Comprehension:
Amines are derivatives of ammonia obtained by replacement of one, two or all the three hydrogen atoms by alkyl and / or aryl groups. Nitrogen atom of amines like ammonia is trivalent and carries an unshared pair of electrons. Orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amines. The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. Due to the presence of unshared pair of electrons, the angle C-N-E (where E is C or H) is less than tetrahedral angle 109.5°. Amines are classified as primary (1°), secondary (2°) and tertiary (3°) amines. All amines are basic in nature and basicity differs between primary amines, secondary amines and tertiary amines. In aryl amines, –NH2 group is directly attached to the benzene ring. C6H5NH2 is simplest example of arylamine. Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
Arrange the following compounds in the decreasing order of their boiling points.
A. n-C4H9NH2
B. C2H5CH(CH3)2
C. (C2H5)2NH
D. n-C4H9OH
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → D, A, C, B
For compounds of similar molar mass, the boiling point depends on the strength of intermolecular hydrogen bonding:
Alcohols vs amines: oxygen is more electronegative than nitrogen, so the O–H…O hydrogen bond is stronger than N–H…N. Alcohols boil higher than amines.
Primary vs secondary vs tertiary amines: a primary amine has two N–H hydrogens for hydrogen bonding, a secondary amine one and a tertiary amine none. So the boiling point order is 1° > 2° > 3°.
Alkanes: non-polar, with only weak van der Waals forces, so they boil lowest.
Matching the given compounds (all with molar mass 72–74 g mol−1):
(D) n-\( \ce{C4H9OH} \): butan-1-ol (alcohol)
(A) n-\( \ce{C4H9NH2} \): butan-1-amine (1° amine)
(C) \( \ce{(C2H5)2NH} \): N-ethylethanamine (2° amine)
(B) \( \ce{C2H5CH(CH3)2} \): 2-methylbutane (alkane)
Shortcut: The alcohol (D) has the highest boiling point. Only Option (3) starts with D.
NCERT (2026-27 edition):
Physical Properties – …
