Problem 164 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following does not react with nitrous acid? (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHCH}_{3}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}\left(\mathrm{CH}_{3}\right)_{2}\) (d) None of these

Step-by-Step Solution

Answer

The compound that does not react with nitrous acid is (c) C_6H_5N(CH_3)_2.

Step 1: Understand the properties of nitrous acid

Nitrous acid, HNO_2, is known to react with primary amines to form diazonium salts. Secondary amines generally react to form N-nitrosamines, and tertiary amines do not typically form stable products with nitrous acid.

Step 2: Analyze the given compounds

The compounds are (a) Aniline (C_6H_5NH_2), (b) N-methylaniline (C_6H_5NHCH_3), (c) N,N-dimethylaniline (C_6H_5N(CH_3)_2).  Aniline is a primary amine, N-methylaniline is a secondary amine, and N,N-dimethylaniline is a tertiary amine.

Step 3: Determine the reactivity with nitrous acid

(a) Aniline reacts with nitrous acid to form a diazonium salt due to being a primary amine. (b) N-methylaniline, being a secondary amine, will react with nitrous acid forming N-nitrosamine. (c) N,N-dimethylaniline, being a tertiary amine, does not react with nitrous acid due to the lack of a hydrogen atom to form a stable compound.

Step 4: Identify the compound that does not react with nitrous acid

Based on the reactivity discussed, C_6H_5N(CH_3)_2 (N,N-dimethylaniline) is the compound that does not react with nitrous acid to form any stable product due to being a tertiary amine.

Key Concepts

Nitrous Acid Reactions

Nitrous acid ( HNO_2 ) is a fascinating molecule that plays a crucial role in organic chemistry, especially in reactions involving different classes of amines. Its reactivity is largely determined by its ability to donate an NO+ ion, making it a powerful reagent in the formation of diazonium salts from primary amines. In general, nitrous acid is not stable as a pure substance and is usually generated in situ by reacting sodium nitrite with a mineral acid, like hydrochloric acid.

• Reacts with primary amines to form diazonium salts.
• Forms N-nitrosamines with secondary amines.
• Typically does not form stable products with tertiary amines.

Understanding these interactions is vital for predicting the results when nitrous acid is introduced to different nitrogen-containing compounds.

Primary Amine Reactivity

Primary amines, like aniline ( C_6H_5NH_2 ), are known for their unique reactivity with nitrous acid. When a primary amine reacts with nitrous acid, a diazonium salt is typically formed. This occurs because the nitrogen atom in the primary amine can easily accommodate the diazonium ion formed in the reaction.
Reaction pattern steps include:

• Formation of nitrosyl cation from nitrous acid.
• Attack on the primary amine leading to the loss of water.
• Formation of a diazonium ion.

This diazonium salt is highly useful as an intermediate in further reactions, especially in aromatic substitution reactions. However, it's important to handle these compounds carefully as they can be unstable and hazardous if not properly managed.

Tertiary Amines

Tertiary amines, like N,N-dimethylaniline ( C_6H_5N(CH_3)_2 ), display a different behavior when exposed to nitrous acid. Unlike primary and secondary amines, tertiary amines do not generally react to form stable products. This is mainly because there are no hydrogen atoms on the nitrogen that can be replaced by a nitrosyl group.
Key characteristics:

• Lacks a hydrogen atom on nitrogen for stable bonding with nitrous acid.
• Tends to form unstable or no products at all.

Thus, tertiary amines are less reactive towards nitrous acid, which is why compounds like N,N-dimethylaniline do not react to form stable compounds when exposed to this reagent.

Diazonium Salt Formation

Diazonium salts are significant intermediates in organic synthesis, often formed when primary aromatic amines react with nitrous acid. These salts act as great intermediates for various reactions  including the Sandmeyer reaction and azo coupling reactions, which are essential processes in organic chemistry.
Formation process includes:

• Reacting a primary amine with nitrous acid.
• Formation of the diazonium group (N+?Cl^-).

These salts have versatile applications but also come with necessary precautions due to their potential explosiveness. Cooling and careful handling can mitigate many of the risks, making the diazonium salt a powerful tool for creating desired organic transformations.