Solvents can be categorized as polar and protic. I understand that to be protic there had to be a significantly polar bond involving a hydrogen atom. However, how is the a hydrogen bond different in any way to another dipole-dipole interaction and therefore, what's the need for the distinction between polar and protic solvents?

When considering the hydration enthalpy $\triangle_{hyd} G^{\theta}$ it is not sufficient to simply use the Born equation: $$\triangle_{solv} G^{\theta}=-\frac{N_Ae^2z^2}{8 \pi \epsilon_0}(1-\frac{1}{\epsilon_r})$$ BECAUSE this only considers Coulombic interactions (i.e hydrogen bonds are not solely coulombic interactions). My textbook seems to suggest that hydrogen bonds are somehow different and more significant. My question is why? Surely there should be no distinction between different polar bonds; what makes $\alpha^{\delta-}-H^{\delta +}$ special?


Hydrogen bonds often have a significant covalent component. This can be seen in many aspects of the hydrogen bond: it is directional, unlike Coulombic interactions, and the bond length is generally significantly shorter than predicted by Coulombic models. There is great variation in the covalent character of hydrogen bonds depending on their environment, with e.g. bond lengths between 2.2 and 4.0 Angstroms known.

Other intermolecular interactions have significant covalent components; there is nothing special about hydrogen per se but it is the most important example.


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