Order nucleophilicity
`(i)OH-" "(ii)HS^(-)" "(iii)Ph-O^(-)" "(iv)C_(2)H_(5)-O^(-)`

Options

A. `I gt ii gt iiigt iv`
B. `ii gt iv gt I gt iii`
C. `ii gt iii gt I gt iv`
D. `iii gt iv gt I gt ii`

Explanation

To determine the order of nucleophilicity for the given species \( \text{OH}^- \), \( \text{HS}^- \), \( \text{Ph-O}^- \), and \( \text{C}_2\text{H}_5\text{-O}^- \), we need to analyze the stability of the negative charge in each species. The less stable the negative charge, the better the nucleophile it will be.

### Step-by-Step Solution:

1. **Analyze \( \text{Ph-O}^- \)**:
- The negative charge on the oxygen is stabilized by resonance with the benzene ring. This resonance delocalizes the negative charge, making it more stable and thus a poorer nucleophile.
- **Conclusion**: \( \text{Ph-O}^- \) is the least nucleophilic.

2. **Compare \( \text{OH}^- \) and \( \text{HS}^- \)**:
- \( \text{OH}^- \) has a negative charge on oxygen, which is more electronegative than sulfur. This makes the negative charge on \( \text{OH}^- \) more stable compared to \( \text{HS}^- \).
- Therefore, \( \text{HS}^- \) is more nucleophilic than \( \text{OH}^- \) because the negative charge on sulfur is less stable.
- **Conclusion**: \( \text{HS}^- \) > \( \text{OH}^- \).

3. **Compare \( \text{C}_2\text{H}_5\text{-O}^- \) with \( \text{HS}^- \)**:
- The negative charge on \( \text{C}_2\text{H}_5\text{-O}^- \) is on oxygen, which is electronegative and stabilizes the negative charge. However, the ethyl group has a +I (inductive) effect, which destabilizes the negative charge slightly.
- \( \text{HS}^- \) is still more nucleophilic than \( \text{C}_2\text{H}_5\text{-O}^- \) because the negative charge on sulfur is less stable than that on oxygen.
- **Conclusion**: \( \text{HS}^- \) > \( \text{C}_2\text{H}_5\text{-O}^- \).

4. **Compare \( \text{C}_2\text{H}_5\text{-O}^- \) with \( \text{OH}^- \)**:
- The ethyl group in \( \text{C}_2\text{H}_5\text{-O}^- \) provides some destabilization to the negative charge due to its +I effect, making it a better nucleophile than \( \text{OH}^- \).
- **Conclusion**: \( \text{C}_2\text{H}_5\text{-O}^- \) > \( \text{OH}^- \).

5. **Final Order of Nucleophilicity**:
- From the comparisons, we can summarize the order of nucleophilicity as follows:
- \( \text{HS}^- > \text{C}_2\text{H}_5\text{-O}^- > \text{OH}^- > \text{Ph-O}^- \)

Thus, the final order of nucleophilicity is:
\[ \text{HS}^- > \text{C}_2\text{H}_5\text{-O}^- > \text{OH}^- > \text{Ph-O}^- \]