Arrange the following compounds in increasing order of boiling point.
(A) Ethanol
(B) Diethyl ether
(C) Ethanal
(D) Ethane
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (D), (C), (B), (A)
Note: None of the given options was correct in the NTA question paper. To provide additional question practice to the students, the question has been slightly modified so that the question has one clear answer.
The boiling point of a molecular compound depends on the strength of its intermolecular forces: hydrogen bonding is the strongest, then dipole–dipole attraction, and the weakest are the van der Waals (London) forces present in all molecules. The London forces grow with the size and molar mass of the molecule, so a large, weakly polar molecule can boil higher than a small, strongly polar one.
(D) Ethane, \( \ce{C2H6} \) (30 g mol−1): non-polar, only weak London forces, so it comes first (b.p. 184 K).
(C) Ethanal, \( \ce{CH3CHO} \) (44 g mol−1): strongly polar C=O group, dipole–dipole attraction but no hydrogen bonding; it comes second (b.p. 294 K).
(B) Diethyl ether, \( \ce{C2H5-O-C2H5} \) (74 g mol−1): weakly polar and no hydrogen bonding, but its much larger molar mass gives stronger London forces that outweigh the higher polarity of ethanal; it comes third (b.p. 308 K).
(A) Ethanol, \( \ce{C2H5OH} \) (46 g mol−1): the O–H group forms intermolecular hydrogen bonds, so it comes last (b.p. 351 K).