Question

Show how to convert toluene to (a) 2,4 -dinitrobenzoic acid and (b) 3,5 -dinitrobenzoic acid.

Short answer

Answer: The conversion of toluene to 2,4-dinitrobenzoic acid and 3,5-dinitrobenzoic acid involves the process of nitration, reduction, and oxidation. For 2,4-dinitrobenzoic acid, the steps are: 1) Nitration of toluene to form 2,4-dinitrotoluene, 2) Reduction of 2,4-dinitrotoluene to form 2,4-diaminotoluene, and 3) Oxidative cleavage of the methyl group to form 2,4-dinitrobenzoic acid. For 3,5-dinitrobenzoic acid, the steps are: 1) Nitration of toluene to form 3,5-dinitrotoluene, 2) Reduction of 3,5-dinitrotoluene to form 3,5-diaminotoluene, and 3) Oxidative cleavage of the methyl group to form 3,5-dinitrobenzoic acid.

Step-by-step solution

Nitration of toluene to form 2,4-dinitrotoluene

To begin, we need to nitrate the toluene molecule so that the 2- and 4- positions on the benzene ring are nitrated. This can be achieved by reacting toluene with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at 0-10 degrees Celsius. After the reaction is completed, we will obtain 2,4-dinitrotoluene.

Reduction of 2,4-dinitrotoluene to form 2,4-dinitrophenylhydrazine

Next, we need to convert the nitro groups of 2,4-dinitrotoluene into amino groups. This can be done by reducing the nitro groups using either catalytic hydrogenation (with a Raney nickel or Pd/C catalyst) or dissolving metal reduction (with zinc and hydrochloric acid). After the reduction is complete, we will obtain 2,4-diaminotoluene.

Oxidative cleavage of the methyl group

We now need to remove the methyl group in 2,4-diaminotoluene to form the desired product, 2,4-dinitrobenzoic acid. This can be done by oxidizing the methyl group using potassium permanganate (KMnO4) in an alkaline medium, such as potassium hydroxide (KOH), and water. After the reaction, an acidic work-up using sulfuric acid (H2SO4) yields the final product: 2,4-dinitrobenzoic acid. Part B: Conversion of Toluene to 3,5-dinitrobenzoic acid

Nitration of toluene to form 3,5-dinitrotoluene

Similarly to part A, we need to nitrate the toluene molecule, this time with the nitro groups at the 3- and 5- positions on the benzene ring. Again, we use a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at around 50-60 degrees Celsius. The reaction yields 3,5-dinitrotoluene.

Reduction of 3,5-dinitrotoluene to form 3,5-dinitrophenylhydrazine

We convert the nitro groups of 3,5-dinitrotoluene into amino groups, just like in part A. This is done through catalytic hydrogenation (using a Raney nickel or Pd/C catalyst) or dissolving metal reduction (using zinc and hydrochloric acid). As the reduction step is completed, we will obtain 3,5-diaminotoluene.

Oxidative cleavage of the methyl group

Oxidizing the methyl group in 3,5-diaminotoluene is the final step in forming the desired product, 3,5-dinitrobenzoic acid. We use potassium permanganate (KMnO4) in an alkaline medium like potassium hydroxide (KOH) and water. Once the reaction is complete, an acidic work-up using sulfuric acid (H2SO4) yields the final product, 3,5-dinitrobenzoic acid.

Key concepts

Nitration of Toluene

Nitration of toluene is a fundamental process in organic chemistry that introduces nitro groups (-NO₂) into the aromatic ring of toluene. It's a type of electrophilic aromatic substitution where toluene reacts with nitric acid, usually in the presence of sulfuric acid which acts as a catalyst.

The nitration process involves the formation of a nitronium ion (NO₂⁺), which is the actual nitrating agent. The temperature during nitration plays a crucial role in dictating which positions on the benzene ring will be nitrated. For instance, at lower temperatures (0-10 degrees Celsius), we can selectively nitrate the 2 and 4 positions to obtain compounds such as 2,4-dinitrotoluene, while moderate temperatures (50-60 degrees Celsius) favor nitration at the 3 and 5 positions.

The nitration reaction is highly exothermic and must be controlled carefully to avoid side reactions. It is also important to note that the presence of the methyl group in toluene makes it more reactive than benzene due to its electron-donating effect, thus making the ring more susceptible to attack by the nitronium ion.

Reduction of Nitro Compounds

Reduction of nitro compounds is an essential step in the synthesis of various aromatic derivatives. The reduction of nitro groups to amino groups on an aromatic ring transforms a nitro compound, like dinitrotoluene, into a diamine compound, such as diaminotoluene.

Two common methods for achieving this reduction are catalytic hydrogenation and dissolving metal reduction. Catalytic hydrogenation involves passing hydrogen gas over a metal catalyst such as Raney nickel or palladium on carbon (Pd/C). This method is highly efficient but requires special equipment. On the other hand, the dissolving metal reduction, often using zinc in hydrochloric acid, is more accessible but can be slower and less selective.

This chemical transformation is crucial as the amino group (-NH₂) serves as a key functional group for further chemical modifications. It's also a strategic step in converting toluene into dinitrobenzoic acid, which involves multiple stages, including the modification of the methyl group.

Oxidative Cleavage of Methyl Group

The oxidative cleavage of the methyl group in aromatic compounds like diaminotoluene is the final step in synthesizing dinitrobenzoic acid. This process involves converting the side chain of an aromatic compound into a carboxylic acid group (-COOH).

Oxidation using potassium permanganate (KMnO₄) in an alkaline medium, such as a solution of potassium hydroxide (KOH), is a classic way to achieve this transformation. The reaction needs to be carefully controlled to prevent over-oxidation, which could damage the benzene ring itself.

After the oxidation is complete, an acidic workup often using sulfuric acid (H₂SO₄) neutralizes the reaction mixture and facilitates the precipitation of the product. The resultant dinitrobenzoic acid is a valuable chemical for various applications, including the development of pharmaceuticals, dyes, and other organic compounds. It's this meticulous sequence of nitrations, reductions, and oxidation that enables the intricate change from toluene to a more complex and functionalized aromatic acid.