Answer the question on the basis of passage given below:
Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively, is replaced by –OH group. The substitution of a hydrogen atom in a hydrocarbon by an alkoxy or aryloxy group forms ethers.
Alcohols and phenols consist of two parts, an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are primarily due to the hydroxyl group. The nature of alkyl and aryl groups simply modifies these properties. Alcohols react both as nucleophiles and electrophiles whereas in phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions.
Few compounds are given below (A to D). Arrange them in the increasing order of their boiling points.
(A) Pentan-1-ol
(B) n-Butane
(C) Pentanal
(D) Ethoxyethane
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) < (D) < (C) < (A)
The boiling point of a compound depends on the strength of the forces between its molecules. For molecules of comparable size, the forces become stronger in this order:
Alkanes: non-polar, held only by weak van der Waals forces.
Ethers: the C–O–C group is only weakly polar, so the dipole-dipole attraction is weak, and ethers cannot form hydrogen bonds among themselves.
Aldehydes and ketones: the C=O group is strongly polar, giving stronger dipole-dipole attraction, but still no hydrogen bonding.
Alcohols: the O–H group forms intermolecular hydrogen bonds, the strongest attraction of the four.
So the boiling points increase as:
alkane < ether < aldehyde < alcohol
\( \ce{CH3CH2CH2CH3} \) < \( \ce{C2H5OC2H5} \) < \( \ce{CH3(CH2)3CHO} \) < \( \ce{CH3(CH2)4OH} \)
Matching the given compounds:
(B) n-butane: alkane, b.p. about 273 K
(D) ethoxyethane: ether, b.p. 308 K
(C) pentanal: aldehyde, b.p. 376 K
(A) Pentan-1-ol: alcohol, b.p. 411 K
Shortcut: n-Butane (B) has the lowest boiling point and the alcohol (A) the highest. Only Option (3) starts with (B) and ends with (A).
NCERT (2026-27 edition):
Physical Properties – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 29-05 S1 26-05 S2 | 21-05 S1 19-05 S2 | 15-05 S1 | 05-06 S1 22-05 S3 21-05 S3 | – |