Isobutylene is obtained when

Options

A. sodium t-butoxide is treated with methyl iodide
B. t-butyl bromide is treated with sodium methoxide
C. t-butyl alcohol is treated with conc. `H_(2)SO_(4)`
D. t-butyl methyl ether is heated with conc. `H_(2)SO_(4)`

Answer

To determine how isobutylene is obtained, we will analyze the provided options step by step.

### Step 1: Analyze Option 1 - Sodium tert-butoxide with Methyl Iodide
- **Reaction**: Sodium tert-butoxide (CH3O-Na+) reacts with methyl iodide (CH3I).
- **Mechanism**: The oxygen in sodium tert-butoxide attacks the methyl iodide, leading to the formation of an ether.
- **Conclusion**: This reaction does not produce isobutylene; instead, it produces an ether. Therefore, **Option 1 is incorrect**.

### Step 2: Analyze Option 2 - tert-butyl bromide with Sodium Methoxide
- **Reaction**: tert-butyl bromide (C4H9Br) reacts with sodium methoxide (CH3O-Na+).
- **Mechanism**: The bromine atom leaves, forming a stable tertiary carbocation (C4H9+). Due to steric hindrance, the methoxide ion cannot attack the carbocation directly.
- **Outcome**: The methoxide ion abstracts a proton from the adjacent carbon, resulting in the formation of isobutylene (C4H8).
- **Conclusion**: This reaction produces isobutylene. Therefore, **Option 2 is correct**.

### Step 3: Analyze Option 3 - tert-butyl Alcohol with Concentrated Sulfuric Acid
- **Reaction**: tert-butyl alcohol (C4H9OH) is treated with concentrated sulfuric acid (H2SO4).
- **Mechanism**: The alcohol group is protonated by sulfuric acid, forming a good leaving group (water). The leaving of water generates a tertiary carbocation.
- **Outcome**: Similar to the previous case, the carbocation loses a proton to form isobutylene.
- **Conclusion**: This reaction also produces isobutylene. Therefore, **Option 3 is correct**.

### Step 4: Analyze Option 4 - tert-butyl-methyl Ether with Concentrated Sulfuric Acid
- **Reaction**: tert-butyl-methyl ether (C4H9OCH3) is treated with concentrated sulfuric acid.
- **Mechanism**: The ether oxygen attacks a proton from sulfuric acid, forming a protonated ether. The leaving of the methyl group leads to the formation of a tertiary carbocation.
- **Outcome**: The carbocation loses a proton, resulting in the formation of isobutylene.
- **Conclusion**: This reaction also produces isobutylene. Therefore, **Option 4 is correct**.

### Final Conclusion:
- **Correct Options**: Options 2, 3, and 4 lead to the formation of isobutylene.
- **Incorrect Option**: Option 1 does not produce isobutylene.