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

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 options is an example of 1,2-elimination, we need to analyze the reactions and the positions from which the groups are eliminated.

### Step-by-Step Solution:

1. **Understanding 1,2-Elimination**:
- In a 1,2-elimination reaction, two groups (usually a leaving group and a hydrogen atom) are removed from adjacent carbon atoms (the first and second carbon). This results in the formation of a double bond between these two carbons.

2. **Identifying the Reaction**:
- The question refers to a reaction where a hydroxide ion (OH⁻) abstracts a beta hydrogen from the beta position of a substrate in the presence of alcoholic KOH.
- For example, if we have a compound where the leaving group (X) is on carbon 1 and a hydrogen on carbon 2, the elimination will yield an alkene.

3. **Analyzing the Options**:
- **Option 1**: If the reaction involves the removal of a leaving group (X) from carbon 1 and a hydrogen from carbon 2, it is a 1,2-elimination.
- **Option 2**: This is a Wurtz reaction, where two halides are removed from carbon 1 and carbon 3, resulting in a 1,3-elimination.
- **Option 3**: If the elimination occurs from carbon 1 and carbon 4, it is a 1,4-elimination.

4. **Conclusion**:
- The correct answer is the option where the elimination occurs from adjacent carbons (1 and 2). This is the only scenario that qualifies as a 1,2-elimination.

### Final Answer:
The example of 1,2-elimination is the reaction where a leaving group is removed from carbon 1 and a hydrogen from carbon 2, leading to the formation of an alkene.