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)

$\underset{\text{n-butylamine }\left(\text{primary amine}\right)}{H_{3}C-C{H}_2-C{H}_2-C{H}_2-N{H}_2}$

$\underset{\text{Diethylamine }\left(\text{Secondary amine}\right)}{C{H}_{3}C{H}_2-NH-C{H}_2-C{H}_3}$

$\underset{\text{ethyldimethylamine }(\text{tertiary amine})}{C{H}_3-\stackrel{C{H}_3}{\stackrel{|}{N}-C_2{H}_5}}$

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 $III<II<I$ .