Problem 83 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound \(\mathrm{C}\) in the following is sequence: \(\mathrm{C}_{7} \mathrm{H}_{8} \stackrel{3 \mathrm{Cl}_{2} / \Delta}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{Br}_{2} / \mathrm{Fe}}{\longrightarrow} \mathrm{B} \stackrel{\mathrm{Zn} / \mathrm{HCl}}{\rightarrow} \mathrm{C}\) (a) p-bromotoluene (b) o-bromotoluene (c) m-bromotoluene (d) 3 -bromo-2, 4,6 -trichlorotoluene

Step-by-Step Solution

Answer

The compound \( \mathrm{C} \) is 3-bromo-2,4,6-trichlorotoluene (option d).

Step 1: Analyze the first reaction

The compound \( \text{C}_7\text{H}_8 \) is toluene. The reaction \( \text{3Cl}_2 / \Delta \) indicates the chlorination of the methyl group in toluene under heat. This converts toluene into 2,4,6-trichlorotoluene.

Step 2: Recognize the second reaction role

In the next step, \( \mathrm{Br}_2 / \mathrm{Fe} \) is used, which is a typical bromination reaction in aromatic systems favoring electrophilic aromatic substitution. This reaction will add a bromine atom to the aromatic ring. Given the bulky chlorine groups at 2,4, and 6 positions, bromination is most likely to occur at position 5, resulting in 3-bromo-2,4,6-trichlorotoluene (since the methyl group directs ortho and para, and para is less hindered).

Step 3: Interpret the third reaction

The last step is \( \mathrm{Zn} / \mathrm{HCl} \), which is a reduction reaction, often used to remove halogens from alkyl positions. However, in this context, it can be understood to reduce the non-aromatic halogen if any, but here, it primarily confirms \( \mathrm{C} \) retains its aryl halogen configuration, not affecting the bromination product from step 2.

Step 4: Determine the final compound

Given the reactions, \( \text{C} \) must be 3-bromo-2,4,6-trichlorotoluene, as no changes occur to the aromatic bromine anywhere, and the structure is consistent with the pathways given by the reactions.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a fundamental type of organic reaction in which an aromatic hydrogen is replaced by an electrophile. This reaction is crucial in aromatic chemistry. It allows the introduction of a variety of functional groups into an aromatic ring. The process typically involves:

• Formation of a strong electrophile, which is usually facilitated by a catalyst like iron (Fe) or aluminum chloride (AlCl₃).
• Attack of the aromatic pi electrons on the electrophile, forming a non-aromatic carbocation intermediate called the arenium ion.
• Re-aromatization of the ring by the loss of a proton, restoring aromaticity.

In the given exercise, the bromination process using Br extsubscript{2} and Fe demonstrates EAS. The position where the bromine is added is influenced by existing substituents, like the chlorine atoms in 2,4, and 6 positions, which are electron-withdrawing groups. This directs the incoming electrophile, bromine, to the most accessible position, often resulting in a para or ortho substitution if there is limited steric hindrance.

Reduction Reaction

Reduction reactions involve the gain of electrons or the loss of oxygen in a chemical species. In organic chemistry, reduction often refers to the process of adding hydrogen (hydrogenation) or removing halogen atoms. It is a critical step in many synthetic pathways, converting functional groups into more useful or less complex forms. In the equation given, Zn/HCl is used after bromination. Here, it's important to note that common reduction reactions involving Zn/HCl typically target nitro groups or other complex additions, facilitating the reduction of selective functional groups without affecting the aromatic bromine product. However, in this context, Zn/HCl supports the stability of the final compound, confirming the brominated structure and ensuring any unwanted transformations are minimized. Its presence confirms that the structure after reduction remains as 3-bromo-2,4,6-trichlorotoluene.

Chlorination of Toluene

Chlorination, specifically of toluene, is characterized by the addition of chlorine atoms to a substrate, where conditions like heat (94) facilitate reaction progress. In the chlorination of toluene, Cl extsubscript{2} reacts with toluene, a methyl-substituted benzene, to form trichlorotoluene. Here, the methyl group acts as an activating substituent for the aromatic ring, directing chlorination to ortho and para positions, thanks to its electron-donating nature. When reacting to conditions mentioned in the exercise, the methyl group, susceptible to radical substitution under heated conditions, results in a chlorination pattern highlighted by chlorine atoms occupying the 2, 4, and 6 positions on the aromatic ring, forming 2,4,6-trichlorotoluene. The controlled addition process here demonstrates regioselectivity influenced by the existing methyl group.

Bromination Reaction

Bromination is a type of electrophilic aromatic substitution that introduces a bromine atom into an aromatic compound. When using Br extsubscript{2} along with a catalyst like iron (Fe), it facilitates the bromination of aromatic rings. Given the discovery of 3-bromo-2,4,6-trichlorotoluene, bromination targets accessible positions on the aromatic ring. In the exercise context, the bromination is strategically processed on trichlorotoluene. The chlorine atoms at positions 2, 4, and 6 make ortho and para positions less favorable due to steric hindrance. The presence of a methyl group, which typically directs ortho/para addition, influences the choice toward the para position as the least hindered. Ultimately, this allows the formation of 3-bromo-2,4,6-trichlorotoluene, showcasing the precision achievable in selective substitution in aromatic chemistry.