Dehydration by conc. `H_(2)SO_(4)` is the most difficult in

Options

A.
B.
C.
D.

Answer

To determine which alcohol is the most difficult to dehydrate using concentrated sulfuric acid (H₂SO₄), we need to analyze the stability of the carbocations formed during the dehydration process. The stability of the carbocation is crucial because the formation of the carbocation is the rate-determining step (RDS) in the dehydration mechanism.

### Step-by-Step Solution:

1. **Understanding Dehydration Mechanism**:
- Dehydration of alcohols typically involves the loss of a water molecule (H₂O) to form an alkene. This process can be facilitated by strong acids like concentrated H₂SO₄.
- The mechanism involves protonation of the alcohol to form a better leaving group (water), followed by the formation of a carbocation.

2. **Carbocation Formation**:
- The first step in the dehydration mechanism is the protonation of the alcohol, resulting in the formation of a protonated alcohol (ROH₂⁺).
- The leaving of water (H₂O) leads to the formation of a carbocation (R⁺).

3. **Evaluating Carbocation Stability**:
- Carbocation stability follows the order: tertiary (3°) > secondary (2°) > primary (1°) > methyl (0°).
- The more stable the carbocation, the easier it is to form, and thus the easier the dehydration reaction will be.

4. **Analyzing Given Options**:
- **Option 1**: A primary alcohol will form a primary carbocation, which is the least stable.
- **Option 2**: A secondary alcohol will form a secondary carbocation, which is more stable than primary but less stable than tertiary.
- **Option 3**: A tertiary alcohol will form a tertiary carbocation, which is the most stable.
- **Option 4**: A compound that cannot form a stable carbocation (like a bridged carbocation) will be very difficult to dehydrate.

5. **Conclusion**:
- The alcohol that forms the least stable carbocation will be the most difficult to dehydrate.
- Based on the analysis, the primary alcohol (Option 1) is the most difficult to dehydrate because it forms a primary carbocation, which is the least stable.

### Final Answer:
The most difficult alcohol to dehydrate by concentrated H₂SO₄ is the primary alcohol.