A chemist plans to prepare 1- bromo-2- pentene by the following reaction
`CH_(3)CH_(2)CH =CHCH_(3) + NBS overset(Delta)(to) CH_(3) CH_(2) CH= CHCH_(2)Br`
This plane is not likely to work because

Options

A. There will be no reaction
B. `CH_(3)underset(Br)underset(|)(CH_(2))CH= CHCH_(2)` will also form
C. `CH_(3)CH_(2)underset(Br)underset(|)(CH) -underset(Br)underset(|)(CH)- CH_(3)` will form
D. `BrCH_(2) CH_(2) CH= CH -CH_(3) ` will form

Answer

To solve the question regarding the preparation of 1-bromo-2-pentene from the given alkene using NBS (N-bromosuccinimide), we need to analyze the reaction and understand why the chemist's plan might not work as intended.

### Step-by-Step Solution:

1. **Identify the Reactants and Products**:
- The reactant is 2-pentene (CH₃CH₂CH=CHCH₃).
- The expected product is 1-bromo-2-pentene (CH₃CH₂CH=CHCH₂Br).

2. **Understand the Role of NBS**:
- NBS is used for bromination, particularly at the allylic positions of alkenes. An allylic position is the carbon atom adjacent to a double bond.

3. **Determine Allylic Positions**:
- In 2-pentene, the double bond is between the 2nd and 3rd carbons. The allylic positions are:
- Carbon 1 (CH₃) is adjacent to the double bond.
- Carbon 2 (CH₂) is part of the double bond.
- Carbon 3 (CH) is adjacent to the double bond.
- Carbon 4 (CH₂) is adjacent to the double bond.
- Therefore, the allylic positions are at Carbon 1 and Carbon 4.

4. **Bromination Mechanism**:
- When NBS is used, it can brominate at either of the allylic positions (C1 or C4). This means that both 1-bromo-2-pentene and 4-bromo-2-pentene could potentially form.

5. **Expected Products**:
- If bromination occurs at C1, the product would be 1-bromo-2-pentene.
- If bromination occurs at C4, the product would be 4-bromo-2-pentene.

6. **Conclusion**:
- Since there are two possible allylic positions for bromination, the reaction will not exclusively yield 1-bromo-2-pentene. Instead, a mixture of products will form, which includes both 1-bromo-2-pentene and 4-bromo-2-pentene.
- Therefore, the chemist's plan to prepare only 1-bromo-2-pentene is not likely to work as intended because multiple products will be formed due to the presence of two allylic positions.

### Final Answer:
The chemist's plan is not likely to work because the reaction will yield a mixture of products (1-bromo-2-pentene and 4-bromo-2-pentene) instead of exclusively forming 1-bromo-2-pentene.