PERMANENT DIPOLE – DIPOLE FORCES, HYDROGEN BONDING, Conditions for hydrogen bonding, Chemistry of Hydrogen bonding

 

(2)      PERMANENT DIPOLE – DIPOLE FORCES:

 

            Permanent dipole – dipole forces are the intermolecular forces that exist in polar molecules which attract each other electrostatic-ally.

             The negative end of one molecule attracts the positive end of the other molecule. Permanent dipole – dipole forces exist in polar solid and polar liquid substances. Crystal lattice due to solid polar molecules having high melting point than non-polar solid substance because extra energy is required to break and overcome the attractive forces due to permanent dipole – dipole interaction. e.g;

        

            In polar liquids, they have high boiling point than non-polar liquids because more energy is required to overcome dipole – dipole forces. e.g;

 

            Silance (SiH4)                        Water (H₂O)

            (Non-polar) <                                    (Polar)

   b.p    -111⁰C                                                100⁰C

 

            In general, the polar molecules which have dipole – dipole forces have high melting points,  boiling points and enthalpy of vaporization. Due to the same forces polar solutes are soluble in polar solvents because energy required to break the polar crystal is compensated by the energy released when polar solute molecules are attracted by polar solvent molecules.

 (3)      HYDROGEN BONDING:

           

            Weak electrostatic forces of attraction between poorly shielded proton of the hydrogen atom and the highly electronegative and small atom of neighboring molecule is called hydrogen bonding.

 

            Conditions for hydrogen bonding: The following conditions are required for the formation of hydrogen bonds.

 

(a)       Hydrogen atom must be bonded to a highly electronegative atom like fluorine, oxygen and nitrogen i.e; F > O > N.

 

(b)       Hydrogen bonding does not occur with larger atoms such as chlorine and sulpher even though the electronegative of these elements are comparable to that of nitrogen. This is because the large size electronegative atom of one molecule can not approach hydrogen atom of mother molecule very closely.

 

(c)       The electronegative atom must have lone pair of electrons.

 

Chemistry of Hydrogen bonding:

 

            A hydrogen atom is unique because it has no inner shielding in bonded form.  When hydrogen atom is bonded to one of the highly electronegative and small size atom like, fluorine, oxygen or nitrogen, the shared electron cloud is greatly distorted towards the electronegative atom. The single proton in the hydrogen  nucleus is thinly shielded by the electron cloud.