H-bonds continued

Methanol is a good hydrogen bonding solvent. Since there is a H atom attached to an O atom, methanol has both an H-bond donor and a H-bond acceptor

Because of O's electronegativity electron density is drawn toward the O. A dipole forms; the O is the negative end and the H is the positive end.

If H-bonding is a dipole-dipole interaction why is it so special, or why is it so strong?

Hydrogen has no core electrons. Since the valence electrons are being drawn away from the H, and there are no core electrons the partially positive H atom can get much closer to the partially negative O atom than any other partially positve atom could. Every other atom involved in a dipole has core electrons. As the dipoles attract each other the core electrons cause repulsion as the dipoles get closer together.

So we have explained why an H-bond is so strong. The atoms can get very close together; thus r is small making E a large negative number. Also the lack of core electrons makes the hydrogen atom strongly charged/

 

Often a H-bond is indicated by a dotted line.