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

Short answer

(c) M-xylene is thermodynamically most stable.

Step-by-step solution

Understanding the Reaction

Toluene is a benzene ring with a methyl group attached. When toluene reacts with methyl chloride in the presence of anhydrous aluminum chloride, it undergoes an electrophilic aromatic substitution reaction, specifically Friedel-Crafts alkylation.

Identifying Ortho, Para, and Meta Positions

The methyl group on toluene is an activating group and directs incoming electrophiles to the ortho and para positions. However, we are interested in why m-xylene is the major product, suggesting something overrides typical ortho/para activation.

Options Evaluation

We must evaluate the options in terms of their chemical effects on the benzene ring. Option (a), the +I effect, indicates electron donation by the methyl group but doesn't explain meta direction. Option (b) suggests the methyl group is meta directing, conflicting with known chemistry without further specification. Option (c) suggests that m-xylene is thermodynamically most stable, a plausible reason for its majority presence. Option (d) involves hyperconjugation, which reinforces ortho/para placement, making it an unlikely reason for meta preference.

Conclusion Based on Stability

The most plausible reason for m-xylene to be the main product is that it is thermodynamically more stable due to less steric repulsion compared to ortho and para isomers during formation. This aligns with option (c).

Key concepts

Friedel-Crafts Alkylation

Friedel-Crafts Alkylation is a key reaction in organic chemistry where an alkyl group is introduced to an aromatic ring. This process involves an electrophilic aromatic substitution, utilizing the catalyst behavior of anhydrous aluminum chloride (AlCl₃) for activation. In the example of toluene reacting with methyl chloride, the methyl group on toluene helps stabilize the positive charge formed during the reaction. This stability facilitates the attachment of the alkyl group from methyl chloride to the aromatic ring.

The reaction proceeds as follows: the aluminum chloride catalyst forms a complex with methyl chloride, generating a more potent electrophile. This electrophile is then attracted to the benzene ring, which due to the electron-donating nature of the toluene's methyl group, prefers to add at the extbf{ortho} and extbf{para} positions under usual conditions. Nevertheless, other factors influence the major product direction in specific cases, as seen in our exercise.

M-xylene Stability

The stability of m-xylene can be understood in terms of thermodynamics and steric factors. M-xylene, one of the isomers of the dimethylbenzene family, features two methyl groups positioned meta to each other on the aromatic benzene ring. While conventionally ortho and para directing groups are more favored for substitution, the case of m-xylene presents a scenario where steric effects play a pivotal role.

The steric interference between two alkyl groups can lead to destabilization in close proximity, as seen in ortho-xylene. In para-xylene, although steric hindrance is minimized, m-xylene holds a thermodynamic advantage in certain reactions. The distance between substituents in m-xylene allows for reduced steric clash during formation, offering a more stable intermediate state during the course of Friedel-Crafts Alkylation. Thus, m-xylene's product predominance is linked to lower steric strain and favorable thermodynamics.

Steric Repulsion in Aromatic Compounds

Steric repulsion is a core concept in understanding the reactivity and directionality of aromatic substitution reactions. It pertains to the repulsive forces acting between atoms or groups that are in close proximity, which can significantly influence chemical behavior and product distributions in reactions involving aromatic compounds.

For example, during Friedel-Crafts Alkylation, the formation of different isomers from toluene often depends on steric considerations. Ortho positions face significantly higher steric hindrance due to the proximity of substituents. Para positions are less affected by steric hindrance due to the distancing of groups. However, under certain conditions, the resultant steric relief that comes with meta substitution can override the typical electronic directing effects observed with electrophilic substitution. Consequently, steric repulsion not only helps explain deviations from expected ortho/para dominated products but also underscores the importance of spatial arrangements in chemical reactions.