Question

Outline all steps in the laboratory synthesis of the following compounds from benzene and/or toluene, using any needed aliphatic or inorganic reagents. Assume that a pure para isomer can be separated from an ortho, para mixture. (a) \(\mathrm{p}\) -nitrotoluene (d) p-bromobenzoic acid (b) \(\mathrm{p}\) -bromonitrobenzene (e) o-iodobenzoic acid (c) m-bromobenzenesulfonic acid (f) \(1,3,5-\) trinitrobenzene (g) 3,5 -dinitrobenzoic acid

Short answer

In summary, to synthesize the given compounds, we performed various reactions on benzene or toluene derivatives using methods like nitration, reduction, oxidation, acetylation, halogenation, and hydrolysis. The key idea is to strategically introduce substituents, control directing effects of those substituents, and carefully choose suitable reagents for every individual step. We synthesized (a) p-nitrotoluene using nitration and p-bromobenzoic acid (d) by a sequence of nitration, reduction, acetylation, bromination, hydrolysis, and oxidation reactions.

Step-by-step solution

Nitration of Toluene

Toluene reacts with nitric acid in the presence of sulfuric acid to initially form a ortho, para mixture. The reaction occurs via electrophilic aromatic substitution.

Separation of para-isomer

The product from the previous step is a mixture of ortho and para isomers. Since it is mentioned that pure para isomer can be separated from the mixture, this nitration reaction generates the desired p -nitrotoluene. ##Synthesis of (d) p-bromobenzoic acid##

Nitration of Benzene

The reaction starts with the nitration of benzene to form nitrobenzene. This reaction is carried out in the presence of nitric acid and sulfuric acid.

Reduction of Nitro Group

Nitrobenzene is then reduced to aniline using a reducing agent such as tin and hydrochloric acid.

Acetylation of aniline

Acetyl chloride acts as an acylating agent to transform the amine into an amide, and thus preventing any further reactivity of the amino group.

Bromination

Bromination with bromine in the presence of acetic acid leads to the formation of p-bromoacetanilide.

Hydrolysis

Hydrolysis of previous compound generates an amine and a carboxylic acid, affording p-bromoaniline and acetic acid.

Oxidation

Finally, an oxidation reaction carried out with acidic potassium permanganate results in oxidation of the amine to a carboxylic group, hence forming p-bromobenzoic acid. This solution will be continued in the next step by the synthesis of the compounds (b), (e), (c), (f), and (g).

Key concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a core principle in organic chemistry, essential for modifying aromatic rings like benzene. During EAS, an electrophile replaces a hydrogen atom on the aromatic ring. The process typically involves three steps: the aromatic ring momentarily loses its aromaticity as it binds with the electrophile, then a hydrogen is removed to restore aromaticity. When carried out with nitric acid and sulfuric acid, this reaction is known as nitration, which introduces a nitro group to the benzene ring.

Understanding this process is fundamental when discussing the conversion of benzene into various substituted aromatic compounds. The position where the new group attaches depends on the directing effects of any existing substituents on the ring; for instance, electron-donating groups like the methyl group in toluene direct new substitutions to the ortho or para positions relative to the methyl group.

Nitration of Benzene

When nitration is specifically applied to benzene, the result is the formation of nitrobenzene. This reaction is a classic example of electrophilic aromatic substitution, using a mixture of nitric and sulfuric acids as the reagents. The nitronium ion (o2+) acts as the electrophile, which the benzene ring attacks, forming a sigma complex. Then, a proton is removed, regaining the system's aromaticity.

The beauty of nitration lies in its selectivity. By controlling reaction conditions, chemists can direct the formation of particular isomers. For instance, nitration of toluene yields a mixture of ortho and para isomers, which can be separated if needed. In a laboratory synthesis scenario, this control is invaluable for producing desired compounds with high specificity.

Reduction of Nitro Groups

The reduction of nitro groups to amino groups is a vital transformation in aromatic chemistry. This change drastically alters the chemical behavior of the molecule, paving the way for subsequent reactions. Common reducing agents include metal and acid combinations, like tin and hydrochloric acid, or other systems such as hydrogenation over a palladium catalyst.

Reduction is not just a simple swap of groups; it involves multiple electron transfers and can proceed through different mechanisms depending on the reagents and conditions. The choice of reducing agent can also influence the reaction's speed and the purity of the product. After reduction, the newly formed amino group can undergo a range of further transformations, such as acetylation, as seen in the synthesis of p-bromobenzoic acid.

Bromination Reaction

Bromination is another form of electrophilic aromatic substitution, where a bromine atom is introduced into the aromatic ring. It typically requires a Lewis acid, like iron(III) bromide, to generate the bromonium ion—a potent electrophile. Bromination can occur with remarkable regioselectivity, with the bromine atom preferentially attaching to positions guided by the existing substituents on the ring.

In the lab, this reaction is carried out cautiously, as elemental bromine is highly reactive and must be handled under proper safety conditions. The outcome can yield various brominated products like p-bromonitrobenzene or p-bromobenzoic acid, with further transformations taking place after this initial bromination step.

Oxidation in Organic Chemistry

Oxidation reactions are incredibly diverse in organic chemistry, encompassing a wide range of processes that increase the oxygen content of a molecule or reduce its hydrogen content. In the context of aromatic compounds, oxidation often targets side chains attached to the ring, transforming alkyl groups into more oxidized forms like carbonyls or carboxylic acids.

One powerful oxidizing agent is potassium permanganate, especially in acidic conditions. It's used in the laboratory to convert aromatic amines to carboxylic acids, as seen in the synthesis of p-bromobenzoic acid. The oxidation state of the carbon increases, while the permanganate is reduced. This versatility and power make oxidation reactions essential tools in the chemist's arsenal, capable of significantly altering a compound's structure and reactivity.