Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Toluene, when treated with \(\mathrm{Br}_{2} / \mathrm{Fe}\), gives \(p\)-bromotoluene as the major product because \(\mathrm{CH}_{3}\) group (a) is para directing [1999-3 Marks] (b) is meta directing (c) activates the ring by hyperconjugation (d) deactivates the ring

Step-by-Step Solution

Answer

(a) is para directing and (c) activates the ring by hyperconjugation.

Step 1: Identify the Reaction Type

The reaction involves electrophilic aromatic substitution of toluene using bromine supported by a Lewis acid catalyst (Fe). This reaction typically results in ortho and para products.

Step 2: Recognize the Directing Effects of the Methyl Group

The methyl group (\(\mathrm{CH}_3\)) on toluene is an electron-donating group. Electron-donating groups are known to be ortho-para directing when it comes to electrophilic aromatic substitutions.

Step 3: Analyze the Activating Effect

The methyl group activates the benzene ring through hyperconjugation. This is because the electron-donating ability enhances the electron density on the ring, thus favoring positions that are ortho and para relative to the methyl group.

Step 4: Conclude Based on Directing and Activating Effects

The bromine will preferentially add to the para position relative to the methyl group due to it being para-directing and activating the ring through hyperconjugation. Thus, the major product is para-bromotoluene.

Key Concepts

Para Directing

In electrophilic aromatic substitution reactions, the substituents attached to the benzene ring can direct incoming electrophiles to specific positions. The methyl group ( \(\text{CH}_3\) ) on toluene is a classic example of a para-directing substituent.

When an electrophile approaches, electron-donating groups, like the methyl group, will typically boost the electron density at the ortho and para positions.
These positions are located right next to (ortho) or directly opposite (para) the substituent group on the benzene ring.

Among these, the para position is often favored due to reduced steric hindrance compared to the ortho position.

• The electric influence of the methyl group fosters a higher electron density at the para position.
• This increased electron density makes the para position more accessible and reactive for electrophile attack.

Hyperconjugation

Hyperconjugation is a stabilizing interaction that results from the overlap of the \( \sigma \)-bond orbitals of a \( \text{C-H} \) bond adjacent to a positively charged carbon or a pi system.

In toluene, the methyl group ( \(\text{CH}_3\)) delocalizes some of its electron density into the benzene ring.
This occurs because the hydrogen atoms of the methyl group can shift their electron cloud towards the \(\pi\) system, thereby increasing the electron density on the benzene ring.

• By stabilizing the positively charged intermediate during the reaction, hyperconjugation enhances the electrophilic reaction's effectiveness.
• This effect is significant in directing the electrophile to more favorable positions (ortho and para) within the ring.

Electron-Donating Groups

In electrophilic aromatic substitution reactions, electron-donating groups (EDGs) play a crucial role in determining the reactivity and orientation of substitution.

Substituents like the methyl group in toluene are classified as EDGs because they push electron density into the benzene ring.
This generally makes the ring more nucleophilic and more likely to undergo further substitution.

• EDGs increase the electron density on the ring, which stabilizes the cationic intermediate formed during the reaction.
• Owing to this electron donation, these groups guide new electrophilic substitutions to the ortho and para positions.
• The overall effect is a more reactive and strategically substituted aromatic compound.