Answer:
Due to hydrogen bond in propylamine.
Step-by-step explanation:
The boiling point or melting point are property of
a) size
b) the intermolecular force of interactions.
As we know that primary and secondary amines form intermoleclar hydrogen bonds, which increases the force of attractions between molecules increasing the boiling point of amines.
However in case of tertiary amines there is no hydrogen directly attached to electronegative nitrogen atoms, thus no possibility of formation of hydrogen bond hence low boiling points.
The structures are shown in the figure.