Question

Toluene on treatment with \(\mathrm{CH}_{3} \mathrm{Cl}\) and \(\mathrm{AlCl}_{3}\) at \(80^{\circ} \mathrm{C}\) gives the major product as (1) o-xylene (2) p-xylenc (3) m-xylenc (4) cthyl benzene

Short answer

The major product is p-xylene.

Step-by-step solution

Identify the reactants

The reactants are toluene \(\text{C}_6\text{H}_5\text{CH}_3\), methyl chloride \(\text{CH}_3\text{Cl}\), and aluminum chloride \(\text{AlCl}_3\). The reaction is carried out at 80°C.

Understand the reaction mechanism

This reaction is a Friedel-Crafts alkylation. The methyl group from \(\text{CH}_3\text{Cl}\) will be added to the benzene ring of toluene in the presence of \(\text{AlCl}_3\).

Determine the directing effects

The \(\text{CH}_3\) group in toluene is an ortho/para-directing activator. This means that the new \(\text{CH}_3\) group will predominantly add to the ortho and para positions relative to the existing methyl group.

Form ortho and para products

The possible major products from adding a \(\text{CH}_3\) group to the ortho and para positions are o-xylene \(\text{o-C}_6\text{H}_4(\text{CH}_3)_2\) and p-xylene \(\text{p-C}_6\text{H}_4(\text{CH}_3)_2\).

Identify the major product

Given the steric hindrance, para-substitution is often favored over ortho-substitution. Hence, the major product will be p-xylene.

Key concepts

toluene chemistry

Toluene is an aromatic hydrocarbon with the chemical formula \( \text{C}_6\text{H}_5\text{CH}_3 \). In simpler terms, it's a benzene ring with a single methyl group (\( \text{CH}_3 \)) attached to it. This makes toluene a good starting material for various chemical reactions, especially in organic synthesis. Understanding the reactivity of toluene is essential for anticipating the products of its reactions. The presence of the methyl group affects how toluene reacts with other chemicals.

• The methyl group (\text{CH}_3) is an electron-donating group.
• This means it pushes electrons into the benzene ring, making the ring more reactive.
• Consequently, toluene typically undergoes electrophilic aromatic substitution reactions more readily than benzene itself.

This increased reactivity makes toluene a prime candidate for reactions like Friedel-Crafts alkylation.

reaction mechanism

Friedel-Crafts alkylation is a common method used to introduce alkyl groups into aromatic compounds, such as toluene. The mechanism generally involves a few straightforward steps:
The reaction starts by generating a highly reactive electrophile. Here, the methyl chloride (\( \text{CH}_3\text{Cl} \)) reacts with aluminum chloride (\( \text{AlCl}_3 \)), a strong Lewis acid, to form a \( \text{CH}_3^+ \) carbocation.

• \( \text{CH}_3\text{Cl} + \text{AlCl}_3 \rightarrow \text{CH}_3^+ + \text{AlCl}_4^- \)

The generated \( \text{CH}_3^+ \) carbocation then attacks the electron-rich benzene ring of toluene, forming a sigma complex. This complex is an intermediate state where the benzene ring is temporarily disrupted.

• The complex then quickly rearranges to restore aromaticity, expelling a proton to give the alkylated product.
• In this case, the new methyl group attaches to either the ortho or para positions relative to the existing \( \text{CH}_3 \) group on the benzene ring.

The overall balanced equation for this reaction looks like:
\( \text{C}_6\text{H}_5\text{CH}_3 + \text{CH}_3\text{Cl} \rightarrow (\text{CH}_3)_2\text{C}_6\text{H}_4 \)

ortho/para-directing activators

The methyl group (\( \text{CH}_3 \)) in toluene is considered an ortho/para-directing activator. This is crucial for Friedel-Crafts alkylation reactions as it influences where new substituents will attach to the benzene ring.

Ortho/para-directing activators are substituents that increase the electron density of the benzene ring through induction and resonance. This increased electron density primarily stabilizes the intermediate formation at the ortho and para positions, making these sites more reactive:

• Ortho positions are adjacent to the existing substituent, meaning two positions around the ring.
• Para position is directly opposite the existing substituent on the benzene ring.

When another \( \text{CH}_3 \) group is introduced via a Friedel-Crafts alkylation, it prefers these positions, thus giving us o-xylene and p-xylene as potential products.

• Ortho- (\text{o-xylene}) and para-xylene (\text{p-xylene}) are positional isomers.
• Meta-positions are generally less favorable for electrophilic substitution when the existing group is an activator, like \( \text{CH}_3 \).

steric hindrance

Steric hindrance plays an essential role in determining the major product of alkylation reactions. Steric hindrance refers to the physical obstruction that atoms or groups of atoms create due to their size.
When introducing another \( \text{CH}_3 \) group to toluene, steric effects come into play:

• Ortho positions (adjacent to the \( \text{CH}_3 \) group) can be more crowded due to the proximity of the substituents.
• Para position (opposite the initial \( \text{CH}_3 \) group) usually provides more space, leading to less steric hindrance.

As a result, para-substitution is generally favored for Friedel-Crafts alkylation of toluene, producing p-xylene as the major product.

• Less steric crowding means the para product is more stable and easier to form.
• This principle helps chemists predict and control the outcomes of such reactions.

Understanding steric hindrance is, therefore, critical for anticipating the products in complex organic reactions like this one.