Problem 12 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The number of primary, secondary and tertiary amines possible with the molecular formula \(\mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N}\) respectively (a) \(1,2,2\) (b) \(1,2,1\) (c) \(2,1,1\) (d) \(3,0,1\)

Step-by-Step Solution

Answer

The correct answer is (c) \(2,1,1\).

Step 1: Understanding the Molecular Formula

The molecular formula given is \( \mathrm{C}_3 \mathrm{H}_9 \mathrm{~N} \), which indicates that we have three carbon atoms, nine hydrogen atoms, and one nitrogen atom to work with. Amines are classified into primary, secondary, and tertiary based on the bonding of the nitrogen atom with carbon atoms.

Step 2: Identifying Primary Amines

Primary amines have the general structure \( \mathrm{RNH}_2 \), where the nitrogen atom is attached to one carbon atom. For \( \mathrm{C}_3 \mathrm{H}_9 \mathrm{~N} \), possible primary amines are:1. Propan-1-amine: Nitrogen is attached to a terminal carbon.2. Propan-2-amine: Nitrogen is attached to the second carbon.Thus, there are 2 primary amines possible.

Step 3: Identifying Secondary Amines

Secondary amines have the general structure \( \mathrm{R}_2\mathrm{NH} \), where the nitrogen atom is attached to two carbon atoms. Considering permutations of carbon atoms and maintaining the molecular formula, the possible secondary amine is:1. \( \mathrm{N{-}methylethylamine} \): One methyl and one ethyl group attached to nitrogen.Thus, there is 1 secondary amine possible.

Step 4: Identifying Tertiary Amines

Tertiary amines have the structure \( \mathrm{R}_3\mathrm{N} \), where the nitrogen atom is attached to three carbon atoms. The possible tertiary amine for \( \mathrm{C}_3 \mathrm{H}_9 \mathrm{~N} \) is:1. \( \mathrm{N{-}trimethylamine} \): Three methyl groups attached to nitrogen.Thus, there is 1 tertiary amine possible.

Step 5: Conclusion and Selection

Based on the analysis, there are 2 primary amines, 1 secondary amine, and 1 tertiary amine possible with the molecular formula \( \mathrm{C}_3 \mathrm{H}_9 \mathrm{~N} \). This corresponds with option (c).

Key Concepts

Primary Amines

Primary amines are a type of amine where the nitrogen atom is bonded to only one carbon atom and two hydrogen atoms. They have the general structure of \( \text{RNH}_2 \). The 'R' group in this formula represents an alkyl or aryl group.
Primary amines are typically more reactive due to the presence of the hydrogen atoms on the nitrogen. This setup allows for various chemical reactions, such as acylation or alkylation, to occur.In the context of the molecular formula \( \text{C}_3 \text{H}_9 \text{N} \), two primary amines can be formed:

• Propan-1-amine: Here, the nitrogen is bonded to the first or terminal carbon in the chain.
• Propan-2-amine: In this structure, the nitrogen is bonded to the second carbon.

These specific configurations allow the amines to maintain the molecular composition while meeting the structural requirements of primary amines.

Secondary Amines

Secondary amines are defined by having a nitrogen atom bonded to two carbon atoms and one hydrogen atom. Their general structure is \( \text{R}_2\text{NH} \), with 'R' groups being alkyl or aryl groups.
In these amines, the nitrogen's lone pair of electrons can participate in bonding, making them slightly less reactive than primary amines, but still quite reactive in typical organic reactions.For the molecular formula \( \text{C}_3 \text{H}_9 \text{N} \), one potential secondary amine is:

• N-methylethylamine: In this compound, the nitrogen is attached to both a methyl group and an ethyl group.

This configuration provides the exact number of hydrogen and carbon atoms while fulfilling the criteria of secondary amine structure.

Tertiary Amines

Tertiary amines occur when a nitrogen atom is bonded to three carbon atoms with no hydrogen atoms directly attached to it. Their standard formula is \( \text{R}_3\text{N} \), where 'R' signifies alkyl or aryl groups.
Tertiary amines typically display the least reactivity among amines, primarily due to the lack of hydrogen atoms bonded to nitrogen. However, their structure makes them significant in applications like solubilizers and catalysts.For the given molecular formula \( \text{C}_3 \text{H}_9 \text{N} \), the possible tertiary amine is:

• N-trimethylamine: This structure involves three methyl groups attached to the nitrogen atom.

Such an arrangement aligns perfectly with the specifications for a tertiary amine and simultaneously adheres to the molecular formula provided.