Question

The major product formed in the dehydrohalogenation reaction of 2-Bromo pentane is Pent-2-ene. This product formation is based on

• A. Huckel’s Rule
• B. Saytzeff’s Rule
• C. Hund’s Rule
• D. Hofmann Rule

Answer 1

The Correct Option is B

To understand why 2-Bromo pentane undergoes dehydrohalogenation to primarily form Pent-2-ene, we need to consider the rules governing elimination reactions, specifically the Saytzeff's rule (also spelled Zaitsev's rule).

1. Understanding Dehydrohalogenation: This reaction involves the removal of a hydrogen atom and a halogen atom (in this case, bromine) from adjacent carbon atoms in an alkyl halide, resulting in the formation of an alkene. The reaction typically follows an elimination pathway.
2. Saytzeff’s Rule Overview: This rule predicts the preferred regioselectivity of elimination reactions. According to Saytzeff's rule, during dehydrohalogenation, the more substituted alkene is favored as the major product. This means that the hydrogen is preferentially removed from the β-carbon with fewer hydrogen atoms (more substituents), leading to a more stable, alkene with greater alkyl substitution.
3. Application to 2-Bromo pentane:
   • The structure of 2-Bromo pentane is CH₃-CHBr-CH₂-CH₂-CH₃.
   • During the elimination process, a base abstracts a hydrogen from one of the β-carbons. There are two possible β-carbons from which hydrogen can be abstracted: one leading to pent-1-ene and the other to pent-2-ene.
   • The formation of pent-2-ene involves the removal of hydrogen from the carbon atom adjacent to the more substituted carbon, leading to a tetrasubstituted alkene.
   • Pent-2-ene is more stable than pent-1-ene due to greater alkyl substitution which stabilizes the resulting alkene.
4. Conclusion: Therefore, the major product formed in the dehydrohalogenation of 2-Bromo pentane is Pent-2-ene, following Saytzeff’s rule because it results in the more stable, more substituted alkene product.

This makes Option: Saytzeff’s Rule the correct answer in explaining the major product formation during the reaction.