Question

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

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

Answer 1

The Correct Option is A

The major product formed in the dehydrohalogenation reaction of 2-\text{Bromo}\text{pentane} is \text{Pent-2-ene}. This reaction is guided by Saytzeff's Rule. Let's break down the reasoning behind this:

1. Concept of Dehydrohalogenation: Dehydrohalogenation is an elimination reaction where a halogen and a hydrogen are removed from adjacent carbon atoms, forming a double bond. This results in the formation of an alkene.
2. Saytzeff's Rule Explained: Saytzeff's (or Zaitsev's) Rule states that in elimination reactions, the more substituted alkene is usually the major product. This is because more substituted alkenes are generally more stable due to hyperconjugation and the inductive effect.
3. Application to 2-Bromo-pentane:
   • When 2-Bromo-pentane undergoes a dehydrohalogenation reaction, the β-hydrogen is removed from either of the two adjacent carbons.
   • Removing a hydrogen from carbon-3 leads to the formation of Pent-2-ene, while removing from carbon-1 would lead to Pent-1-ene.
   • According to Saytzeff's Rule, the formation of Pent-2-ene is favored because it is a more substituted alkene compared to Pent-1-ene.
4. The Reason for Other Options:
   • Hund’s Rule: Relates to electron filling in orbitals and does not apply to this chemical reaction.
   • Hofmann Rule: States that the least substituted alkene is often formed if the base is bulky. Here, Saytzeff's Rule applies better due to substrate choice and typical conditions.
   • Huckel’s Rule: Applies to aromaticity, not elimination reactions.

Therefore, the correct answer is based on Saytzeff's Rule.