Problem 201 - A Complete Resource Book in Chemistry for JEE Main

Question

Toluene in nitrated and the resulting product is reduced with tin and hydrochloric acid. The product so obtained is diazotized and then heated with cuprous bromide. The reaction mixture so formed contains \(\quad\) [2008] (a) Mixture of o-and p-bromotoluenes (b) Mixture of o- and p-dibromobenzenes (c) Mixture of o- and p-bromoanilines (d) Mixture of o- and m-bromotoluenes

Step-by-Step Solution

Answer

(c) Mixture of o- and p-bromoanilines

Step 1: Nitration of Toluene

Toluene undergoes nitration to form a mixture of ortho and para nitrotoluenes. The reaction involves the substitution of a nitro group (NO₂) onto the benzene ring of toluene. Given the methyl group is an ortho/para director, we get both ortho and para substituted products.

Step 2: Reduction to Amines

The ortho and para nitrotoluenes are reduced using tin (Sn) and hydrochloric acid (HCl) to form ortho and para toluidines. This reaction replaces the nitro group with an amino group (NH₂).

Step 3: Diazotization

The resulting ortho and para toluidines are treated with sodium nitrite (NaNO₂) and hydrochloric acid (HCl) to form diazonium salts. This step involves converting the amino group into a diazonium group (-N₂⁺Cl⁻).

Step 4: Sandmeyer Reaction with Cuprous Bromide

The diazonium salts are then heated with cuprous bromide (CuBr), which leads to the substitution of the diazonium group with a bromine atom, forming ortho and para bromoanilines. This is known as a Sandmeyer reaction.

Key Concepts

Nitration of Toluene

Nitration of toluene is a fascinating reaction in organic chemistry where a nitro group (NO₂) is introduced to the benzene ring of toluene. Toluene is essentially a benzene ring with a methyl group. Due to the presence of the methyl group, which is an ortho/para director, the nitration primarily yields ortho and para nitrotoluenes. This means that the nitro group attaches either next to the methyl group (ortho position) or opposite it (para position). These reactions are typically carried out using a mixture of concentrated nitric acid and sulfuric acid as the nitrating agents. The presence of the methyl group enhances the reactivity of the benzene ring and directs the incoming nitro group to these positions.
Understanding the mechanism here is crucial as it shows how different substituents influence the reactivity and orientation of benzene substituents. This concept is a fundamental pillar in guiding the synthesis of complex aromatic compounds.

Reduction to Amines

Once we have ortho and para nitrotoluenes from nitration, the next step is their reduction to amines. In this context, the transformation replaces the nitro group (NO₂) with an amino group (NH₂). Such reductions are commonly achieved using metal catalysts like tin (Sn) along with hydrochloric acid (HCl).

The process involves transferring electrons, typically in acidic conditions, which help break the nitro group’s double bonds and replace it with a single bond to an amino group. This transformation forms compounds known as ortho and para toluidines. It's an important step in organic synthesis, allowing chemists to move from a nitro group, often used in explosives and industrial chemical reactions, to an amino group, a common functional group in drugs and natural products.
This reaction not only illustrates a critical transition in functional groups but also highlights the utility of reduction reactions in modifying the structure and reactivity of aromatic compounds.

Diazotization

Diazotization is a unique and valuable reaction in organic chemistry where an amino group (NH₂) is transformed into a diazonium salt (-N₂⁺Cl⁻). This is typically achieved by treating the amine with sodium nitrite (NaNO₂) in acidic conditions, often using hydrochloric acid (HCl).

The reaction proceeds by the formation of a nitrous acid in situ, which then reacts with the amine to form the diazonium group. These groups are highly reactive intermediates and open the door to subsequent transformations. The beauty of diazotization is that it makes many synthetic routes possible, whether it be coupling reactions to form azo dyes or substitutions like the Sandmeyer Reaction.
This step demonstrates how functional groups can be used as stepping stones in organic chemistry, facilitating the crafting of increasingly complex molecular structures.

Organic Chemistry Reactions

Organic chemistry is a world of transformations, where molecules are shuffled and reshaped constantly. Among the vast array of reactions, the Sandmeyer reaction is particularly notable. This type of reaction, named after Traugott Sandmeyer, is a valuable method to perform halogenation (introduction of a halogen atom like bromine) on aromatic rings through diazonium salts.

In the context described here, after forming diazonium salts from toluidines, the Sandmeyer reaction uses cuprous bromide (CuBr) to substitute the diazonium group with a bromine atom. The chemist's goal is often to achieve a more complex aromatic structure, which might be utilized in dye manufacturing, pharmaceuticals, or material science.

• The reaction is typically carried out at low temperatures to prevent spontaneous decomposition of the diazonium compound.
• It represents one of the many ingenious ways chemists use reactive intermediates like diazonium salts for synthetic advantage.

Understanding these kinds of reactions enriches our grasp of synthetic pathways and highlights the practical utility of seemingly intricate chemical processes.