For polar molecules, when does hydrogen bonding occur?

1 Answer
Sep 5, 2017

Answer:

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.