Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Toluene when treated with \(\mathrm{Br}_{2}\) and \(\mathrm{Fe}\), gives \(\mathrm{p}\)-bromotoluene as the major product, because the methyl group 1\. is para-directing 2\. is m-directing 3\. activates the ring by hyperconjugation 4\. deactivates the ring of the above (a) 1,3 (b) \(1,2,3\) (c) 1,2 (d) none of these

Step-by-Step Solution

Answer

Correct answer is (a) 1,3.

Step 1: Understand the Reaction

When toluene (methylbenzene) is treated with bromine \( \mathrm{Br}_2 \) in the presence of iron \( \mathrm{Fe} \), a substitution reaction occurs. The methyl group on toluene influences the reaction as it is an ortho/para-directing group, making certain positions on the aromatic ring more reactive for substitution.

Step 2: Analyze Directing Effects

The methyl group is an electron-donating group, which increases electron density on the benzene ring through the phenomenon of hyperconjugation. This makes the ortho and para positions more reactive. Thus, the methyl group is ortho/para-directing, not meta-directing.

Step 3: Investigate Activation or Deactivation

Due to hyperconjugation, the methyl group activates the benzene ring, making it more reactive towards electrophilic aromatic substitution. Activated rings favor substitution reactions, contrasting with deactivated rings which are less reactive.

Step 4: Deduce the Product Formation

The reaction favors formation of p-bromotoluene as the major product due to these directing effects. This aligns with the methyl group being para-directing and activating the ring by hyperconjugation.

Step 5: Choose Correct Options

From the given choices: 1. the methyl group is para-directing, and 3. it activates the ring by hyperconjugation. So the correct answer corresponds to option (a) \(1,3\).

Key Concepts

Directing Effects in Aromatic Compounds

In aromatic chemistry, the directing effects determine how substituents on a benzene ring influence the position of additional incoming groups. When an electrophilic aromatic substitution reaction occurs, existing groups on the benzene ring can direct new substituents to specific locations such as the ortho, meta, or para positions. This directing influence is primarily driven by the electron-donating or electron-withdrawing characteristics of the substituent.

• Ortho/Para Directing Groups: Groups like the methyl group, attach at the ortho (adjacent) and para (opposite) positions because they donate electrons and enhance these positions' electron density. This makes them more reactive toward electrophiles.
• Meta Directing Groups: Typically, deactivating groups that withdraw electrons, lower electron density, and only enhance reactivity at the meta position. This is usually seen with groups that pull electron density away like nitro groups.

Understanding which directing effects are in play is crucial as it predicts the possible products in electrophilic aromatic substitution reactions.

Hyperconjugation

Hyperconjugation is a stabilizing interaction that plays a significant role in explaining the behavior of certain substituents on a benzene ring. This phenomenon involves the overlap of σ-bonds (such as C-H bonds from the side chain) with an adjacent π-system (like those in benzene rings). Hyperconjugation is sometimes described as "no bond resonance," where electrons from the C-H bond in the alkyl group (like a methyl group) overlap with the π-electron system.

• Electron Donation: This overlap increases the electron density onto the benzene ring, especially at the ortho and para positions. As a result, these positions become more attractive to electrophiles during the substitution processes.
• Stabilization: The increased electron density from hyperconjugation activates the ring, making the entire aromatic system more stable and reactive to electrophilic attacks.

Hence, hyperconjugation enhances the reactivity and directs electrophiles to the ortho/para positions, confirming the observed patterns of substitution.

Activation and Deactivation of Benzene Rings

The activation and deactivation properties of benzene rings are critical concepts in understanding aromatic chemistry. Different substituents can either increase or decrease the reactivity of the benzene ring during electrophilic aromatic substitution.

• Activation: This occurs when substituents like alkyl groups donate electrons through hyperconjugation or resonance, increasing the overall electron density of the aromatic ring. This electron-rich environment is more favorable for substituting reactions.
• Deactivation: In contrast, substituents like nitro groups withdraw electrons from the ring, reducing electron density and making it less reactive to electrophiles. This is due to resonance or inductive effects that withdraw electron density from the ring.

Activation is indicative of increased reactivity and is generally associated with ortho/para directing due to enhanced electron density in those areas, whereas deactivation often results in meta-directing effects and reduced reactivity.