Alkyl halides undergo nucleophilic substitution reactions in which halogen atom is replaced by other atom.
`RX + Nu:^(-) to Nu-R + X^(-)`
These reactions follow `S_(N)1` and `S_(N)2` type mechanism, in which `S_(N)1` takes place in two steps while `S_(N)2` takes place in single step. Due to their tendency to undergo substitution by a large number of nucleophiles, they form a variety of products.
Which of the following has highest nucleophilicity?

Options

A. `SH^(-)`
B. `H_(2)O`
C. `OH^(-)`
D. `F^(-)`

Answer

To determine which of the given nucleophiles has the highest nucleophilicity, we need to analyze the options based on their structure and electronegativity. Here’s a step-by-step solution:

### Step 1: Identify the Nucleophiles
The question provides several nucleophiles. Let's assume the options are:
1. \( \text{SH}^- \) (Thiol)
2. \( \text{H}_2\text{O} \) (Water)
3. \( \text{OH}^- \) (Hydroxide)
4. \( \text{F}^- \) (Fluoride)

### Step 2: Understand Nucleophilicity
Nucleophilicity refers to the ability of a species to donate a pair of electrons to an electrophile. Stronger nucleophiles are typically less electronegative, as they hold onto their electrons less tightly and can donate them more readily.

### Step 3: Analyze Each Nucleophile
- **\( \text{SH}^- \)**: Sulfur is less electronegative than oxygen and fluorine, which means it can donate its lone pair more easily.
- **\( \text{H}_2\text{O} \)**: Water is a neutral molecule and has lone pairs on oxygen, but it is more electronegative than sulfur, making it a weaker nucleophile.
- **\( \text{OH}^- \)**: Hydroxide has a negative charge and is a stronger nucleophile than water, but still less than \( \text{SH}^- \) because of the electronegativity of oxygen.
- **\( \text{F}^- \)**: Fluoride is highly electronegative and holds onto its electrons tightly, making it a very weak nucleophile despite having a negative charge.

### Step 4: Compare Nucleophilicity
Based on the above analysis:
- \( \text{SH}^- \) is the least electronegative and can donate its electrons easily.
- \( \text{OH}^- \) is a stronger nucleophile than \( \text{H}_2\text{O} \) but weaker than \( \text{SH}^- \).
- \( \text{F}^- \) is the weakest nucleophile due to its high electronegativity.

### Conclusion
Thus, the nucleophile with the highest nucleophilicity among the options is \( \text{SH}^- \).

### Final Answer
**The nucleophile with the highest nucleophilicity is \( \text{SH}^- \).**

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