Which of the following nucleophiles has highest tendency for `S_(N)2` reactions?

Options

A. MeOH
B. `MeCH_(2)OH`
C. `Me_(2)CHOH`
D. `Me_(3)COH`

Answer

To determine which nucleophile has the highest tendency for \( S_N2 \) reactions, we need to analyze the stability of the carbocations formed during these reactions. The \( S_N2 \) mechanism involves a nucleophile attacking an electrophile, leading to the formation of a transition state and then a product where the leaving group departs.

### Step-by-Step Solution:

1. **Understanding \( S_N2 \) Reactions:**
- In \( S_N2 \) reactions, the nucleophile attacks the electrophile, resulting in the formation of a transition state. The reaction proceeds with a backside attack, leading to the inversion of configuration at the carbon center.

2. **Identifying the Role of Carbocation Stability:**
- The stability of the carbocation intermediate formed during the reaction is crucial. The more stable the carbocation, the more favorable the \( S_N2 \) reaction will be.

3. **Types of Carbocations:**
- Carbocations can be classified based on the number of alkyl groups attached to the positively charged carbon:
- **Primary Carbocation (1°):** Less stable (e.g., \( CH_3^+ \))
- **Secondary Carbocation (2°):** More stable than primary (e.g., \( CH_3CH_2^+ \))
- **Tertiary Carbocation (3°):** Most stable (e.g., \( (CH_3)_3C^+ \))

4. **Comparing Given Nucleophiles:**
- Let's analyze the nucleophiles provided:
- \( CH_3OH \) (Methanol) - forms a primary carbocation.
- \( CH_3CH_2OH \) (Ethanol) - forms a primary carbocation.
- \( CH_3C(CH_3)OH \) (Isopropanol) - forms a secondary carbocation.
- \( (CH_3)_3COH \) (Tert-Butanol) - forms a tertiary carbocation.

5. **Stability Order of Carbocations:**
- The stability order of the carbocations is:
- Tertiary (3°) > Secondary (2°) > Primary (1°)
- Therefore, the tertiary carbocation formed from \( (CH_3)_3COH \) is the most stable.

6. **Conclusion:**
- Since the tendency for \( S_N2 \) reactions is directly proportional to the stability of the carbocation, the nucleophile with the highest tendency for \( S_N2 \) reactions is \( (CH_3)_3COH \) (Tert-Butanol).

### Final Answer:
- The nucleophile with the highest tendency for \( S_N2 \) reactions is **\( (CH_3)_3COH \)**.