Question

The following amines are having same molecular masses. (i) n-C\(_4\)H\(_9\)NH\(_2\) (ii) (C\(_2\)H\(_5\))\(_2\)NH (iii) C\(_2\)H\(_5\)N(CH\(_3\))\(_2\). The correct order of boiling point of the above amines is

• A. (i) $>$ (ii) $>$ (iii)
• B. (ii) $>$ (iii) $>$ (i)
• C. (iii) $>$ (i) $>$ (ii)
• D. (ii) $>$ (i) $>$ (iii)
• E. (i) $>$ (iii) $>$ (ii)

Hint:

For isomeric amines, the boiling point order is: 1\(^\circ\) $>$ 2\(^\circ\) $>$ 3\(^\circ\).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The boiling point of a substance depends on the strength of the intermolecular forces between its molecules. For amines of comparable molecular mass, the main intermolecular forces to consider are hydrogen bonding and van der Waals forces. The extent of hydrogen bonding is the dominant factor.
Step 2: Detailed Explanation:
Let's classify the given amines:
(i) n-C\(_4\)H\(_9\)NH\(_2\) (Butan-1-amine): This is a primary (1\(^\circ\)) amine. It has two hydrogen atoms attached to the nitrogen atom (-NH\(_2\)). This allows it to form extensive intermolecular hydrogen bonds, acting as both a hydrogen bond donor and acceptor.
(ii) (C\(_2\)H\(_5\))\(_2\)NH (Diethylamine): This is a secondary (2\(^\circ\)) amine. It has one hydrogen atom attached to the nitrogen atom (-NH-). It can still form hydrogen bonds, but to a lesser extent than a primary amine because it has fewer N-H bonds.
(iii) C\(_2\)H\(_5\)N(CH\(_3\))\(_2\) (N,N-Dimethylethanamine): This is a tertiary (3\(^\circ\)) amine. It has no hydrogen atoms attached to the nitrogen atom. Therefore, it cannot form hydrogen bonds with other tertiary amine molecules (it cannot act as a hydrogen bond donor). It can only act as a hydrogen bond acceptor with other molecules like water. The main intermolecular forces are dipole-dipole interactions and van der Waals forces.
Since all three amines are isomers (C\(_4\)H\(_{11}\)N, molecular mass = 73 g/mol), their van der Waals forces are comparable. The deciding factor is hydrogen bonding.
The strength of hydrogen bonding follows the order: Primary amine> Secondary amine> Tertiary amine.
Consequently, the energy required to overcome these forces (and thus the boiling point) will follow the same order.
Boiling Point Order: n-C\(_4\)H\(_9\)NH\(_2\)> (C\(_2\)H\(_5\))\(_2\)NH> C\(_2\)H\(_5\)N(CH\(_3\))\(_2\).
This corresponds to the order (i)> (ii)> (iii).
Step 3: Final Answer:
The correct order of boiling points is (i)> (ii)> (iii). This corresponds to option (A).