Question

Primary amine have higher bp than isomeric secondary and tertiary amine.why?

Answer 1

• The reason for the higher boiling points of the primary amines is that they can form hydrogen bonds with each other as well as van der Waals dispersion forces and dipole-dipole interactions. Hydrogen bonds can form between the lone pair on the very electronegative nitrogen atom and the slightly positive hydrogen atom in another molecule.
• The boiling point of the secondary amine is a little lower than the corresponding primary amine with the same number of carbon atoms. 
• Secondary amines still form hydrogen bonds, but having the nitrogen atom in the middle of the chain rather than at the end makes the permanent dipole on the molecule slightly less. 
• The lower boiling point is due to the lower dipole-dipole attractions in the dimethylamine compared with ethylamine.
• But in tertiary amine, due to absence of hydrogen, no hydrogen and formation takes place. This is the reason that primary amine has higher b.p.