Why do primary amines have higher boiling point than tertiary amines?

Explanation

Out of `1^(@), 2^(@) and 3^(@)` amines (of equal molecular masses), `1^(@)` amines have highest boiling while `3^(@)` amines have lowest boiling point. For example
`underset("n-Propylamine"(1^(@)))(CH_(3)CH_(2)CH_(2)NH_(2))underset("Ethylmethylamine"(2^(@)))(CH_(3)-CH_(2)-NH-CH_(3))underset(("Trimethylamine("3^(@)))(CH_(3)-oversetoverset(CH_(3))(|)(N)-CH_(3))` have equal molecular masses (59 g `mol^(-1)`).
Out of these three amines, n-propylamine have highest boiling while trimethyl amine have lowest boiling point. This is because H-bonding is most extensive in `1^(@)` amines in which two H-atoms are directly attached to N which take part in H-bonding. In case of `3^(@)` amine no H-bonding is possible as no H-atom is directly attached to nitrogen. As we know H-bonding leads to stronger intermolecular forces and hence higher boiling points.