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

KEAMKEAM 2011Amines

Solution:

The boiling points of amines depends upon their degree of association and the degree of association is further depends upon the extent of H-bonding. Since $ 1{}^\circ $ amines have two, $ 2{}^\circ $ amines have one while 3° amines have no hydrogen atoms linked to nitrogen, therefore among isomeric amines, $ 1{}^\circ $ amines have the highest, while $ 3{}^\circ $ amines have the lowest boiling point. Thus, the increasing order of boiling point is
$ \underset{N,\text{ }N-dimethylethylamine\text{ }(III)}{\mathop{C{{H}_{3}}\overset{\begin{smallmatrix} C{{H}_{3}} \\ | \end{smallmatrix}}{\mathop{N}}\,C{{H}_{2}}C{{H}_{2}}}}\, $
$ \underset{diethylamine\text{ }(II)}{\mathop{ < C{{H}_{3}}C{{H}_{2}}NHC{{H}_{2}}-C{{H}_{3}}}}\, $ $ \underset{n-butylamine\text{ }(I)}{\mathop{ < C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}C{{H}_{2}}N{{H}_{2}}}}\, $