Elimination of HBr from 2-bromobutane results in the formation of :

Question

Accepted Answer

Elimination of HBr from 2-bromobutane is a classic example of a dehydrohalogenation reaction, which typically follows the Zaitsev's rule. According to Zaitsev's rule, the more substituted alkene is generally the major product in elimination reactions. This is because more substituted alkenes are more stable due to hyperconjugation and the inductive effect.

1. **Explanation for Option 2 (Predominantly 2-butene):**
   - In the elimination of HBr from 2-bromobutane, the base abstracts a proton from the β-carbon that has the fewest hydrogens, leading to the formation of the more substituted alkene. In this case, the removal of a hydrogen from the carbon adjacent to the carbon bearing the bromine atom results in the formation of 2-butene.
   - 2-butene is more stable than 1-butene because it is a more substituted alkene. It has two alkyl groups attached to the double-bonded carbons, which stabilize the double bond through hyperconjugation and the inductive effect.
   - Therefore, the reaction predominantly yields 2-butene, making Option 2 the correct answer.

2. **Explanation for why other options are incorrect:**
   - **Option 1 (Equimolar mixture of 1- and 2-butene):** This option is incorrect because the reaction does not produce an equimolar mixture. The Zaitsev's rule predicts that the more substituted alkene (2-butene) will be the major product, not an equal mixture.
   - **Option 3 (Predominantly 1-butene):** This option is incorrect because 1-butene is the less substituted and less stable alkene compared to 2-butene. According to Zaitsev's rule, the reaction favors the formation of the more substituted alkene, which is 2-butene.
   - **Option 4 (Predominantly 2-butyne):** This option is incorrect because the reaction involves the elimination of HBr to form an alkene, not an alkyne. The formation of an alkyne would require a different set of conditions and reagents, which are not present in this reaction.

In conclusion, the elimination of HBr from 2-bromobutane predominantly results in the formation of 2-butene due to the stability provided by the more substituted double bond, in accordance with Zaitsev's rule. This supports Option 2 as the correct answer.