Why does hydrogen bonding occur?

1 Answer
anor277
Dec 28, 2017

Because of bond polarity.....

Explanation:

When hydrogen is bound to a strongly electronegative atom, i.e. fluorine, or nitrogen, or water, the heteroatom polarizes electron density towards itself to give charge separation, and a resultant molecular dipole....i.e.

#stackrel(""^+delta)H-stackrel(delta^-)O-stackrel(delta^+)H#...and this special instance of dipole-dipole interaction, which occurs in the bulk solvent BETWEEN individual water molecules, constitutes a potent intermolecular force.

#stackrel(""^+delta)H-stackrel(delta^-)O-stackrel(delta^+)Hcdotsstackrel(""^(-)delta)OH_2cdotsOH_2#

And the best metric of this is of course the normal boiling point, which for water is (dohh!) #100# #""^@C#. Given the size of the water molecule, this boiling point is absurdly high. Compare the boiling points of #H_2S#, #-60# #""^@C#; #H_2Se#, #-41.3# #""^@C#; #H_2Te#, #-2.2# #""^@C#.

In the lower hydrides, the extent of hydrogen bonding is not so great in that the element hydrogen bonds are not so polar. And thus the unusual order of boiling points.

We could draw similar inferences from the boiling point of the hydrogen halides, and the Group 15 hydrides....#HF#, #"boiling point"=19.5# #""^@C#, and #NH_3#, #"boiling point"=-33.3# #""^@C#; these are elevated with respect to lower Group hydrides, #HCl#, and #PH_3#...both room temperature gases.

Note that this type of dipole-dipole interaction is usually treated under a separate umbrella, i.e. #"intermolecular hydrogen bonding."#

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