Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N}\) cannot represent (a) quaternary salt (b) \(1^{\circ}\) amine (c) \(2^{\circ}\) amine (d) \(3^{\circ}\) amine

Step-by-Step Solution

Answer

(a) Quaternary salt cannot be represented by \(\mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N}\).

Step 1: Understanding Molecular Formula

The molecular formula given is \(\mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N}\). It contains 3 carbon atoms, 9 hydrogen atoms, and 1 nitrogen atom.

Step 2: Identifying the Quaternary Ammonium Salt

A quaternary ammonium salt is formed when a nitrogen atom is bonded to four carbon groups, usually taking the form of \(\text{NR}_4^+\). This structure cannot be formed using only three carbon atoms as provided by \(\mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N}\).

Step 3: Identifying the Types of Amines

Amino groups can be associated with different numbers of carbon groups: \(1^{\circ}\) amines (primary) have one carbon group, \(2^{\circ}\) amines (secondary) have two carbon groups, and \(3^{\circ}\) amines (tertiary) have three carbon groups attached to a nitrogen atom.

Step 4: Applying the Formula to Each Type

1. Primary amine can be \((\text{C}_3\text{H}_7\text{NH}_2)\), matching the formula. 2. Secondary amine can be \((\text{C}_2\text{H}_5\text{NH}\text{CH}_3)\) or other combinations, matching the formula.3. Tertiary amine can be \((\text{CH}_3\text{NH}\text{CH}_2\text{CH}_3)\), matching the formula.

Key Concepts

Molecular Formula

The molecular formula, such as \( \mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N} \), provides crucial information about a molecule's composition. It tells us the number of each type of atom in a single molecule. In this formula, we see three carbon atoms, nine hydrogen atoms, and one nitrogen atom.
This molecular configuration hints at the structure of the molecule and suggests what it could potentially form or recognize as different organic compounds.
In organic chemistry, such formulas help identify possible functional groups and predict the molecule's physical and chemical behavior. Understanding a molecular formula is the first step in analyzing what type of compound or compounds could be represented, providing a gateway to further exploration of its reactive properties.

Amines

Amines are organic compounds derived from ammonia \((\text{NH}_3)\) but with one or more hydrogen atoms replaced by a carbon-containing group. They can be classified into different types based on the number of carbon groups attached to the nitrogen atom.

• Primary amines \((1^{\circ})\): Have one alkyl group attached, represented as \(\mathrm{RNH}_2\).
• Secondary amines \((2^{\circ})\): Have two alkyl groups attached, represented as \(\mathrm{R}_2\mathrm{NH}\).
• Tertiary amines \((3^{\circ})\): Have three alkyl groups attached, shown as \(\mathrm{R}_3\mathrm{N}\).

Amines are essential building blocks in organic chemistry, participating in various reactions and forming the backbone for numerous biological compounds like amino acids. The given formula \(\mathrm{C}_3\mathrm{H}_9\mathrm{~N}\) can represent all three types of amines by arranging different combinations of carbon and hydrogen around nitrogen, emphasizing the versatility of amine structures.

Quaternary Ammonium Salts

Quaternary ammonium salts are distinct from typical amines because they feature a nitrogen atom bonded to four carbon-containing groups, forming a positive charge denoted as \(\mathrm{NR}_4^{+}\). This structure is what gives them their unique set of properties.
In the given molecular formula \(\mathrm{C}_{3} \mathrm{H}_{9} \mathrm{~N} \), it's not possible to form a quaternary ammonium salt. The reason is that there aren't enough carbon atoms available to create four covalent bonds with the nitrogen atom.
These salts find applications primarily as surfactants and fabric softeners due to their ability to alter surface tension properties. Understanding how these salts are structured and why they require a different carbon count than provided in \(\mathrm{C}_3\mathrm{H}_9\mathrm{~N}\) is key to appreciating their role in both industrial and household products.