Butene when treated with chlorine at about `500^(@)C` forms:

Options

A. `MeCH_(2)CHCl-CH_(2)Cl`
B. `MeCH(Cl)-CH=CH_(2)`
C. `ClCH_(2)CH_(2)CH=CH_(2)`
D. `MeC(Cl)_(2)CH=CH_(2)`

Explanation

To solve the question of what butene forms when treated with chlorine at about 500°C, we can follow these steps:

### Step 1: Identify the Structure of Butene
Butene is an alkene with the formula C4H8. The structure of butene can be represented as:
\[ \text{CH}_3\text{CH}_2\text{CH}=\text{CH}_2 \]
This structure indicates that there is a double bond between the last two carbon atoms.

### Step 2: Understand the Reaction with Chlorine
When butene is treated with chlorine (Cl2) at high temperatures (around 500°C), a substitution reaction occurs. The chlorine can react with the allylic hydrogen atoms (the hydrogen atoms attached to the carbon adjacent to the double bond).

### Step 3: Identify the Allylic Hydrogen
In the structure of butene, the allylic hydrogens are those attached to the carbon atoms next to the double bond. In this case, the carbon atoms adjacent to the double bond are:
- Carbon 2 (CH2) has 2 hydrogens.
- Carbon 3 (CH) has 1 hydrogen.

### Step 4: Chlorination of Allylic Hydrogen
When chlorine is introduced, one of the allylic hydrogens will be replaced by a chlorine atom. This leads to the formation of a product where chlorine is attached to the allylic carbon (Carbon 3).

### Step 5: Determine the Product Structure
After replacing the allylic hydrogen with chlorine, the structure of the product becomes:
\[ \text{CH}_3\text{CCl}\text{H}\text{CH}=\text{CH}_2 \]
This can be numbered as follows:
1. CH3 (Carbon 1)
2. CCl (Carbon 2)
3. CH (Carbon 3, where the chlorine is attached)
4. CH2 (Carbon 4, part of the double bond)

This compound is known as 3-chloro-1-butene.

### Step 6: Select the Correct Option
From the options given in the question:
1. MeCH2CHClCH2Cl
2. MeCHClCH=CH2
3. ClCH2CH2CH=CH2
4. MeCCl2CH=CH2

The correct structure corresponding to our product (3-chloro-1-butene) is:
**Option 2: MeCHClCH=CH2**

### Final Answer
Butene when treated with chlorine at about 500°C forms **3-chloro-1-butene**, corresponding to **Option 2: MeCHClCH=CH2**.

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