Which of the following arylamines can not from a diazonium salt on reaction with sodium nitrite in hydrochloric acid? (a) 4-Chloro-2-nitroaniline (b) N-Ethyl-2-methylaniline (c) m-Ethylaniline (d) p-Aminoacetophenone

The formation of diazonium salts is a cornerstone of aromatic chemistry and is widely used in the synthesis of various aromatic compounds. The process involves the conversion of a primary aromatic amine, such as aniline, into a diazonium salt. This transformation is achieved by treating the amine with sodium nitrite (NaNO₂) and an acid, typically hydrochloric acid (HCl), under cold conditions, usually 0-5°C. During this reaction, the nitrogen-nitrogen triple bond (N₂+) of the diazonium group is formed, replacing the hydrogen atom of the amine group (-NH₂).

The reaction mechanism starts with the generation of nitrous acid (HNO₂), which then forms a nitrosonium ion (NO⁺). This electrophile attacks the lone pair of electrons on the nitrogen atom of the amine, eventually leading to the loss of water and the formation of the diazonium cation. The resulting diazonium salt is an important intermediate because it can participate in a variety of subsequent reactions, such as Sandmeyer reactions, azo coupling, and replacement by other groups.

The reactivity of anilines in diazotization reactions largely depends on the structure of the compound and the substituents present on the aromatic ring. Primary aromatic amines, where the amino group is directly attached to the benzene ring without additional substituents on the nitrogen, are typically very reactive towards diazotization. This is due to the electron-donating nature of the amino group that activates the ring, making it more reactive to electrophilic attack.

However, the presence of electron-withdrawing groups, like nitro (-NO₂) or carbonyl groups, can decrease the reactivity of the aniline by reducing the electron density on the nitrogen, although they might not prevent diazotization if the amino group remains primary. In the case of N-alkylated anilines, where the nitrogen has one or more alkyl groups attached, diazotization cannot occur because these are not primary amines. Substitution of the amino group for secondary or tertiary amines alters the hydrogen bonding and electron availability, thus preventing the formation of the diazonium ion.

Amino group substitution in diazotization refers to the specific replacement of the amino group with a diazonium group during the formation of diazonium salts. This step is essential because it significantly alters the chemistry of the original aromatic amine. Once the diazonium salt is formed, it has the potential to undergo various nucleophilic substitution reactions. These reactions can lead to the introduction of a wide range of functional groups in place of the diazonium group, such as halides, cyanides, and hydroxyl groups.

One should note that the presence of different substituents on the aromatic ring affects not only the ability of the amine to undergo diazotization but also the reactivity of the resulting diazonium salt towards different nucleophiles. Electron-donating groups tend to stabilize the diazonium ion, making it more susceptible to nucleophilic attack, whereas electron-withdrawing groups can decrease its reactivity. Understanding both the factors that influence diazotization and the potential for subsequent substitution reactions is crucial for predicting and controlling the outcomes of the synthesis of aromatic compounds.