Q44 / 150 · MHT CET 2026 (15 April Shift 2) · Chemistry
Which of the following amines is not represented by formula $\text{C}_3\text{H}_9\text{N}$ ?
- A$1^\circ$ amine
- B$2^\circ$ amine
- C$3^\circ$ amine
- Correct answerDquaternary ammonium salt
Solution
The molecular formula $\text{C}_3\text{H}_9\text{N}$ corresponds to saturated acyclic amines.
The possible structural isomers are:
Primary ($1^\circ$) amines: $\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$ (propan-1-amine) and $\text{CH}_3\text{CH}(\text{NH}_2)\text{CH}_3$ (propan-2-amine).
Secondary ($2^\circ$) amine: $\text{CH}_3\text{CH}_2\text{NHCH}_3$ (N-methylethanamine).
Tertiary ($3^\circ$) amine: $(\text{CH}_3)_3\text{N}$ (N,N-dimethylmethanamine).
A quaternary ammonium salt requires the nitrogen atom to be bonded to four alkyl groups. The simplest quaternary ammonium salt is the tetramethylammonium ion, which contains four carbon atoms ($\text{C}_4\text{H}_{12}\text{N}^+$).
Thus, $\text{C}_3\text{H}_9\text{N}$ cannot represent a quaternary ammonium salt.
Answer: quaternary ammonium salt
The possible structural isomers are:
Primary ($1^\circ$) amines: $\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$ (propan-1-amine) and $\text{CH}_3\text{CH}(\text{NH}_2)\text{CH}_3$ (propan-2-amine).
Secondary ($2^\circ$) amine: $\text{CH}_3\text{CH}_2\text{NHCH}_3$ (N-methylethanamine).
Tertiary ($3^\circ$) amine: $(\text{CH}_3)_3\text{N}$ (N,N-dimethylmethanamine).
A quaternary ammonium salt requires the nitrogen atom to be bonded to four alkyl groups. The simplest quaternary ammonium salt is the tetramethylammonium ion, which contains four carbon atoms ($\text{C}_4\text{H}_{12}\text{N}^+$).
Thus, $\text{C}_3\text{H}_9\text{N}$ cannot represent a quaternary ammonium salt.
Answer: quaternary ammonium salt
Hence, the correct answer is option: 'D'.