JEE Main 2022 · 27 June · evening · Single correct · +4 / −1
The correct order of increasing intermolecular hydrogen bond strength is :
- (a) HCN < H₂O < NH₃
- (b) HCN < CH₄ < NH₃
- (c) CH₄ < HCN < NH₃
- (d) CH₄ < NH₃ < HCN
Answer and solution
Answer (c) CH₄ < HCN < NH₃
Solution
Chemistry ma’am · 3 steps
Ask two things of each molecule: is its H polar enough to give, and is there a lone pair to accept?
- Hydrogen bond needs. A hydrogen bond needs a donor: an H attached to a strongly electronegative atom. It also needs an acceptor: a lone pair on an electronegative atom of the next molecule. The more polar the X–H bond, the stronger the hydrogen bond.
Molecule Donor H Acceptor lone pair Hydrogen bonding CH₄ C–H on an sp³ carbon, almost non-polar none none HCN C–H, but the sp carbon (50% s character) is fairly electronegative one on N weak NH₃ N–H, highly polar one on N strong H₂O O–H, the most polar of these two on O stronger than NH₃ - Why HCN beats CH4. The sp carbon in HCN pulls electron density towards itself. This makes its H slightly acidic, so it forms weak C–H···N hydrogen bonds. CH₄ has neither a polar C–H bond nor a lone pair.
- So. The increasing order is CH₄ < HCN < NH₃, which is option (c).
The idea to keep. Hydrogen bond strength follows the polarity of the X–H bond. An sp C–H can give a weak hydrogen bond. An sp³ C–H, as in CH₄, gives none at all.
Answer from the NTA final answer key. Solution by PrepWiser.