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: (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

Short answer

(a) Mixture of o- and p-bromotoluenes.

Step-by-step solution

Understand the Initial Reaction

The reaction begins with the nitration of toluene, which primarily gives ortho and para-nitrotoluene due to the activating methyl group, which directs electrophilic aromatic substitution to the ortho and para positions.

Reduction of Nitro Group

The nitro group on the nitrotoluene is reduced to an amino group by using tin and hydrochloric acid, resulting in the formation of ortho and para-aminotoluene (or toluidines).

Diazotization of Aminotoluene

The ortho and para-aminotoluenes undergo diazotization in the presence of nitrous acid, forming diazonium salts (diazonium ion is stable at low temperatures).

Substitution by Cuprous Bromide

The diazonium salts are then heated with cuprous bromide (Sandmeyer reaction), which replaces the diazonium group with a bromine atom. This leads to the formation of ortho and para-bromotoluenes.

Final Verification

Review the complete reaction process and confirm that the only brominated products formed are ortho and para-bromotoluenes, resulting from the initial orientation of the methyl group and subsequent reactions.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a key reaction in aromatic chemistry. It describes a process where an aromatic ring, like benzene, interacts with an electrophile to substitute a hydrogen atom on the ring with another atom or group. This reaction preserves the aromaticity, meaning the stable ring system is retained.
This process is integral in transformations involving aromatic compounds. One classic example is the nitration of toluene.

• The starting molecule, toluene, is an aromatic compound with a methyl group attached.
• The methyl group acts as an electron-donating entity, enhancing the electron density of the benzene ring.

As a result, the ring becomes more reactive and predisposed to substitution at the ortho and para positions.
In the context of the problem, identifying the electrophile, the nitronium ion ( O_2^+ ), is crucial. This ion is responsible for attacking the aromatic ring and facilitating the substitution.

Nitration of Toluene

Nitration is a subtype of electrophilic aromatic substitution specifically involving the introduction of a nitro group ( NO_2 ) to the aromatic ring. Toluene's nitrogenation begins by generating the nitronium ion through the reaction of nitric acid and sulfuric acid. The mechanism of this inclusion involves several steps, starting with the electrophilic attack at the most activated positions on the toluene molecule.

• Due to the electron-donating nature of the methyl group, nitration predominantly occurs at the ortho and para positions relative to the methyl group.
• Thus, the major products formed are ortho-nitrotoluene and para-nitrotoluene.

These positions are favored because they exhibit higher electron density, making them more reactive toward electrophiles. This selectivity is crucial in predicting the nature and position of subsequent reactions.

Sandmeyer Reaction

The Sandmeyer reaction is pivotal in organic synthesis, allowing for the substitution of diazonium salts. In the problem, after diazotization, ortho and para-aminotoluenes are transformed into diazonium salts using nitrous acid.
This process is selective, ensuring that the aromatic ring remains intact while the diazonium salt makes the aromatic nitrogen available for substitution.
Upon heating these salts with cuprous bromide, the diazonium group is effectively replaced by a bromine atom.

• This allows the formation of ortho and para-bromotoluenes.
• The Sandmeyer reaction is highly valued due to its ability to introduce a variety of functional groups, such as halides, onto the aromatic ring.

This formation is essential to the problem at hand, representing a strategic synthetic step in manipulating aromatic compounds.

Reduction of Nitro Compounds

Reduction of nitro compounds is an important transformation in synthetic chemistry, particularly when converting a nitro group to an amino group. In the context of the discussed exercise, ortho and para-nitrotoluene undergo reduction with tin and hydrochloric acid. The role of tin in this reduction is as a reducing agent.

The process effectively transforms the nitro groups present into amino groups, resulting in ortho and para-aminotoluenes.

• This transformation is significant because it enables further chemical modification of the aromatic ring.
• The amino group can be readily modified into other functional groups, such as the diazonium ion.

By understanding this transformation, students can appreciate how initial nitration leads to a cascade of reactions resulting in the final substituted aromatic product, all while maintaining the aromatic structure. This comprehension is critical for anticipating the outcomes of multi-step reactions.