Question

n-Butylamine (I), diethylamine (II) and N,N-dimethylethyl amine (III) have the same molar mass. The increasing order of their boiling point is

• A. III < II < I
• B. I < II < III
• C. II < III < I
• D. II < I < III

Answer 1

Answer: (A)

$\underset{\text{n-butylamine } \left(\text{primary amine}\right)}{H_{3} C - C H_{2} - C H_{2} - C H_{2} - N H_{2}}$

$\underset{\text{Diethylamine } \left(\text{Secondary amine}\right)}{C H_{3} C H_{2} - N H - C H_{2} - C H_{3}}$

$\underset{\text{ethyldimethylamine } \left(\right. \text{tertiary amine} \left.\right)}{C H_{3} - \overset{C H_{3} \, }{\overset{\left|\right.}{N} - C_{2} H_{5}}}$

Primary amines have two hydrogen atoms available for hydrogen bond formation, while $2^\circ $ amines have only one hydrogen atom for hydrogen bond formation. $3^\circ $ amines do not have intermolecular association due to absence of hydrogen atom.

Thus, order of boiling point of isomeric amines is as follows

$3^\circ < 2^\circ < 1^\circ $

or $III < II < I$ .