Rank the following anions in order of decreasing nucleophilicity in a polar protic solvent (most $\rightarrow$ least nucleophilic).
To rank the given anions in order of decreasing nucleophilicity in a polar protic solvent, we need to understand the behavior of nucleophiles in such solvents. Polar protic solvents (like water, alcohols) have hydrogen bonding capability, which can affect the nucleophilicity of anions.
In polar protic solvents, nucleophilicity generally increases down the periodic table. This is because larger atoms are more polarizable and less hindered by hydrogen bonding, allowing them to be better nucleophiles despite being less basic. Let's analyze each anion:
- \(\text{CH}_3\text{CH}_2\text{CH}_2\text{O}^−\)
- \(\text{CH}_3\text{CH}_2\text{CH}_2\text{S}^−\)
- \(\text{CH}_3\text{CH}_2\text{C(O)O}^−\)
For these anions:
- 2: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{S}^−\) is the conjugate base of a thiol. Sulfur is larger and more polarizable than oxygen, leading to higher nucleophilicity in polar protic solvents.
- 1: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{O}^−\) is an alkoxide ion. Oxygen is less polarizable than sulfur, resulting in lower nucleophilicity.
- 3: \(\text{CH}_3\text{CH}_2\text{C(O)O}^−\) is a carboxylate ion. The resonance stabilization makes it the least nucleophilic.
Therefore, the correct order of nucleophilicity is 2 > 1 > 3.
Conclusion: The correct answer is 2 > 1 > 3 as sulfur-based anions are more nucleophilic in polar protic solvents than oxygen-based anions, and resonance-stabilized ions like carboxylates are the least nucleophilic.
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