Problem 67 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

The correct statement is (a). N,N-dimethylaniline reacts with nitrous acid to form p-nitroso-N,N-dimethylaniline.

Step 1: Analyze Option (a)

N,N-dimethylaniline, when reacted with nitrous acid, tends to undergo nitrosation at the para position of the aniline ring if sterically possible. The reaction involves the formation of a nitroso group at the para position, leading to p-nitroso-N,N-dimethylaniline. Hence, option (a) is correct.

Step 2: Analyze Option (b)

In the bromination of p-toluidine, the amino group is highly activating and ortho-para directing, leading to the substitution at the ortho and para positions relative to the amino group. Generally, the product is 2,4,6-tribromo-4-methylaniline, not 3,5-dibromo-4-methylaniline. So, option (b) is incorrect.

Step 3: Analyze Option (c)

Aliphatic amines are generally more basic than ammonia due to the electron-donating effect of the alkyl groups which increase the electron density on the nitrogen. Therefore, option (c) is incorrect.

Step 4: Analyze Option (d)

Aliphatic primary amines, when treated with nitrous acid at low temperatures, do not form stable diazonium salts; instead, they usually decompose to give alcohols or other products. Hence, option (d) is incorrect.

Key Concepts

N,N-dimethylaniline

N,N-dimethylaniline is an aromatic compound in which an aniline molecule (C6H5NH2) is modified by replacing the hydrogen atoms of the amino group with two methyl (2CH3) groups. This modification results in the formation of a secondary amine known as a tertiary amine due to the presence of the two methyl groups attached to the nitrogen atom. These two methyl groups make N,N-dimethylaniline an electron-rich molecule, enhancing its reactivity in electrophilic aromatic substitution reactions. One classic reaction is with nitrous acid. When reacted, the molecule tends to undergo nitrosation at the para position, provided steric hindrance allows it. This reaction creates p-nitroso-N,N-dimethylaniline, where a nitroso group (-NO) replaces a hydrogen atom at the para position of the aromatic ring.

Bromination of p-toluidine

Bromination refers to the introduction of bromine atoms into a compound. In the case of p-toluidine (an aromatic amine), this involves the addition of bromine to the aromatic ring. The amino group in p-toluidine is highly activating and directs incoming bromine atoms to the ortho and para positions relative to its position. This is because the amino group donates electrons into the aromatic ring, increasing the electron density at these positions. Due to this electron-rich environment, bromination happens readily at multiple sites. As a result, instead of forming 3,5-dibromo-4-methylaniline, p-toluidine typically yields 2,4,6-tribromo-4-methylaniline under typical bromination conditions. This makes its bromination a useful reaction in synthesizing certain derivatives used in dyes and pharmaceuticals.

Amine Basicity

Basicity in amines is typically attributed to the lone pair of electrons on the nitrogen atom. The presence of this lone pair means amines can accept protons (H+) easily, a fundamental characteristic of bases. Aliphatic amines, which have alkyl groups attached to the nitrogen, are generally more basic than ammonia (NH3). This is because alkyl groups are electron-releasing through inductive effects, increasing electron density on the nitrogen, and making the lone pair more available for protonation. This enhanced basicity makes aliphatic amines very reactive and useful in a wide range of applications, including as base-rich reagents and building blocks in organic synthesis.

Diazonium Salts Formation

Diazonium salts are compounds that consist of a diazonium group (N2+) attached to a an aromatic ring. They are typically formed through the reaction of primary aromatic amines with nitrous acid. However, in the case of primary aliphatic amines, a stable diazonium salt doesn't form under normal conditions. Instead, when treated with nitrous acid at low temperatures, these compounds tend to decompose, often forming alcohols or other side products. This is because the diazonium salts derived from aliphatic amines are not stable, unlike their aromatic counterparts. Aromatic diazonium salts are stable due to resonance stabilization over the aromatic ring, a characteristic not available to aliphatic compounds.