Which is a better nucleophile in aqueous solution?

• In water (a protic solvent), small, highly basic ions like RO⁻ are strongly solvated by hydrogen bonding.
• Larger, less basic ions like RS⁻ are less solvated and therefore more available to attack electrophiles.

Answer: RS⁻ is the better nucleophile in aqueous solution.

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Which is a better nucleophile in DMSO?

• DMSO is a polar aprotic solvent — it does not strongly solvate anions.
• Hence, nucleophilicity parallels basicity in aprotic solvents.
• Since RO⁻ is the stronger base, it is also the stronger nucleophile in DMSO.

Answer: RO⁻ is the better nucleophile in DMSO.

a. Br⁻ or Cl⁻ in H₂O → Br⁻

• In protic solvents larger, more polarizable anions are less strongly solvated and therefore better nucleophiles: I⁻ > Br⁻ > Cl⁻ > F⁻.

b. Br⁻ or Cl⁻ in DMSO → Cl⁻

• In polar aprotic solvents nucleophilicity follows basicity (smaller, harder = better): Cl⁻ is a better nucleophile than Br⁻ in DMSO.

c. CH₃O⁻ or CH₃OH in H₂O → CH₃O⁻

• The anion (methoxide) is far more nucleophilic than neutral methanol, even though it is somewhat solvated in water.

d. CH₃O⁻ or CH₃OH in DMSO → CH₃O⁻

• In a polar aprotic solvent the anionic nucleophile is even more reactive (less solvated), so methoxide wins by a larger margin.

e. HO⁻ or ⁻NH₂ in H₂O → HO⁻

• Practical point: ⁻NH₂ is so basic that it is protonated by water (it doesn’t exist appreciably in aqueous solution), so hydroxide is the available nucleophile.

f. HO⁻ or ⁻NH₂ in DMSO → ⁻NH₂

• In an aprotic medium where ⁻NH₂ can exist, the stronger base (⁻NH₂) is the stronger nucleophile.

g. I⁻ or Br⁻ in H₂O → I⁻

• Same protic-solvent rule: the larger, more polarizable I⁻ is less solvated and therefore the better nucleophile in water.

h. I⁻ or Br⁻ in DMSO → Br⁻

• In polar aprotic solvents nucleophilicity follows basicity/charge density: the smaller Br⁻ is the stronger nucleophile than I⁻.