Question

For polar molecules, when does hydrogen bonding occur?

Answer 1

Well, for a start, the polar molecule BY SPECIFICATION, must contain hydrogen atoms bound to a electronegative element......to engage in hydrogen bonding.....

Explanation:

And three molecules may be used as exemplars: `H_2O`; `NH_3`; and `HF`....and a special type of bond polarity, of charge separation operates......

For each molecule we could represent the dipoles, the charge separation this way: `stackrel(delta+)H-stackrel(delta-)O-stackrel(delta+)H`; `stackrel(delta-)Nstackrel(delta+)H_3`; `stackrel(delta+)H-stackrel(delta-)F`. In the bulk solvent, the dipoles line up in solution, and in aggregate this constitutes a potent intermolecular force.

If you interrogate the boiling points of these molecules (and you should, because as a physical scientist always must consider the actual data), certainly you will find the boiling points anomalously high. Certainly they are high compared to homologous, `H_2S`, `PH_3`, and `HCl` for which hydrogen bonding does not operate so strongly, and dispersion forces (as might be expected for larger, many electron, molecules) are not large enuff to compensate.