Which of the following is an example of 1,2 -elimination ?

Options

A. `CH_(3)-underset(Br)underset(|)(CH)-CH_(3) overset("Reagent")(rarr)CH_(4)-CH=CH_(2)`
B.
C. `H_(2)underset(X)underset(|)(C)-CH_(2)-CH_(2)-underset(Y)underset(|)(CH_(2))overset("Reagent")(rarr)H_(3)C-CH=CH-CH_(3)`
D. All of these

Answer

To determine which of the given reactions is an example of 1,2-elimination, we need to analyze each option based on the definition of 1,2-elimination. This type of elimination involves the removal of atoms or groups from adjacent carbon atoms.

### Step-by-Step Solution:

1. **Understanding 1,2-elimination**:
- 1,2-elimination, also known as alpha-beta elimination, involves the removal of atoms or groups from two adjacent carbon atoms in a molecule.

2. **Analyzing Option A**:
- The reaction given is CH3CHBrCH3 in the presence of a reagent, resulting in an alkene.
- Here, bromine (Br) is eliminated from the second carbon, and a hydrogen (H) is eliminated from the first carbon.
- Since the elimination occurs from two adjacent carbons (1 and 2), this reaction is indeed a 1,2-elimination.

3. **Analyzing Option B**:
- The reactant is CH2-X CH2-CH2-Y, which leads to the formation of a three-membered ring.
- In this case, X is eliminated from the first carbon and Y from the third carbon.
- Since the elimination occurs from non-adjacent carbons (1 and 3), this is a 1,3-elimination, not a 1,2-elimination.

4. **Analyzing Option C**:
- The reactant is CH2X CH2CH2CH2Y, which also leads to the formation of an alkene.
- Here, X is eliminated from the first carbon and Y from the fourth carbon.
- This results in a 1,4-elimination, as the elimination occurs from carbons that are not adjacent.

5. **Conclusion**:
- Based on the analysis, only Option A demonstrates 1,2-elimination, while Options B and C do not.

### Final Answer:
- The correct answer is **Option A**, which is an example of 1,2-elimination.