Arrange the following Nucleophiles in the decreasing order of their a…

The correct answer is Option (2) → (C) > (D) > (B) > (A)

The strength of a nucleophile depends on how available its electron pair is:

• Larger, more polarisable atom: sulphur is bigger and less electronegative than oxygen, so its lone pair is held less tightly. Thiolates (\( \ce{RS^{-}} \)) are stronger nucleophiles than alkoxides (\( \ce{RO^{-}} \)).

• Localised vs delocalised charge: in an alkoxide the negative charge sits on one oxygen; in a carboxylate it is spread over two oxygens by resonance, so the carboxylate is a weaker nucleophile.

• Further delocalisation: benzoic acid is a stronger acid than butanoic acid, so the benzoate ion is a weaker base and a weaker nucleophile than the butanoate ion.

• (C) \( \ce{CH3CH2CH2CH2S^{-}} \) — 1st (strongest). Butanethiolate: the charge is on the large, polarisable sulphur atom, whose lone pair is held least tightly.

• (D) \( \ce{CH3CH2CH2CH2O^{-}} \) — 2nd. Butoxide: the charge is localised on one small oxygen atom, so it is weaker than the thiolate but not spread out.

• (B) \( \ce{CH3CH2CH2COO^{-}} \) — 3rd. Butanoate: the charge is shared by two oxygen atoms through resonance, so the electron pair is less available.

• (A) \( \ce{C6H5COO^{-}} \) — 4th (weakest). Benzoate: a carboxylate too, and the conjugate base of benzoic acid, a stronger acid than butanoic acid, so it is the weakest base and nucleophile.

Shortcut: Options (2) and (3) both start with (C) and end with (A); the alkoxide (D) is a stronger nucleophile than the carboxylate (B), which leaves Option (2).

Note: The question is from beyond the NCERT textbook.