Question

2. Write ALL the types of intermolecular attractions that can occur between the molecules listed below. You may assume that there are “many” molecules of each type present in solution. Use molecular dipole moments to justify your answers.

a) CH2Cl2 and CH2Cl2

b) CH2Cl2 and CH4

c) CCl4 and CCl4

Answer 1

There are three main types of intermolecular forces of attraction

(i) van der Waals forces: These interactions occur between all molecules at all times

(ii) Dipole-dipole forces: Shown by molecules with dipole moments

(iii) hydrogen bonding: The molecule must have hydrogen atom bonded to a very electronegative atom auch as O, N, F.

All the three molecules given in the question are shown below.

(a)  In dichloromethane (CH₂Cl₂), chlorines are bonded to carbon atom. Chlorine has more electronegativity than carbon. It will polarise the C-Cl bond by pulling the electrons towards the chlorine atoms. In addition, carbon and hydrogen do have a very small electronegativity difference. Due to the difference in electronegativities, chlorine becomes partially negative (? - ) and carbon partially positive (? + ).

As dichloromethane is tetrahedral, the two chlorines are not exactly opposite to each other (as shown in figure above), so the dipoles don’t cancel out each other. So, it has a permanent dipole.  

Two dichloromethane molecules will have dipole dipole interaction in addition to the Vander Wall Forces of attraction.

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(b) CH2Cl2 and CH4

Methane is a non polar molecule as four hydrogen atoms are arranged in a tetrahedron around the carbon atoms. Methane molecules will have Vander Wall forces of interactions. Dichloromethane has a permanent dipole, interactions between two dichloromethane (dipole - dipole) are discussed above.

Since, methane is not a dipole, therefore, Vander Wall forces of interactions will be there in dichloromethane and methane molecules.

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(c) CCl₄ is a non polar molecule with a dipole moment of zero. CCl₄ molecule is tetrahedral, i.e. the angle between any two chlorine atoms is the same throughout the molecule. Even though each chlorine atom has a high electronegativity value than carbon atoms, still the overall polarity of the molecule is zero. This is becuase all dipole moments cancel out each other.

H-bonding is not possible as there are no hydrogens in the molecule.

Only intermolecular forces of interaction which can be operating in this case would be Vander Wall forces of attraction.