Intermolecular forces: Explain Hydrogen bonding


HYdrogen bond is a force exerted between between an H atom bonded to an F,O,N atom in one molecule and an unshared pair on the F,O or N atom of a neighboring molecule:

         : X ---- H....:X - H                    X=N,O or F

                     ^  Hydrogen bond

Hydrogen bonding can also be defined as a weak to moderate attractive force that exist between a hydrogen atom covalently bonded  to a very electronegative atom, X and a lone pair of electrons on another small, electronegative atom Y, represented in the below strctral drawing by a series of dots.

   ---X---- H .....Y ---

Usually, hydrogen bonding is seen in case where X and Y are the atoms F,O or N. 

There are two reasons why hydrogen bonds are stronger than ordinary dipole forces:

1. The difference in electronegativity between hydrogen and flourine,oxygen or nitrogen is quite large.It causes the bonding electrons in molecules such as HF,H2O and NH3 to be primarily associated with the more electronegative atom.So,hydrogen atom, insofar as its interection with a neighbouring molecule is concerned,behaves almost like a bare proton.

2. The small size of hydrogen atom allows the unshared pair of an F,O, or N atom of one molecule to approach the H atom in another very closely.It is significant that hydrogen bonding occurs only with three nonmetals ,all of which have small atomic radii.

Consider the boiling points for the hydrides of the group VI A elements.There is an increase in boiling points for H2S,H2Se and H2Te which is consistent with the fact that london forces were the only forces present in these molecule but you find that H2O has much higher boiling point then rest of the hydrides of group VI A which supports the view that hydrogen bonding exists in H2O but is essentially absent in H2S,H2Se and H2Te.

Lets see a solved example:

Sample problem 1 : Would you expect to find the hydrogen bonds in (a) acetic acid (b) diethyl ether (c) hydrazine?


Step 1: For hydrogen bonding  to occur,hydrogen must be bonded to F,O or N.

Step 2: The best way to figure out if hydrogen is bonded to any one of these atom, draw the lewis strucutre.

(a) There should be hydrogen bond in acetic acid because an H atom is bonded to oxygen.

(b) All hydrogen atoms are bonded to carbon in diethyl ether,so there should be no hydrogen bonds.

(c) In hydrazine hydrogen is bonded to nitrogen,hydrogen bonding can occur between neighbouring N2H4 molecules.The boiling point of  hydrazine is much higher than that of molecular O2,which has the same molar mass.