (pKa 15.7) or the hydrocarbons (pKa 43). Hence acetate is the most stable. Ranking: acetate >> OH⁻ > allyl⁻ approx benzyl⁻.

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