Problem 194 - A Complete Resource Book in Chemistry for JEE Main

Question

Elimination of bromine from 2 -bromobutane results in the formation of (a) equimolar mixture of 1 and 2 -butene (b) predominantly 2 -butene (c) predominantly 1-butene (d) predominantly 2 -butyne

Step-by-Step Solution

Answer

(b) Predominantly 2-butene due to Zaitsev's rule.

Step 1: Understanding the Reaction Type

The elimination of bromine from 2-bromobutane involves a dehydrohalogenation process, which typically follows an E2 mechanism. In this process, a hydrogen atom is removed along with the leaving group, which is bromine in this case, resulting in the formation of an alkene.

Step 2: Identifying the Possible Alkenes

The structure of 2-bromobutane is CH3-CH(Br)-CH2-CH3. The elimination can occur in two possible ways: removing a hydrogen from carbon-1 to form 1-butene (CH2=CH-CH2-CH3) or removing a hydrogen from carbon-3 to form 2-butene (CH3-CH=CH-CH3).

Step 3: Applying Zaitsev's Rule

Zaitsev's rule states that the more substituted alkene is the major product in an elimination reaction. In this case, 2-butene is more substituted than 1-butene because it is a disubstituted alkene, whereas 1-butene is monosubstituted.

Step 4: Determining the Major Product

Based on Zaitsev's rule, 2-butene (CH3-CH=CH-CH3) will be formed predominantly as it is more stable compared to 1-butene. The stability is due to the increased substitution of the double bond, which leads to hyperconjugation and better electron distribution.

Key Concepts

E2 Mechanism Explained

When we talk about elimination reactions, the E2 mechanism stands out as a key process, especially in organic chemistry. "E2" means bimolecular elimination, which unfolds in a single step. Here, the reaction involves two molecules coming together to form the transition state. This unity is why we say it is a single-step reaction.
In the E2 mechanism, a base plays a critical role in "grabbing" a hydrogen atom that is beta to the leaving group. What’s beta, you ask? It just means it's adjacent. As the base snatches away this hydrogen, the leaving group (a halogen like bromine) departs at the same time. This entire concerted action leads to the formation of a double bond, giving rise to an alkene.

• The elimination occurs at the same moment the base forms a bond with the hydrogen.
• The transition state involves both the base and the substrate.
• This mechanism favors strong bases and high temperatures.

The beauty of the E2 reaction lies in its simplicity and elegance - all changes happen simultaneously, making it efficient and swiftly productive in creating alkenes.

Zaitsev's Rule in Action

Zaitsev's Rule is a guiding principle in elimination reactions, providing a clue about which alkene is likely to dominate the product mix. This rule stipulates that the more substituted alkene will emerge as the major product.
What does "more substituted" mean? It's all about the number of carbon groups attached to the carbon atoms of the double bond. More carbon attachments usually result in a more stable structure. In the case of 2-bromobutane, Zaitsev's Rule helps predict that 2-butene is the major output since it has more carbon substituents.

• Zaitsev's Rule highlights stability due to substituent effects.
• It posits that alkyl groups can stabilize a double bond through hyperconjugation and electron donation.

In layman's terms, think of substituents like supportive friends - the more you have, the more stable you feel. This stability arises because the "friends" help to evenly distribute the charge around the double bond. Thus, according to Zaitsev's Rule, 2-butene, with its greater "support system," is the predominant product.

Alkene Stability and Substitution

Alkenes aren't created equal; their stability can vary based on several factors. The number of substituents attached to the double-bonded carbons significantly affects this stability. The underlying logic is connected to how these substituents influence electron distribution - more substituents typically equate to better electron dispersal, which translates to stability.
An important principle underpinning alkene stability is hyperconjugation, where electrons in bonds adjacent to the double bond start "mingling" with it, thus stabilizing the molecule. This mix leads to a lesser concentration of any one charge, which enhances stability.

• More alkyl substituents lead to higher alkene stability.
• Stability results from better resonance and electron mobility.

This rationale of "more is better" explains why, in elimination reactions, the more substituted product - like 2-butene over 1-butene in our example - tends to be predominant. It’s not merely about quantity but about the quality of stability conferred by hyperconjugation and electronic effects.