Chemistry · Amines · NEET
A primary amine (R–NH2) has two N–H bonds. A secondary amine (R2NH) has one N–H bond. A tertiary amine (R3N) has zero N–H bonds because all three positions on nitrogen carry carbon groups. This count is the whole story: only an N–H hydrogen can be donated for hydrogen bonding to another amine molecule.
A hydrogen bond needs a hydrogen attached to N, O, or F on one molecule (a donor) and a lone pair on another. Tertiary amines still have the nitrogen lone pair (acceptor), but they have no N–H to donate. So two tertiary amine molecules cannot hold each other by hydrogen bonding, and only weak van der Waals forces are left. That is why 3° amines boil lowest.
Yes. Boiling point also rises with molecular mass because heavier molecules have stronger van der Waals forces. The 1° > 2° > 3° rule is about hydrogen bonding, so it holds cleanly only when the amines have similar masses (for example the three isomers of C3H9N: propan-1-amine, N-methylethanamine, trimethylamine). If masses differ a lot, mass can override the N–H effect.
Yes. Solubility and boiling point are different questions. A tertiary amine has no N–H to donate, so it cannot hydrogen bond amine-to-amine (low boiling point). But its nitrogen lone pair can still ACCEPT a hydrogen bond from a water O–H, so lower tertiary amines are still soluble in water. Do not mix up the two ideas.
A primary amine has two N–H bonds, so each molecule can act as a hydrogen-bond donor at two points and can link into a larger network. A secondary amine has only one N–H, so it forms fewer bonds. This extra network in primary amines needs the most heat energy to break, giving the highest boiling point of the three.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For amines of similar molecular mass the order is 1° > 2° > 3°. Primary amines boil highest because they have two N–H bonds for hydrogen bonding, secondary have one, and tertiary have none.
No. Tertiary amines have no N–H bond to donate, so they cannot form hydrogen bonds with one another. They are held together only by weak van der Waals forces, which is why they boil lowest among the three.
Nitrogen is less electronegative than oxygen, so the N–H bond is less polar than O–H. Amine hydrogen bonds are therefore weaker than alcohol hydrogen bonds, so amines boil lower than comparable alcohols.
Propan-1-amine (a primary amine) has the highest boiling point among the C3H9N isomers because it has two N–H bonds. Trimethylamine (tertiary, no N–H) has the lowest.
Yes. NEET tests intermolecular forces and hydrogen bonding through assertion-reason and comparison questions. Knowing that boiling point follows the N–H count (1° > 2° > 3°) prevents easy mistakes.
