CB14 Dipole – dipole interactions influence the substances’ properties

Aim: To show that dipole – dipole interactions influence the substances’ properties

 

   Dipole – dipole interactions also influence the properties of a substance. This illustration shows boiling point as a function of the number of substituted halogen atoms for CH4 and CCl4.

CCl4 :
Substitution of Br for Cl in CClxBr(4-x) hardly changes the dipole moment of the molecule. The difference in electronegativity between Cl and Br is small, so the molecule stays more or less apolar. The boiling point increases linearly as Br is substituted; this is to be expected since the molecular mass is also increasing.

CH4 :
Substitution of F for H has a distinct influence on the dipole moment. CH4 and CF4 are apolar whilst CH3F, CH2F2 and CHF3 have large dipole moments. As a result the boiling points do not increase linearly as F is substituted (red dotted line). The dipole – dipole interactions give rise to higher boiling points than would be expected from the molecular masses.