Chemistry · Amines · NEET
Yes, but only if the amine has an N-H bond. Primary (R-NH2) and secondary (R2NH) amines have N-H bonds, so they can form hydrogen bonds with each other. Tertiary amines (R3N) have no N-H bond, so they cannot form hydrogen bonds among their own molecules. The bond forms because nitrogen is more electronegative than hydrogen, making N-H slightly polar.
Alkanes only have weak van der Waals forces. Amines with N-H bonds have both van der Waals forces AND hydrogen bonds. Hydrogen bonds are stronger, so more heat energy is needed to pull the molecules apart. That is why an amine boils higher than an alkane of almost the same molar mass (NCERT Table 9.2).
For amines of comparable molar mass: primary > secondary > tertiary (1deg > 2deg > 3deg). A primary amine has two N-H bonds so it forms the most hydrogen bonds, a secondary amine has one N-H bond so it forms fewer, and a tertiary amine has zero N-H bonds so it forms none. More hydrogen bonds means a higher boiling point.
They still have van der Waals (dispersion) forces between molecules, which every molecule has. These forces alone are weak, so a tertiary amine boils lower than the 1deg and 2deg amines but it still has a real boiling point. The missing part is only the extra hydrogen bonding, not all attraction.
Yes. A tertiary amine cannot hydrogen bond with another tertiary amine (no N-H), but its lone pair on nitrogen can still accept a hydrogen bond from a water O-H. This is why lower amines, including tertiary ones, dissolve in water. This point is different from the boiling point discussion, which is about amine-amine bonds only.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Primary amines. Each primary amine (R-NH2) has two N-H bonds, so it can form the most intermolecular hydrogen bonds, giving it the highest boiling point of the three classes at comparable mass.
A hydrogen bond of this type needs a hydrogen attached to nitrogen (N-H). Tertiary amines have all three positions filled by carbon groups (R3N), so there is no N-H hydrogen to donate. Hence no amine-amine hydrogen bonding.
Lower. Nitrogen is less electronegative than oxygen, so the N-H hydrogen bond is weaker than the O-H hydrogen bond. This is covered in the next concept, why amines boil lower than alcohols.
Yes. Lower amines dissolve in water because their N-H or lone pair can hydrogen bond with water. Solubility drops as the alkyl part gets larger, because the water-repelling (hydrophobic) part increases (NCERT).
