A vicinal dihalide is not formed in the reaction-

Options

A. `HOCH_(2)–CH_(2)OHoverset(PBr_(3))rarr`
B. `CH_(3)–CH=CH_(2) overset(Br_(2))`
C. `CH-=CH overset(HBr)rarr overset(HBr)rarr`
D. `CH_(3)–CH=CHBr underset("Peroxide")overset("HBr")rarr`

Answer

To determine which reaction does not form a vicinal dihalide, we need to analyze each option provided in the question. A vicinal dihalide is characterized by two halogen atoms attached to adjacent carbon atoms. Let's go through the options step by step.

### Step 1: Analyze the first reaction
**Reaction:** CH2OH + PBr3

- **Mechanism:** In this reaction, PBr3 acts as a reagent that facilitates nucleophilic substitution. The bromide ion (Br-) from PBr3 will replace the hydroxyl group (OH) on the carbon.
- **Product Formation:** The product formed will be CH2Br and CH2Br, resulting in two bromine atoms on adjacent carbon atoms.
- **Conclusion:** This reaction forms a vicinal dihalide.

**Hint:** Look for the presence of halogens on adjacent carbons to identify vicinal dihalides.

### Step 2: Analyze the second reaction
**Reaction:** CH2=CH2 + Br2

- **Mechanism:** The addition of Br2 to an alkene occurs via an electrophilic addition mechanism. The bromine atoms will add to the double-bonded carbons.
- **Product Formation:** The product will be CH2Br-CHBr, which has bromine atoms on adjacent carbons.
- **Conclusion:** This reaction also forms a vicinal dihalide.

**Hint:** Electrophilic addition of halogens to alkenes typically results in vicinal dihalides.

### Step 3: Analyze the third reaction
**Reaction:** CH≡C + HBr

- **Mechanism:** The addition of HBr to an alkyne follows an electrophilic addition mechanism. The hydrogen will add to one carbon and the bromine to the other.
- **Product Formation:** The product will be CH2=CHBr. In this case, the bromine is added to one carbon, and the hydrogen to the other, resulting in a double bond rather than a vicinal dihalide.
- **Conclusion:** This reaction does not form a vicinal dihalide; instead, it forms a terminal alkene.

**Hint:** Consider the structure of the product formed after the reaction to identify if it meets the criteria for vicinal dihalides.

### Step 4: Analyze the fourth reaction
**Reaction:** CH3-CH=CHBr (with HBr in the presence of peroxide)

- **Mechanism:** The addition of HBr in the presence of peroxide follows a free radical mechanism, which leads to anti-Markovnikov addition.
- **Product Formation:** The product will be CH3-CHBr-CH2Br, resulting in bromine atoms on adjacent carbons.
- **Conclusion:** This reaction forms a vicinal dihalide.

**Hint:** Free radical mechanisms can lead to unexpected products; always check the final structure.

### Final Conclusion:
After analyzing all the reactions, we find that the reaction involving **CH≡C + HBr** does not form a vicinal dihalide. Therefore, the correct answer is option **C**.