Problem 125 - A Complete Resource Book in Chemistry for JEE Main

Question

Toluene reacts with methyl chloride in presence of anhydrous aluminium chloride to form mainly m-xylene. This is because (a) \(\mathrm{CH}_{3}\) - group has +I effect (b) \(\mathrm{CH}_{3}-\) group is meta directing (c) m-xylene is thermodynamically most stable of the other xylenes (d) hyperconjugation effect of \(\mathrm{CH}_{3}\) - group

Step-by-Step Solution

Answer

(c) m-xylene is thermodynamically most stable of the other xylenes

Step 1: Identify the Reaction Type

The reaction described involves a Friedel-Crafts alkylation, where toluene, an aromatic compound with a methyl group, reacts with methyl chloride in the presence of anhydrous AlCl₃ to introduce a new methyl group.

Step 2: Understand the Directing Effect of the CH3 Group

The methyl group in toluene is an activating group due to its +I (inductive) effect, making the aromatic ring electron-rich and directing the incoming electrophile to the ortho and para positions.

Step 3: Identify Given Observational Outcome

According to the exercise, the main product is m-xylene, suggesting that the methyl group in this scenario primarily directs the new group to the meta position.

Step 4: Analyze Possible Reason for Meta-Substitution

Normally, the methyl group directs to ortho and para positions. However, steric hindrance and stability factors might favor the formation of meta products in specific conditions or due to experimental setups.

Step 5: Evaluate Answer Choices

(a) The +I effect increases electron density at ortho and para positions, not meta, ruling out this option as the primary cause. (b) Methyl is typically ortho/para directing, thus not naturally meta directing. (c) The formation of m-xylene as primary suggests it might be the most stable under the severe conditions favoring meta. (d) Hyperconjugation stabilizes ortho/para positions, not meta.

Step 6: Conclusion

Based on the understanding that under these specific reaction conditions m-xylene becomes the major product, this indicates thermodynamically m-xylene might be the most stable in these specific conditions.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a fundamental type of reaction for aromatic compounds like benzene. This process replaces a hydrogen atom on the aromatic ring with an electrophile — an electron-seeking species. The aromaticity of the benzene ring, characterized by its stable electron cloud, remains intact after the substitution.
This stability gives aromatic rings their unique reactivity compared to aliphatic compounds.

• The typical steps involve forming a highly reactive cationic intermediate named the arenium ion.
• The electrophile attacks the electron-rich aromatic ring, temporarily disrupting its aromaticity.
• A base then deprotonates the intermediate, restoring the aromaticity while substituting an electrophile on the ring.

This reaction type is central to Friedel-Crafts alkylation, where an alkyl group is introduced to an aromatic ring with the help of a catalyst like AlCl₃. The reaction is optimized by ensuring the aromatic ring is electron-rich, often achievable through activating substituents.

Directing Effects in Aromatic Substitution

Directing effects in aromatic substitution are crucial to understanding where a new group will add on an aromatic ring. This effect focuses on how groups already present on the ring (substituents) affect the position of new substituents.
Substituents generally fall into two categories: activating and deactivating. Activating groups, such as a methyl group, increase the electron density on the ring through the +I (inductive) effect, making certain positions more reactive to electrophiles.

• Activating groups often direct additional groups to ortho (adjacent) or para (opposite) positions due to increased electron density there.
• Deactivating groups would direct new groups to meta positions, as they reduce electron density at ortho and para positions.

In our scenario with toluene, the methyl group is typically ortho/para directing. However, the observation of m-xylene points to possible steric hindrance or another stabilizing factor altering the expected directing effect, favoring a meta product.

Thermodynamic Stability in Chemical Reactions

Thermodynamic stability in chemical reactions refers to the stability of the products compared to the reactants, defined by the reaction's energy dynamics. When a product is more stable, it means lower energy, which is often the thermodynamic preference in chemical processes. Such stability directly impacts the final product composition at the reaction’s conclusion.
For reactions involving electrophilic aromatic substitution, you usually aim to produce thermodynamically stable compounds, where structures often benefit from resonance and minimal steric hindrance.

• In the case of the formation of m-xylene from toluene, despite the methyl group being an ortho/para director, the conditions favor the meta product.
• This suggests that the m-xylene formed under these specific reaction conditions must be thermodynamically favored due to lower energy state, potentially because of steric factors or interaction within the reaction environment.

Such factors make the major product the one that embodies the best balance between reaction conditions and inherent structural stability.