Permanent dipole-dipole attractions

Permanent dipole-dipole attractions

1. Polar molecules have attractions between temporary dipoles as well as a permanent dipole-dipole attraction arising from the dipole on one polar molecule interacting with the dipole of another

2. A polar molecule will have a higher melting and boiling point than a non-polar molecule of similar molar mass

   • For example Br2 and ICl have similar molar masses of 160 g mol-1 and 162.5 g mol-1, respectively, and therefore similar-sized electron clouds, but the boiling point of the polar ICl molecule is 34oC higher than the non-polar Br2 molecule

3. The strength of permanent dipole-dipole interactions depends on the polarity of the covalent bonds as well as the shape of the molecule

4. The more polar the molecule, the stronger the permanent dipole-dipole attractions and the higher the melting and boiling points

5. Thus ethanal, CH3CHO, has a higher boiling point than ethane, CH3CH3, because, although they are similar in molar mass, ethanal is more polar than ethane because of the presence of the polar C=O bond