Problem 180 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3} \stackrel{\mathrm{HBr}}{\longrightarrow}\) product \((\mathrm{s})\). Number of products possible in the above reaction is

Step-by-Step Solution

Answer

Two products are possible.

Step 1: Identify the Reaction Type

The given compound is a conjugated diene, and the reaction involves adding HBr to it. This is an example of an electrophilic addition reaction, specifically a hydrohalogenation.

Step 2: Potential Sites for Addition

In conjugated dienes, the reaction can occur at multiple sites due to resonance. Identify the positions where the electrophile (H⁺ from HBr) can attack. The pi bonds between the first and second carbons and between the third and fourth carbons are potential sites for attack.

Step 3: Initial Protonation Step

The H⁺ ion from HBr can add to either end of the diene, creating a carbocation intermediate. Discuss the possible carbocations that can form. The addition can happen at either terminal double bond, generating a more stable allylic carbocation intermediate.

Step 4: Nucleophilic Attack

Br⁻, the nucleophile, will attack the carbocation formed in the previous step. Due to the allylic carbocation, resonance can lead to the formation of different positional isomers.

Step 5: Review of Resonance Structures

Analyze the resonance structures of the allylic carbocation to determine where the Br⁻ could subsequently add. Two key resonance structures can lead to different products.

Step 6: Identify the Products

From the resonance-stabilized intermediates, determine the possible structures of the final products. There are two main products formed due to the reaction.

Step 7: Count the Products

Based on the resonance and possible addition locations, count the distinct, stable products. The reaction results in two distinct products.

Key Concepts

Conjugated Diene

Conjugated dienes are organic compounds containing two alternating double bonds connected by a single bond. This unique structure allows more flexibility and stability through resonance. In the given reaction, the molecule has the formula \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3} \) which is a classic example of a conjugated diene.

What makes conjugated dienes special is their ability to stabilize through delocalization of electrons. When an electrophilic addition reaction occurs, these electrons are shared across the double bonds, providing more sites for chemical attack. This occurs because of their alternating pattern, unlike isolated dienes which have non-alternating double bonds.

Here's what to look for in conjugated dienes:

• Paired double bonds separated by one single bond.
• Potential to participate in reactions at multiple sites.
• Ability to stabilize intermediates via resonance.

This setup is crucial in reactions like hydrohalogenation, where the electrophile adds to one of the pi bonds first, creating a versatile reactive intermediate.

Hydrohalogenation

Hydrohalogenation is a type of electrophilic addition reaction where a hydrogen halide, such as HBr, adds across the double bonds in an unsaturated molecule like a diene. It begins when the hydrogen (H⁺) from HBr attacks one of the double bonds of the conjugated diene, initiating the process.

In this reaction with HBr, the main steps include:

• Protonation of the diene, leading to the formation of a carbocation intermediate.
• Subsequent attack by the bromine ion \( \mathrm{Br}^{-} \) on the carbocation to form the final product.

This process typically follows Markovnikov's rule, where the hydrogen atom bonds to the less substituted carbon atom of the double bond, enhancing the chance of more stable carbocation formation.

Hydrohalogenation reactions with conjugated dienes are especially interesting because they can result in multiple products due to the resonance in the intermediate carbocations, further influenced by the position of the double bonds.

Carbocation Intermediate

A carbocation intermediate is an essential step in the electrophilic addition reaction like hydrohalogenation. In this process, a positively charged carbocation forms after the initial addition of H⁺ to one of the double bonds in the conjugated diene.

In conjugated systems, these carbocations are typically more stable due to resonance, which spreads the positive charge over multiple atoms. Here's how carbocation intermediates work in this context:

• The addition of H⁺ generates a primary carbocation that's adjacent to another unsaturated bond.
• Resonance stabilization can distribute the positive charge, forming multiple allylic carbocation structures.

Each potential carbocation structure allows for different pathways for the nucleophilic attack by \( \mathrm{Br}^{-} \), leading to multiple potential products. The stability of the carbocation is a guiding factor in determining the preferred reaction pathway and consequently, the distribution of the reaction products.

Resonance Structures

Resonance structures are a fundamental concept when dealing with conjugated dienes. They describe the different ways electrons can be distributed in molecules to stabilize intermediates like carbocations during reactions.

In the electrophilic addition reaction involving \( \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3} \) with HBr, resonance plays a critical role:

• The initial carbocation formed can be represented by multiple resonance structures.
• Each resonance structure distributes the positive charge over different positions in the molecule.

These alternate resonance forms are not real molecules but rather a visual method to show the theoretical distribution of electrons.

The stability provided by these resonance structures enables the possible formation of different products. They allow overlap of p orbitals across the molecule's structure, ensuring delocalization of charge and contributing to the overall stability of the reaction intermediates.