Question

In an elimination reaction, Hoffmann product dominates over Sytzeff product if : (A) Leaving tendency of leaving group is poor (B) Bulkier base is used (C) Higher stability of hoffmann product over Sytzeff product (D)All

Short answer

In an elimination reaction, Hoffmann product dominates over the Sytzeff product if a bulkier base is used, as it leads to higher steric hindrance, favoring the formation of the less substituted (Hoffmann) product. Therefore, the correct answer is (B) Bulkier base is used.

Step-by-step solution

Option A: Leaving tendency of leaving group is poor.

In an elimination reaction, the leaving group is an important factor in determining the reaction outcome. However, the tendency of leaving group does not directly influence the predominance of Hoffmann or Sytzeff products. The leaving group affects the rate of the reaction but not the regioselectivity.

Option B: Bulkier base is used.

The bulkiness of the base plays a significant role in determining the outcome of an elimination reaction. A bulkier base leads to higher steric hindrance, which favors the formation of the less substituted (Hoffmann) product. Thus, using a bulkier base increases the chances of Hoffmann product to dominate over the Sytzeff product in an elimination reaction.

Option C: Higher stability of Hoffmann product over Sytzeff product.

The stability of the Hoffmann and Sytzeff products determines their predominance in the reaction. Generally, the more stable product is formed in greater amounts. If the Hoffmann product is more stable than the Sytzeff product, the reaction will favor the formation of the Hoffmann product.

Option D: All.

Considering the analysis of each option, we can conclude that only Option B (Bulkier base is used) directly influences the predominance of Hoffmann product over the Sytzeff product in an elimination reaction. Therefore, Option D (All) is not correct. In conclusion, the correct answer is: (B) Bulkier base is used

Key concepts

Hoffmann Product

The Hoffmann product is an outcome of an elimination reaction where the less substituted alkene is formed. This happens when the base used in the reaction is large and bulky, making it difficult for it to get close enough to extract a hydrogen from the more substituted carbon.
This results in the removal of a hydrogen atom from the less hindered, or less substituted, carbon, leading to the formation of the Hoffmann product.

• Occurs in elimination reactions.
• Forms the less substituted alkene.
• Favored by bulky bases.

This path is preferable with bulky bases due to steric hindrance, where the spatial arrangement of atoms or groups slows down or prevents a chemical reaction. Thus, when sterics are a concern, the Hoffmann product dominates.

Sytzeff Product

In contrast to the Hoffmann product, the Sytzeff product refers to the more substituted alkene formed during an elimination reaction. Generally, reactions favor the Sytzeff product when smaller bases are used, as these can easily remove the hydrogen from the more substituted carbon.
The Sytzeff rule suggests that more stable (more substituted) alkenes are favored. This means that in most cases, without steric hindrance, reactions will naturally lean towards forming the Sytzeff product.

• Common outcome in elimination reactions.
• Formed from the more substituted alkene.
• Typically favored without steric hindrance.

The inherent stability of the more substituted alkene makes it a preferred product under standard conditions when bulky bases don't interfere.

Bulkier Base

The bulkiness of the base is a crucial factor that shifts the balance towards forming the Hoffmann product in elimination reactions. A bulkier base creates steric impediments that prevent it from approaching the more substituted carbon, thus preferring the less substituted one.
In simple terms, the larger the base, the harder it is to maneuver close enough to the crowded regions of the molecule:

• Bulky bases tend to favor the less substituted Hoffmann product.
• Creates steric hindrance, impacting reaction site.
• Leads to a preference for extracting hydrogens from less substituted carbons.

This principle can be contrasted with smaller, more nimble bases, which can easily extract hydrogens from any carbon and typically lead to the formation of the more substituted Sytzeff product.