Problem 13 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound which one reaction with aqueous nitrous acid at low temperature produces an oily nitrosamine is (a) methylamine (b) ethylamine (c) triethylamine (d) diethylamine

Step-by-Step Solution

Answer

(d) diethylamine

Step 1: Understanding the Reaction

Nitrous acid (0{ ext{HNO}_20) reacts with primary amines to form unstable diazonium salts, which decompose. Secondary amines, however, react with nitrous acid to form stable nitrosamines, often as oily compounds. Tertiary amines do not react with nitrous acid in a similar fashion.

Step 2: Identify the Type of Amines

Examine the options: (a) Methylamine is a primary amine (one alkyl group attached to nitrogen). (b) Ethylamine is also a primary amine. (c) Triethylamine is a tertiary amine. (d) Diethylamine is a secondary amine (two alkyl groups attached to nitrogen).

Step 3: Determine the Suitable Reaction

Primary amines like methylamine and ethylamine will form diazonium salts with nitrous acid, not oily nitrosamines. Tertiary amines like triethylamine are generally unresponsive in this context. Only secondary amines such as diethylamine react with nitrous acid to produce stable nitrosamines.

Step 4: Select the Correct Compound

From the understanding of the reaction mechanism, diethylamine (d) is identified as the compound that forms an oily nitrosamine when reacting with aqueous nitrous acid at low temperatures.

Key Concepts

Amine Classification

Amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. They are classified based on the number of carbon-containing groups attached to the nitrogen atom.

• Primary Amines: these have one alkyl or aryl group attached. The general formula looks like RNH₂, where R is the alkyl group.
• Secondary Amines: two alkyl or aryl groups are connected to the nitrogen. The formula is R₂NH.
• Tertiary Amines: all three hydrogen atoms in ammonia are replaced, resulting in R₃N.

Understanding this classification is essential in predicting reactions that amines undergo with various reagents such as nitrous acid.

Nitrous Acid Reactions

Nitrous acid, denoted as HNO₂, is a weak and unstable acid that reacts uniquely with different types of amines.

• Primary Amines: react with nitrous acid to form diazonium salts, which are generally unstable and decompose, especially in water.
• Secondary Amines: they form stable nitrosamines, often noticed as oily substances due to their hydrocarbon content. These compounds are significant due to potential carcinogenic properties.
• Tertiary Amines: usually do not react with nitrous acid, because they lack an N-H bond required for the reaction.

The distinction in reaction outcomes stems from the different structures and the number of alkyl groups present in the amine molecules.

Primary Amines

Primary amines are the simplest type of amines, with the general formula RNH₂. Here, R represents a single alkyl or aryl group. Examples include methylamine (CH₃NH₂) and ethylamine (C₂H₅NH₂).

• These amines are the most reactive with nitrous acid, forming diazonium ions (RN₂⁺), which are critical intermediates in organic synthesis.
• The diazonium ions are quite unstable, especially in aqueous solutions, leading to rapid decomposition.
• This instability makes primary amines unsuitable for forming oily nitrosamines with nitrous acid.

In practical scenarios, handling diazonium compounds requires controlled conditions due to their potential to release nitrogen gas violently.

Secondary Amines

Secondary amines contain two alkyl or aryl groups attached to the nitrogen atom, giving them the R₂NH structure. Diethylamine (C₂H₅)₂NH is a classic example.

• When these amines react with nitrous acid, they form nitrosamines, compounds that typically appear oily and can be persistent.
• Nitrosamines have clinical importance due to safety concerns, as some members of this class are carcinogenic.
• The presence of an N-H bond alongside two carbon chains makes the reaction with nitrous acid result in stable nitrosamines.

Handling secondary amines with nitrous acid requires caution due to the potential health risks associated with nitrosamine formation.

Tertiary Amines

Tertiary amines, characterized by the formula R₃N, bear three alkyl or aryl groups attached to the nitrogen atom. An example is triethylamine (N(C₂H₅)₃).

• These amines cannot form stable compounds with nitrous acid because all hydrogen atoms replaced with carbon groups.
• Tertiary amines generally do not participate in nitrous acid reactions as there is no N-H bond to facilitate such interactions.
• This inertness can be useful since they remain unreactive in systems where nitrous acid is present.

Tertiary amines' stability makes them suitable for many applications, but they do not contribute to nitrosamine production in the presence of nitrous acid.