Which of the following statements is true in aqueous medium?

Options

A. `CH_(2)CH_(2)S^(-)` is a stronger base and more nucleophilic than `CH_(3)CH_(2)O^(-)`
B. `CH_(3)CH_(2)S` is a stronger base but is less nucleophilic than `CH_(3)CH_(2)O^(-)`
C. `CH_(3)CH_(2)S^(-)` is a weaker base but is more nucleophilic than `CH_(3)CH_(2)O^(-)`
D. `CH_(3)CH_(2)S^(-)` is a weaker base and less nucleophilic than `CH_(3)CH_(2)O^(-)`

Explanation

To solve the question regarding which statement is true in an aqueous medium concerning the basicity and nucleophilicity of the given species, we will analyze the two species: C2H5O^- (ethoxide ion) and C2H5S^- (ethanethiolate ion).

### Step-by-Step Solution:

1. **Understanding Basicity**:
- Basicity refers to the ability of a compound or ion to donate its electron pair to an electrophile. The more electron-dense the species, the stronger the base.
- In our case, we have C2H5O^- and C2H5S^-.

2. **Comparing Basicity**:
- C2H5O^- has the negative charge on oxygen, which is more electronegative than sulfur. This means that oxygen holds onto its electrons more tightly, making it a stronger base.
- C2H5S^- has the negative charge on sulfur. Sulfur has a vacant d-orbital, which can delocalize the negative charge, reducing its basicity.
- Therefore, the order of basicity is:
\[ \text{C2H5O}^- > \text{C2H5S}^- \]

3. **Understanding Nucleophilicity**:
- Nucleophilicity is the ability of a nucleophile to attack a positive center. In an aqueous medium, solvation plays a significant role.
- When these ions are in water, they become solvated. The more negative charge a species has, the more it will be solvated.
- C2H5O^- has a higher negative charge density than C2H5S^-. Thus, it will undergo more solvation, which decreases its nucleophilicity.

4. **Comparing Nucleophilicity**:
- Since C2H5O^- is more solvated, its ability to attack electrophiles decreases compared to C2H5S^-.
- Therefore, the order of nucleophilicity is:
\[ \text{C2H5S}^- > \text{C2H5O}^- \]

5. **Conclusion**:
- We have established that C2H5O^- is a stronger base but a weaker nucleophile compared to C2H5S^-.
- The correct statement based on our analysis is:
- C2H5S^- is a weaker base but is more nucleophilic than C2H5O^-.

### Final Answer:
The correct statement is: **C2H5S^- is a weaker base but is more nucleophilic than C2H5O^-**.

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