In which of the following dehydration by conc. `H_(2)SO_(4)` no rearrangement is favourable ?

Options

A.
B.
C.
D.

Answer

To determine which dehydration by concentrated \( H_2SO_4 \) does not favor rearrangement, we need to analyze the carbocation stability formed during the dehydration process. The dehydration of alcohols typically involves the following steps:

### Step 1: Protonation of Alcohol
The first step in the dehydration mechanism is the protonation of the alcohol by concentrated sulfuric acid. This converts the alcohol into a better leaving group (water).

### Step 2: Formation of Carbocation
After protonation, the alcohol loses a water molecule, leading to the formation of a carbocation. The stability of this carbocation is crucial in determining whether rearrangement is favorable.

### Step 3: Analyzing Carbocation Stability
- **Option 1:** If the carbocation formed is a primary carbocation, it is less stable and unlikely to rearrange.
- **Option 2:** If the carbocation is secondary or tertiary, it can rearrange to form a more stable carbocation (e.g., from secondary to tertiary).
- **Option 3:** If the carbocation is part of a cyclic structure, it may undergo ring expansion or rearrangement to achieve a more stable configuration.

### Step 4: Identify the Correct Option
Based on the analysis of the carbocations formed in each option:
- In **Option 1**, the carbocation formed is primary and does not favor rearrangement due to its instability.
- In **Option 2**, the carbocation can rearrange to a more stable tertiary carbocation.
- In **Option 3**, the carbocation can also rearrange due to ring expansion.
- In **Option 4**, the carbocation formed is secondary and can rearrange to a more stable tertiary carbocation.

### Conclusion
Thus, the option where dehydration by concentrated \( H_2SO_4 \) does not favor rearrangement is **Option 1**.

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