Rank the following anions in order of decreasing nucleophilicity in a polar protic solvent (most $\rightarrow$ least nucleophilic).
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In polar protic solvents (like water or alcohols):
- Down a group, nucleophilicity increases because larger atoms are less solvated ($\text{S}^- > \text{O}^-$).
- Among similar nucleophilic atoms, localized charges are much more nucleophilic than resonance-stabilized (delocalized) charges ($\text{R-O}^- > \text{R-COO}^-$).
Step 1: Understanding the Concept:
Nucleophilicity refers to the rate at which a species attacks an electrophile. It is affected by the atom's size, electronegativity, resonance stabilization, and the solvent used.
In polar protic solvents (like water or alcohols), the solvent molecules form strong hydrogen bonds with small, highly electronegative anions.
This "solvation shell" surrounds the anion, making it less reactive because it must break these bonds to reach the electrophile. Step 2: Detailed Explanation: Comparison 1: $O^-$ vs $S^-$ (1 vs 2):
Oxygen and Sulfur are in the same group. Sulfur is larger and less electronegative than Oxygen.
In a polar protic solvent, the small $O^-$ ion is heavily solvated by hydrogen bonds. This significantly reduces its effective nucleophilicity.
The larger $S^-$ ion is poorly solvated because it cannot form strong hydrogen bonds and is highly polarizable. Consequently, it is much freer to attack the electrophile.
Thus, Thiolate (2) is more nucleophilic than Alkoxide (1). ($2>1$). Comparison 2: Alkoxide vs Carboxylate (1 vs 3):
In the propoxide ion (1), the negative charge is localized on a single oxygen atom. This makes it a strong base and a strong nucleophile.
In the propanoate ion (3), the negative charge is delocalized over two oxygen atoms through resonance. This resonance stabilization makes the anion much more stable, less basic, and consequently a much weaker nucleophile.
Localized charges are always more nucleophilic than delocalized charges on the same type of atom.
Thus, Propoxide (1) is more nucleophilic than Propanoate (3). ($1>3$). Step 3: Final Answer:
Combining the rankings, we get the order: $2>1>3$.
The propanethiolate is most nucleophilic (polarizability/low solvation), followed by propoxide (localized charge), and propanoate is the least nucleophilic (resonance stabilization).
This matches option (D). Step 4: Final Verification:
The result $2>1>3$ is consistent with standard nucleophilicity trends in protic environments.
