It is important to realize that hydrogen bonding exists in addition to van der Waals attractions. The hydrogen bonding in the ethanol has lifted its boiling point about 100☌. Methoxymethane (without hydrogen bonding) The boiling points of ethanol and methoxymethane show the dramatic effect that the hydrogen bonding has on the stickiness of the ethanol molecules: ethanol (with hydrogen bonding) Except in some rather unusual cases, the hydrogen atom has to be attached directly to the very electronegative element for hydrogen bonding to occur. In methoxymethane, the lone pairs on the oxygen are still there, but the hydrogens are not sufficiently + for hydrogen bonds to form. The hydrogen bonding is limited by the fact that there is only one hydrogen in each ethanol molecule with sufficient + charge. Hydrogen bonds have about a tenth of the strength of an average covalent bond, and are being constantly broken and reformed in liquid water. Hydrogen bonding can occur between ethanol molecules, although not as effectively as in water. However, ethanol has a hydrogen atom attached directly to an oxygen here the oxygen still has two lone pairs like a water molecule. The van der Waals attractions (both dispersion forces and dipole-dipole attractions) in each will be similar.

The ubiquity and flexibility of hydrogen bonds make them the most important physical interaction in systems of biomolecules in aqueous solution.

It is an interaction that is weaker than the covalent bond and stronger than the van der Waals interaction. They have the same number of electrons, and a similar length. The hydrogen bond has justifiably been termed the ‘master key of molecular recognition’.