JEE Main 2025 · 23 January · evening · Single correct · +4 / −1
Given below are two statements :
Statement (I) : The boiling points of alcohols and phenols increase with increase in the number of C -atoms.
Statement (II) : The boiling points of alcohols and phenols are higher in comparison to other class of compounds such as ethers, haloalkanes.
In the light of the above statements, choose the correct answer from the options given below :
- (a) Statement I is false but Statement II is true
- (b) Both Statement I and Statement II are true
- (c) Statement I is true but Statement II is false
- (d) Both Statement I and Statement II are false
Answer and solution
Answer (b) Both Statement I and Statement II are true
Solution
Chemistry ma’am · 3 steps
Judge each statement separately: size drives Statement I, hydrogen bonding drives Statement II.
- Statement I. Adding carbon atoms makes the molecule bigger, with more surface area and more electrons. Van der Waals forces get stronger, so the boiling point rises: ethanol boils at 351 K, propan-1-ol at 370 K. Statement I is true.
- Statement II. Alcohols and phenols have an O-H group, so their molecules hold each other by intermolecular hydrogen bonds. Ethers and haloalkanes have no H on oxygen or a halogen, so they cannot hydrogen bond with themselves.
Compound Molar mass (g mol⁻¹) Boiling point Main attraction CH₃CH₂CH₂OH 60 370 K hydrogen bonding CH₃OCH₂CH₃ 60 281 K dipole–dipole CH₃CH₂CH₂Cl 78.5 320 K dipole–dipole - So. Even against a heavier haloalkane, the alcohol boils higher, so Statement II is true. Both statements are true, which is option (b).
The idea to keep. Within one family, boiling point rises with size. Across families of similar mass, hydrogen bonding through O-H wins over plain dipole–dipole forces.
Answer from the NTA final answer key. Solution by PrepWiser.