The figure shows the intermolecular hydrogen bonding in an amine (R = alkyl group). The dotted lines are hydrogen bonds.
How many hydrogen atoms on the nitrogen of each molecule are available for hydrogen bond formation, and how does the intermolecular association of this amine compare with that of an isomeric secondary amine?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Two; more than in the secondary amine
Each nitrogen atom in the figure carries one R group and two hydrogen atoms, so the amine is R–NH2, a primary amine. Both N–H hydrogens form hydrogen bonds with the nitrogen of neighbouring molecules.
A secondary amine, R2NH, has only one such hydrogen, so the intermolecular association of the primary amine is more. This is why a primary amine boils higher than its isomeric secondary amine.
Why the other options are incorrect
Option (1) One; less than in the secondary amine — Incorrect. Each nitrogen in the figure has two hydrogen atoms, not one, and with two hydrogens the association is more, not less.
Option (3) Two; less than in the secondary amine — Incorrect. The count is right, but two N–H hydrogens give more hydrogen bonds than the single N–H hydrogen of a secondary amine, so the association is more.
Option (4) One; more than in the secondary amine — Incorrect. One N–H hydrogen is the case of a secondary amine itself; the nitrogen atoms in the figure each carry two hydrogens.
NCERT (2026-27 edition):
9.5 Physical Properties – “Primary and secondary amines are engaged in intermolecular association due to hydrogen bonding between nitrogen of one and hydrogen of another molecule. This intermolecular association is more in primary amines than in secondary amines as there are two hydrogen atoms available for hydrogen bond formation in it. … Intermolecular hydrogen bonding in primary amines is shown in Fig. 9.2.”
Fig. 9.2: Intermolecular hydrogen bonding in primary amines