The major product in the following reaction is:

Question

Accepted Answer

In the given reaction, we have a compound $CH_{3}-CH_{2}-CH_{2}-CH-CH_{3}$ undergoing a reaction with alcoholic KOH. This is a classic example of an elimination reaction, specifically dehydrohalogenation, where the alcoholic KOH acts as a strong base to remove a hydrogen halide from the molecule, resulting in the formation of an alkene.

1. **Explanation for Option 4 ($CH_{3}-CH_{2}-CH=CH-CH_{3}$):**
   - The reaction with alcoholic KOH typically leads to an elimination reaction (E2 mechanism), where a hydrogen atom is removed from the β-carbon (the carbon adjacent to the carbon bearing the leaving group, which is usually a halogen in such reactions). 
   - In this case, the most stable alkene is formed by removing a hydrogen from the β-carbon that results in the formation of a double bond between the second and third carbon atoms in the chain. This leads to the formation of $CH_{3}-CH_{2}-CH=CH-CH_{3}$, which is a more substituted and thus more stable alkene due to hyperconjugation and the inductive effect.
   - This is why Option 4 is the correct answer.

2. **Explanation for why other options are incorrect:**
   - **Option 1 ($CH_{3}-CH_{2}-CH_{2}-CH(OH)-CH_{3}$):** This option suggests the formation of an alcohol. However, the reaction conditions (alcoholic KOH) favor elimination rather than substitution, making the formation of an alcohol unlikely.
   - **Option 2 ($CH_{3}-CH_{2}-CH_{2}-CH=CH_{2}$):** This option represents an alkene, but it is less substituted compared to the alkene in Option 4. According to Zaitsev's rule, the more substituted alkene is favored in elimination reactions, making this option less favorable.
   - **Option 3 ($CH_{3}-CH_{2}-CH_{2}-CH_{2}-CH_{2}OH$):** This option suggests the formation of a primary alcohol, which is not consistent with the elimination reaction conditions provided by alcoholic KOH. The reaction conditions do not support the addition of an OH group to the chain.

In conclusion, the reaction conditions and the mechanism involved strongly support the formation of the more substituted alkene as described in Option 4, making it the correct answer.