Question

A single compound is formed by alkylation of 1,4-dimethylbenzene. Write the equation for the reaction of this compound with \(\mathrm{CH}_{3} \mathrm{Cl}\) and \(\mathrm{AlCl}_{3}\). What is the structure and name of the product?

Short answer

The reaction produces 1,2,4-trimethylbenzene.

Step-by-step solution

Understand the Reaction

This is an example of an alkylation reaction, specifically a Friedel-Crafts alkylation. In this reaction, 1,4-dimethylbenzene (para-xylene) will react with methyl chloride (\(\mathrm{CH}_3\mathrm{Cl}\)) in the presence of a catalyst, aluminum chloride (\(\mathrm{AlCl}_3\)). The alkyl group, in this case, a methyl group, will be added to the aromatic ring.

Analyze the Formation of the Electrophile

When methyl chloride (\(\mathrm{CH}_3\mathrm{Cl}\)) reacts with aluminum chloride (\(\mathrm{AlCl}_3\)), it forms a more reactive electrophile, \(\mathrm{CH}_3^+\), also known as a carbocation.

Determine the Position of Alkylation

In the case of 1,4-dimethylbenzene, the alkylation will occur at the available ortho or para positions relative to one of the existing methyl groups. Since para-xylene is already symmetrically substituted at both para positions, the methyl group will most likely attack one of the ortho positions, which is favored due to steric hindrance at the meta positions.

Draw the Structure of the Product

By adding a methyl group to an ortho position of 1,4-dimethylbenzene, we obtain 1, 2, 4-trimethylbenzene as the product. The added methyl group attaches to one of the carbons adjacent to an existing methyl group.

Write the Reaction Equation

The overall reaction can be depicted as: \[ \mathrm{C_6H_4(CH_3)_2} + \mathrm{CH_3Cl} \xrightarrow{\mathrm{AlCl_3}} \mathrm{C_6H_3(CH_3)_3} + \mathrm{HCl} \] This equation represents the starting materials, para-xylene and methyl chloride, reacting in the presence of aluminum chloride catalyst to form 1, 2, 4-trimethylbenzene and hydrochloric acid (HCl) as a byproduct.

Key concepts

alkylation reaction

The alkylation reaction is a pivotal mechanism in organic chemistry, particularly for modifying aromatic compounds. This reaction involves adding one or more alkyl groups to an aromatic ring. In the context of Friedel-Crafts alkylation, the process is facilitated by a strong Lewis acid catalyst, often aluminum chloride ( AlCl_3 ). This catalysis increases the reactivity of the alkylating agent, enabling the transfer of an alkyl group to the aromatic system.
Friedel-Crafts alkylation is widely used in synthetic chemistry due to its ability to craft complex molecules from simpler precursors. By altering the structure of the aromatic compound, the reaction contributes substantially to the development of advanced pharmaceuticals, dyes, and polymers.

1,4-dimethylbenzene

1,4-dimethylbenzene, also known as para-xylene, is an aromatic hydrocarbon with two methyl groups on opposite sides of the benzene ring. Its symmetrical arrangement is particularly noteworthy. This symmetry simplifies the compound's chemical properties and the prediction of reaction sites during an alkylation reaction.
Para-xylene plays a crucial role in both industrial and laboratory settings. It serves as a precursor to terephthalic acid, necessary for producing polyester fibers and resins. Its symmetrical nature makes it a relevant focus in organic chemistry, especially in Friedel-Crafts reactions, where new substituents are added to its framework.

electrophile formation

In the context of Friedel-Crafts alkylation, electrophile formation is an essential step. The electrophile is a positively charged species that seeks electrons from other entities during a reaction. For the alkylation of 1,4-dimethylbenzene, the electrophile forms when methyl chloride ( CH_3Cl ) reacts with aluminum chloride ( AlCl_3 ).
This interaction results in the generation of a methyl carbocation ( CH_3^+ ), a highly reactive species ready to attack the aromatic ring. The creation of this electrophile is crucial, as it dictates the site of reaction on the benzene ring, influencing the final structure of the alkylation product.

methyl group addition

The addition of a methyl group during alkylation affects the structural and chemical attributes of the original compound. In the case of the reaction involving 1,4-dimethylbenzene and methyl chloride, the methyl group is introduced to an ortho position relative to one of the original methyl groups on the aromatic ring.
This ortho substitution is considered to minimize steric hindrance, as para positions are already occupied. The resultant compound, 1,2,4-trimethylbenzene, exhibits unique properties, advantageous in various applications such as solvents and intermediates in chemical synthesis. Understanding these positional preferences is vital in anticipating the products of alkylation reactions and designing synthesis routes in organic chemistry.