Identify the correct statement among the following: (a) \(\mathrm{n}, \mathrm{n}\) -dimethylaniline reacts with nitrous acid to give \(\mathrm{p}\) -nitroso- \(\mathrm{N}, \mathrm{N}\) -dimethyl aniline (b) Bromination of p-toluidine produces 3,5 -dibromo, 4-methylaniline (c) Aliphatic amines are less basic than ammonia (d) Aliphatic primary amines combine with nitrous acid under ice-cold conditions to form stable diazonium salts

The nitrosation reaction is a chemical process where a nitroso group (-NO) is introduced into a molecule, typically an amine. In the context of aromatic amines such as aniline derivatives, this reaction involves the treatment with nitrous acid (HNO2). However, the expected product is not a p-nitroso compound as some might predict. Instead, the reaction leads to the formation of diazonium ions, pivotal intermediates often used in the synthesis of a variety of organic compounds. These diazonium ions further react to form diverse groups, ranging from hydroxyl to halogen entities.
This process is quite sensitive to the conditions, such as temperature and concentration, which is why it's commonly carried out under cold conditions to control the reaction rate and avoid decomposition of the reactive intermediates. The incorrect assumption that N,N-dimethylaniline would result in nitroso compound formation underlines the importance of understanding the distinct behavior of various amines in the nitrosation reaction.

Electrophilic aromatic substitution is a fundamental type of reaction where an electrophile replaces a hydrogen atom in the benzene ring. This reaction is characteristic of compounds containing a benzene ring, like aniline or substituted aniline such as p-toluidine. Substituents on the ring can influence the reaction, directing the incoming electrophile to specific positions on the ring, known as ortho, meta, or para, relative to the substituent.

In the exercise at hand, p-toluidine, which has a methyl and an amino group, directs the incoming electrophile (bromine in this case) to the ortho and para positions because both substituents are activating and ortho/para-directing. However, since the para position is occupied, bromination occurs at the ortho positions exclusively. Recognizing the patterns of electrophilic aromatic substitution is vital for predicting and understanding the products of such reactions.

Basicity of amines is a measure of the compounds' ability to accept protons (H+) and, in general, the electron-donating ability of the nitrogen's lone pair. The basicity can be significantly influenced by the structure of the amine. Aliphatic amines, where the nitrogen atom is bonded to at least one alkyl group, tend to be more basic than ammonia due to the inductive effect of the alkyl group. This effect enhances the electron density of the nitrogen atom, making its lone pair more readily available to engage with protons.

This contrasts with the assumption made in the exercise that aliphatic amines would be less basic than ammonia. Such misconceptions highlight the need for clarity about the structural factors affecting basicity, and how substituents, whether alkyl or aryl, influence the electron availability associated with the nitrogen atom.

Diazonium salt formation represents a critical transformation in organic chemistry that involves the conversion of primary aromatic amines into diazonium salts through a reaction with nitrous acid, typically under cold conditions. Diazonium salts have the general formula R-N2+X−, where R is an aryl group and X is a halide or other anion. These salts are stable only at low temperatures and can participate in a variety of reactions, serving as intermediates in the synthesis of aromatic compounds with different functional groups.

Contrary to aromatic amines, aliphatic amines react with nitrous acid to produce unstable diazonium ions that often decompose quickly. The stability of diazonium salts is thus greatly influenced by the nature of the amine. Awareness of the differing reactivity and stability of diazonium compounds between aromatic and aliphatic amines is crucial for students undertaking synthesis in organic chemistry.