The major product obtained by the mononitration of \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{SO}_{2}-\mathrm{C}_{6} \mathrm{H}_{5}\) is

The process of mononitration of benzene is a classic example of electrophilic aromatic substitution, a reaction in which benzene acts as a nucleophile to substitute one of its hydrogen atoms for an electrophile, in this case, a nitro group (\textbf{NO}\(_2\)). Here's how it works:

Firstly, a nitrating mixture, typically composed of nitric acid (\textbf{HNO}\(_3\)) and sulfuric acid (\textbf{H}\(_2\)\textbf{SO}\(_4\)), generates the active nitronium ion (\textbf{NO}\(_2^+\)). Next, benzene, with its high electron density and stable pi-electron cloud, forms a complex with the electrophilic nitronium ion. This electrophile attacks one of the hydrogen atoms on the benzene ring and temporarily disrupts the aromatic system, generating an intermediate species. Finally, the aromaticity is restored when the hydrogen proton is removed, resulting in the substituted product, nitrobenzene.

It's crucial that this reaction happens under controlled conditions because the presence of excess nitric acid could lead to further substitution, giving di- or trinitro compounds instead of the desired mononitro derivative.

Within the scope of electrophilic aromatic substitution reactions, ortho-/para-directing groups significantly influence the final position of the introduced electrophile on the benzene ring. These are groups that, due to the presence of lone pair electrons or an electronegative element, can donate electron density through resonance. This action enriches the ortho and para positions with electrons and makes them more attractive to incoming electrophiles.

Examples include functional groups such as \textbf{-OH}, \textbf{-OCH}\(_3\), and \textbf{-NH}\(_2\). When these groups are present on a benzene ring, they increase the likelihood of the electrophile attaching at either the position adjacent to (ortho) or opposite (para) the functional group. The ability for these groups to activate the ring is why they're termed 'activating groups.' Besides resonance, inductive effects can also play a role, where electron-donating groups release electron density into the ring through sigma bonds.

The ortho-/para-directing nature of a group can be a competitive factor when designing synthesis pathways, as other substituents on the benzene can affect the expected outcome of a reaction.

When considering the mononitration of a complex molecule with multiple functional groups, we must assess steric hindrance in nitration. Steric hindrance occurs when the size of groups attached to the benzene ring interferes with the approach of the electrophile. This is a physical obstruction rather than an electronic effect, affecting the reaction's outcome by making certain positions on the ring less accessible.

In step 3 of our step-by-step solution, we determined that the para position was less sterically hindered, leading to the major product having a nitro group at this location. This is a common theme in nitration reactions, where the bulkier ortho position is often less favored. The presence of large substituents could completely inhibit reaction at the ortho position, making the para product the most favorable and sometimes the only possible product.

Understanding steric hindrance is key in predicting reaction outcomes, especially since it does not rely solely on electronic effects. It's a practical consideration that's crucial for obtaining high yields of the desired product in synthetic chemistry.