Amines
Amines are important class of organic compounds which are basic in nature. They are generally derived by replacing one or more hydrogen atoms of ammonia molecule by alkyl / aryl group(s). In nature, they are generally found in proteins, vitamins, alkaloids and hormones. Two biologically active compounds namely adrenaline and ephedrine contain secondary amino group and are used to increase blood pressure.
Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp3 hybridized and the geometry of amine is pyramidal. Due to the presence of unshared pair of electrons, the angle C-N-E (where E is C or H) is less than 109.5°.
Inductive effects, solvation effects, steric effects etc. affect the basic strength of amines. Amines are classified as primary (1°), secondary (2°), and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. Amines are engaged in intermolecular hydrogen bonding. Aliphatic and aromatic primary amines show positive carbylamine reaction. Amines can be prepared from various compounds like reduction of nitro, nitriles, amides etc. compounds. A colourless crystalline solid benzene diazonium chloride can be obtained from aniline by reacting with sodium nitrite and hydrochloric acid at 273-278 K.
Select the correct order of boiling points of isomeric amines
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → n-C4H9NH2 > (C2H5)2NH > C2H5N(CH3)2
The passage states that amines are engaged in intermolecular hydrogen bonding. This bonding holds the molecules together in the liquid, so it raises the boiling point. A hydrogen bond needs an N–H hydrogen on one molecule and the lone pair of N on another. The three amines here are isomers, all C4H11N with the same molar mass (73 g mol−1), so the number of N–H hydrogens decides the order:
n-C4H9NH2 — highest boiling point. Butan-1-amine is a primary amine with two N–H hydrogens, so it forms the most hydrogen bonds per molecule.
(C2H5)2NH — middle. N-Ethylethanamine is a secondary amine with only one N–H hydrogen, so its intermolecular association is less than in the primary amine.
C2H5N(CH3)2 — lowest boiling point. N,N-Dimethylethanamine is a tertiary amine with no N–H hydrogen. Its molecules cannot hydrogen bond with one another and are held only by weaker forces. The boiling points are not equal in any pair, since each amine has a different number of N–H hydrogens.
NCERT (2026-27 edition):
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. Tertiary amines do not have intermolecular association due to the absence of hydrogen atom available for hydrogen bond formation. Therefore, the order of boiling points of isomeric amines is as follows:
Primary > Secondary > Tertiary”