Are These Intermolecular Interactions Hydrogen Bond?
We have 2 criteria for the interaction between 2 molecules to be considered hydrogen bond.
1. Hydrogen is bonded to electronegative fluorine, oxygen or nitrogen
2. Electronegative fluorine, oxygen or nitrogen is partial negative charge
Some students might find criteria 2 redundant, since F/O/N must be partial negative charge if it is bonded to hydrogen as mentioned in criteria 1.
This is true for interaction between pure species such as HF to HF, H2O to H2O or NH3 to NH3.
However in a mixture when 2 different species interact with each other, we can appreciate criteria 2 better.
Let's consider the following 5 examples.
A. CH4 and H2O
Criteria 1 is not met since hydrogen is bonded to non-electronegative carbon. C-H bond is considered non-polar.
Criteria 2 is met since oxygen is bonded to hydrogen hence oxygen is partial negative.
Therefore this interaction is not hydrogen bond.
B. HF and NH3
Criteria 1 is met since H is bonded to F.
Criteria 2 is met since N is bonded to H hence N is partial negative.
This interaction is hydrogen bond.
C. H2O and COH2
Criteria 1 is met since H is bonded to O.
Criteria 2 is met since O is bonded to less electronegative C hence O is partial negative.
Notice it's not a requirement for O to be bonded to H, it just needs to be bonded to a less electronegative element in order to be partial negative.
Hence this interaction is hydrogen bond.
D. NH3 and N2
Criteria 1 is met since H is bonded to N.
Criteria 2 is not met since N is bonded to same element hence the N-N triple bond is nonpolar.
N in N2 will not attract H in NH3.
Therefore this interaction is not hydrogen bond.
E. H2O and OF2
Criteria 1 is met since H is bonded to O.
Criteria 2 is not met since oxygen is bonded to more electronegative F hence O is partial positive and will not attract H in H2O.
Therefore this interaction is not hydrogen bond.
Topic: Intermolecular forces, Physical Chemistry, A Level Chemistry, Singapore
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