Number of molecules from the following which can exhibit hydrogen bonding is _________. (nearest integer)

Solution

Chemistry ma’am · 3 steps

Look for H sitting directly on N, O or F; then remember that o-nitrophenol bonds with itself.

1. The condition. A hydrogen bond needs an H attached directly to a small, very electronegative atom (N, O or F). It also needs a lone pair on another N, O or F to attract it. H bonded to carbon does not count.
2. Two kinds. The bond can form between two molecules (intermolecular) or between two groups of the same molecule (intramolecular). Both count as hydrogen bonding.

   Molecule	H on N, O or F?	Hydrogen bonding
   CH₃OH	yes, O–H	intermolecular
   H₂O	yes, O–H	intermolecular
   C₂H₆	no, only C–H	none
   C₆H₆	no, only C–H	none
   o-nitrophenol	yes, O–H next to NO₂	intramolecular, O–H to an O of NO₂
   HF	yes, F–H	intermolecular
   NH₃	yes, N–H	intermolecular

3. So. Only C₂H₆ and C₆H₆ fail, because all their hydrogens are on carbon. That leaves 7 - 2 = 5 molecules that show hydrogen bonding.

The common slip: 4. Counting only intermolecular hydrogen bonds leaves out o-nitrophenol and gives 4. Its intramolecular O–H···O bond is still a hydrogen bond, so the count is 5.

The idea to keep. Hydrogen bonding needs H on N, O or F, never H on C. When the donor and acceptor groups sit side by side in one molecule, as in o-nitrophenol, the bond forms inside the molecule and still counts.

Answer from the NTA final answer key. Solution by PrepWiser.