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Chapter 4: Problem 140

Which of the following statements is correct? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a stronger base, but weaker nucleophile than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\). (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base, but more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a stronger base and more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\). (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base and less nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\).

Short Answer

Expert verified
Answer: (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base, but more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\).

Step by step solution

01

Compare the basicity of oxygen and sulfur

Notice that oxygen is more electronegative than sulfur. Due to their respective positions on the periodic table, oxygen has a higher tendency to attract electrons, while sulfur is more polarizable. Since basicity involves accepting protons, a more electronegative atom (oxygen) will have a weaker ability to accept protons, making the corresponding anion a weaker base.
02

Compare the nucleophilicity of oxygen and sulfur

Nucleophilicity depends on the ability of an atom to donate electrons to a suitable electrophile. Sulfur is more polarizable and can stabilize the negative charge over a larger volume, resulting in a higher nucleophilicity compared to oxygen. In polar aprotic solvents (like ethers or nitriles), the trend in nucleophilicity is mainly determined by the size and polarizability of the atoms. In these solvents, sulfur has a higher nucleophilicity than oxygen.
03

Relate the basicity and nucleophilicity to the given statements

From our previous analysis, we can conclude that ethylthiolate (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\)) is a weaker base but more nucleophilic than ethoxide (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)).
04

Choose the correct statement

Based on our previous conclusions, we can now identify the correct statement. Statement (b) states that "\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base, but more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)", which is consistent with our analysis. Therefore, statement (b) is the correct answer.

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