Discussion
The following amines are having same molecular masses.
(i) n-C4H9NH2 (ii) (C2H5)2NH (iii) C2H5N(CH3)2
The correct order of boiling point of the above amines is
Answer: A
The order is (i) > (ii) > (iii).
The three amines have the same molar mass (73), so the difference in boiling point comes from intermolecular hydrogen bonding through N-H bonds. The primary amine, n-butylamine, has two N-H hydrogens per molecule and forms the most hydrogen bonds. The secondary amine, diethylamine, has one. The tertiary amine, ethyldimethylamine, has no hydrogen on nitrogen and cannot hydrogen bond with itself at all.
Their boiling points are about 351 K, 329 K and 310 K respectively. Every other option puts the tertiary amine above at least one of the others, or the secondary above the primary, which reverses the hydrogen bonding trend.
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