`CH_3-underset(CH_2)underset(|)CH-CH=CH_2+HBrrarrA` (predominant), A is

Options

A. `CH_3-underset(CH_3)underset(|)CH-underset(Br)underset(|)CH-CH_3`
B. `CH_3-underset(CH_3)underset(|)CH-CH_2-CH_2Br`
C. `CH_3-overset(Br)overset(|)underset(CH_3)underset(|)C-CH_2-CH_3`
D. None of the above

Answer

To solve the problem, we need to determine the predominant product (A) formed when the given alkene reacts with HBr. Let's break down the steps involved in this reaction:

### Step 1: Identify the Alkene Structure
The given alkene is CH₃-CH₂-CH=CH₂. This structure consists of a four-carbon chain with a double bond between the third and fourth carbon atoms.

### Step 2: Understand the Reaction with HBr
When an alkene reacts with HBr, the reaction proceeds via an electrophilic addition mechanism. The double bond acts as a nucleophile and attacks the hydrogen (H⁺) from HBr, leading to the formation of a carbocation.

### Step 3: Formation of the Carbocation
1. The double bond between the carbon atoms breaks, and H⁺ adds to one of the carbons involved in the double bond.
2. The other carbon will now have a positive charge, forming a carbocation.
3. In this case, the more stable carbocation will be favored.

### Step 4: Determine the Stability of Carbocations
- If H⁺ adds to the terminal carbon (C1), a primary carbocation (1°) is formed.
- If H⁺ adds to the second carbon (C2), a secondary carbocation (2°) is formed.
- If H⁺ adds to the third carbon (C3), a tertiary carbocation (3°) is formed.

The tertiary carbocation is the most stable due to hyperconjugation and inductive effects from surrounding alkyl groups.

### Step 5: Nucleophilic Attack by Br⁻
Once the carbocation is formed, the bromide ion (Br⁻) will attack the positively charged carbon. Since we formed a tertiary carbocation, Br⁻ will attack this carbon.

### Step 6: Write the Final Product
The final product (A) will be formed by attaching Br to the carbon where the carbocation was formed. The structure of the product will be:
- CH₃-CH(Br)-CH₂-CH₃

### Final Answer
The predominant product A is **CH₃-CH(Br)-CH₂-CH₃**.