Question

The nitrosation of N,N-dimethylaniline takes place through the attack of electrophile

• A. nitronium ion
• B. protonated nitrous acid
• C. nitrous acid
• D. nitrite ion
• E. nitrosonium ion

Hint:

Do not confuse the nitrosonium ion (\(NO^+\)) used in nitrosation with the nitronium ion (\(NO_2^+\)) used in nitration!

Answer 1

Answer: (E)

Concept: Nitrosation is an electrophilic aromatic substitution reaction. N,N-dimethylaniline is a tertiary aromatic amine. Because the amino group is highly activating, it reacts with weak electrophiles.

Step 1: Generation of the electrophile.
The reaction uses sodium nitrite (\(NaNO_2\)) and hydrochloric acid (\(HCl\)).
• Nitrous acid (\(HNO_2\)) is formed first.
• Under acidic conditions, the hydroxyl group of \(HNO_2\) is protonated and leaves as a water molecule. \[ H-O-N=O + H^+ \rightarrow H_2O^+-N=O \rightarrow H_2O + ^+N=O \]

Step 2: Identify the ion.
The species \(^+N=O\) is called the nitrosonium ion. This is the active electrophile that attacks the aromatic ring.

Step 3: Substitution pattern.
Since the dimethylamino group is bulky and strongly ortho/para directing, the nitrosonium ion attacks the para position to form p-nitroso-N,N-dimethylaniline (a green colored compound).