Question

Treatment with methyl chloride and \(\mathrm{AICI}_{3}\) at \(0^{\circ}\) converts toluene chiefly into o- and p-xylenes; at \(80^{\circ}\), however, the chief product is \(\mathrm{m}\) -xylene. Furthermore, either o- or \(\mathrm{p}\) -xylene is readily converted into \(\mathrm{m}\) -xylene.by treatment with \(\mathrm{AICI}_{3}\) and \(\mathrm{HCl}\) at \(80^{\circ}\). How do you account for this effect of temperature on orientation? Suggest a role for the HCl.

Short answer

Temperature significantly impacts the Friedel-Crafts alkylation of toluene. At 0°C, the reaction primarily results in ortho and para forms of xylene, showing a preferential attachment of the methyl group to the sites ortho and para to the already existing methyl group on toluene. However, at higher temperature of 80°C, the reaction favors the formation of m-xylene, presumably due to 'thermal equilibration', a process where situations of higher energy favor creating the most stable product. In this case, m-xylene is more thermodynamically stable than o-xylene and p-xylene due to a more evenly distributed electron cloud. In addition to temperature, \( \mathrm{HCl} \) also plays a significant role in the reaction. When o- or p-xylene is treated with \( \mathrm{AlCl}_{3} \) and \( \mathrm{HCl} \) at 80°C, a conversion to m-xylene occurs. It's likely that the \( \mathrm{HCl} \) in conjunction with the \( \mathrm{AlCl}_{3} \) catalyzes a process of demethylation and remethylation at the more thermodynamically stable meta position, thus favoring the transformation of o- or p-xylene into m-xylene. Therefore, both temperature and \( \mathrm{HCl} \) effectively control the orientation of the methyl groups in the products of these reactions.

Step-by-step solution

Impact of Temperature

At 0˚C, toluene is primarily converted into the ortho and para forms of xylene. This indicates a preferential attachment of the methyl group to the sites ortho and para to the already existing methyl group on toluene. But when the temperature is raised to 80˚C, the major product of the reaction changes to m-xylene. This change might be due to a process called 'thermal equilibration', where conditions of higher energy (like increased temperature) favor the formation of the most stable product. M-xylene is more thermodynamically stable than ortho- and para-xylenes due to the more evenly distributed electron cloud.

The Role of HCl in the Conversion

In the second part of the question, either o- or p-xylene is converted into m-xylene by treating with \(\mathrm{AlCl}_{3}\) and \(\mathrm{HCl}\) at 80˚C. The role of \(\mathrm{AlCl}_{3}\) and \(\mathrm{HCl}\) at this temperature can best be explained as facilitators of isomerization to form the most stable compound. It seems the \(\mathrm{HCl}\) with \(\mathrm{AlCl}_{3}\) at 80˚C favors demethylation and remethylation at the more thermodynamically stable meta position, converting either o- or p-xylene to m-xylene.

Encompassing Conclusion

To conclude, temperature plays a critical role in deciding the major product in the conversion of toluene to xylene. At lower temperatures, the kinetic product (ortho- and para-) is favored, while at higher temperatures, the thermodynamically stable product (meta-) is formed. In addition, \(\mathrm{HCl}\) acts as an agent that aids the transformation of o- or p-xylene to m-xylene, probably by favoring demethylation and remethylation.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a fundamental reaction mechanism in organic chemistry, particularly for aromatic compounds like benzene and its derivatives. In this reaction, an electrophile replaces a hydrogen atom on the aromatic ring. The process is typically catalyzed by a substance like aluminum chloride (\(\mathrm{AlCl}_3\)), which enhances the electrophilicity of the incoming group by assisting in forming a carbocation intermediate.
This mechanism is crucial in Friedel-Crafts Alkylation, where the benzene ring undergoes substitution by an alkyl group. During the EAS process, the aromatic ring initially attracts the positively charged electrophile, forming a sigma complex.
The intermediate loses a proton, restoring the aromaticity to the ring. The result is a substituted aromatic compound, such as the xylenes formed from toluene. The placement (ortho, meta, para) of substituents on the ring

• depends on the nature of the substituent already present and
• the reaction conditions (like temperature).

Understanding EAS helps explain why, under different conditions, the same starting material can produce different isomers as seen in the transformation involving methyl chloride and \(\mathrm{AlCl}_3\) in the original exercise.

Isomerization

Isomerization is the process whereby a molecule is transformed into another molecule with the same molecular formula but a different arrangement of atoms. This process is key in the conversion of o- and p-xylenes to m-xylene under specific conditions.
In the given exercise, treatment with \(\mathrm{AlCl}_3\) and \(\mathrm{HCl}\) at high temperatures results in the rearrangement of xylene isomers.
This rearrangement favors the formation of the thermodynamically more stable m-xylene.
Isomerization often involves the breaking and reforming of bonds, which can be catalyzed or sped up by the presence of certain acids like \(\mathrm{HCl}\), especially at high temperatures where energy barriers to transformation are more easily overcome.

• This transformation enhances the stability of the compound and
• is usually reversible, demonstrating the dynamic nature of chemical reactions.

The role of isomerization in organic reactions highlights the importance of catalysts and temperature in determining the final product distribution.

Thermodynamic vs Kinetic Control

Thermodynamic and kinetic control are concepts that refer to the conditions under which a chemical reaction occurs, influencing which products will dominate. Under kinetic control, the reaction pathway that leads to the fastest forming product is favored.
In the case of the initial transformation of toluene at low temperatures (0˚C), the ortho- and para-xylenes are kinetically favored due to their rapid formation.

• These products, however, are not necessarily the most stable.
• They are just quicker to form under these conditions.

Thermodynamic control, on the other hand, involves the formation of the most stable product over time, often requiring higher temperatures to achieve activation energy.
At 80˚C, m-xylene is favored because it is the more stable product, having greater thermodynamic stability due to distribution of electronic cloud minimizing steric hindrance. Thus, reactions can lead to different predominant products based on whether conditions favor kinetic or thermodynamic control.

Role of Catalysts in Organic Reactions

Catalysts play a pivotal role in many organic reactions by lowering the activation energy needed to reach the transition state, thus accelerating the reaction without being consumed.In Friedel-Crafts Alkylation and rearrangements like those demonstrated in the original exercise, \(\mathrm{AlCl}_3\) is integral as a catalyst.
It enhances the electrophilicity of groups like methyl chloride, making the reaction more feasible.
The presence of \(\mathrm{HCl}\) with \(\mathrm{AlCl}_3\) also aids isomerization by providing a medium for the rearrangement of xylene isomers to occur.

• This combined action allows for efficient conversion processes and
• facilitates the transformation to thermodynamically stable products.

The understanding of catalytic roles explains how the same reagents can lead to different products under varying conditions, highlighting their utility in synthetic chemistry and industrial applications.