Question

Identify the correct statement among the following. (a) \(\mathrm{n}, \mathrm{n}\)-dimethylaniline reacts with nitrous acid to give p-nitroso-N,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

Short answer

The correct statement is (a).

Step-by-step solution

Understanding the Reaction of N,N-Dimethylaniline with Nitrous Acid

N,N-dimethylaniline is a secondary aromatic amine. When it reacts with nitrous acid, it produces p-nitroso-N,N-dimethylaniline. This is a result of the electrophilic substitution that takes place at the para position of the aromatic ring due to the activating nature of the methyl group.

Examining Bromination of p-Toluidine

p-Toluidine has an amino group, which is an activating group when considering electrophilic aromatic substitution reactions. Bromination of p-toluidine usually occurs readily due to this activation, but the statement claims that it forms 3,5-dibromo-4-methylaniline, which is a bit misleading as the initial bromination at para- and ortho- positions are more favored.

Comparing Basicity of Aliphatic Amines and Ammonia

Aliphatic amines are generally more basic than ammonia. This is because the alkyl group in aliphatic amines is electron-donating, which increases the electron density on the nitrogen, enhancing its ability to donate a lone pair of electrons compared to ammonia.

Analyzing Aliphatic Primary Amines with Nitrous Acid

Aliphatic primary amines react with nitrous acid under ice-cold conditions to form diazonium salts. However, these are typically unstable and rapidly decompose, especially at room temperature. Therefore, claiming they form stable diazonium salts is incorrect.

Key concepts

Electrophilic Aromatic Substitution

In organic chemistry, Electrophilic Aromatic Substitution (EAS) refers to the process where an electrophile replaces a hydrogen atom in an aromatic system. This process is particularly fascinating because of the unique stability of aromatic compounds, like benzene, which can undergo substitution relatively smoothly due to their electron-rich nature.
When we talk about the reaction of compounds like N,N-dimethylaniline with nitrous acid, this is a classic example of EAS. N,N-dimethylaniline is a secondary aromatic amine. It has an amine group that's a strong activator due to the lone pair of electrons on the nitrogen, and the methyl groups further enhance this activation.
The reaction with nitrous acid results in the formation of p-nitroso-N,N-dimethylaniline. Why at the para position? The para position is more accessible for the incoming electrophile due to the electron-donating effect of the methyl groups, which stabilizes the intermediate carbocation forming in the reaction.

• Activating groups like -NH(CH₃)₂ increase electron density on the ring.
• Reactions often favor para substitution for steric and electronic reasons.

Understanding EAS is crucial for predicting how aromatic compounds react with different electrophiles.

Amines Basicity

Amines are fascinating molecules when it comes to their basicity. Their basic nature arises from the lone pair of electrons on the nitrogen atom, which can accept a proton. Comparing aliphatic amines and ammonia reveals interesting insights into their basic behavior.
Aliphatic amines possess alkyl groups, which are electron-donating via the inductive effect. This donation of electron density towards the nitrogen enhances its electron pair donation capability, making aliphatic amines generally **more** basic than ammonia.
Here's why aliphatic amines are usually more basic:

• The alkyl groups push electron density onto nitrogen, making it more reactive.
• This increased electron density makes it easier for the amine to donate electrons, thus acting as a stronger base compared to ammonia.

Knowing the basicity of amines is crucial, not just for acid-base reactions, but for understanding their reactivity in organic synthesis.

Diazonium Salts

Diazonium salts are some of the most interesting intermediates in organic synthesis. These salts are typically formed from the reaction of primary aromatic amines with nitrous acid under cold conditions.
When an aliphatic primary amine reacts with nitrous acid, the story changes significantly. While aromatic diazonium salts ( ext{arenegroups}-N₂⁺) are relatively stable under cold conditions, the aliphatic diazonium ions are quite unstable and decompose rapidly.

• Formation of diazonium salts from aliphatic amines is less common due to their instability.
• They are often employed as intermediates for further chemical transformations in synthetic pathways.

Understanding the stability of these salts helps in predicting their behavior and implementing them in further chemical reactions, although in the case of aliphatic amines, maintaining low temperatures is crucial to prevent decomposition.

Nitrous Acid Reaction

The reaction of amines with nitrous acid is an area rich with intriguing chemistry. When primary amines, especially aromatic ones, are treated with nitrous acid ( ext{HNO₂}), they are converted into diazonium salts. This process involves the formation of a nitrosating agent, usually in acidic conditions, which then reacts with the amine.
Aliphatic amines, when treated with nitrous acid, experience a completely different reaction pathway than their aromatic counterparts. Instead of forming stable diazonium salts, aliphatic diazonium salts are unstable and prone to rapid decomposition, releasing nitrogen gas and forming alcohols or other byproducts depending on the conditions.

• Primary aromatic amines yield relatively stable diazonium salts.
• Aliphatic amines create unstable diazonium salts that decompose, highlighting the differences between aliphatic and aromatic systems.

This fundamental difference underlies many synthetic strategies and explains why specific products form when nitrous acid interacts with different types of amines.