Module 2

Lesson 1.2 Properties of Halogenated Hydrocarbons 



Key Concepts


The physical properties of the halogenated hydrocarbons differ considerably from those of corresponding alkanes. The presence of the halogen functional group is responsible for the unique physical and chemical properties of this family.

  1. Physical Properties of Halogenated Hydrocarbons

    1. Boiling point


      Halogenated hydrocarbons have higher boiling points than alkanes with the same number of carbons. Why is this?


      Consider the following molecules:


      		  Methane       CFC Molecule

      Chemical Formula
                     		 CH4  CCl2F2

      Boiling Point (°C)

             		 
      -161


       -29.8


      Methane is a non-polar molecule and, as such, exhibits only London Dispersion forces. In contrast, dichlorodifluoromethane is a polar molecule, thanks to its electronegative halogen atoms. Because it is a polar molecule, dichlorodifluoromethane possesses the relatively strong "dipole-dipole" forces. In addition, because it has more electrons, dichlorodifluoromethane demonstrates stronger London forces than methane. Remember that the stronger the intermolecular forces, the higher the boiling point!

    2. Solubility


      Because they are more polar, halogenated hydrocarbons are more soluble in water than are alkanes. However, generally speaking, their solubility in water is relatively low due to the fact that halogenated hydrocarbons are unable to form hydrogen bonds with water molecules. However, halogenated hydrocarbons tend to have good solubility in non-polar solvents such as hexane.

  2. Chemical Properties of Halogenated Hydrocarbons
    The presence of the halogen atom determines the chemical properties of halogenated hydrocarbons. What reactions involve halogenated hydrocarbons? You will explore this question in Lesson 2.

Check Your Understanding


Complete Practice Question 3 on page 419 and Practice Question 10 on page 423 of the textbook.

Check your work by clicking on the link below.

Page 419 Practice Question 3
  1. Chloroethene has a higher boiling point than ethene because it has stronger   intermolecular forces. Firstly, because it is polar, it exhibits dipole-dipole forces. Secondly, it has more electrons, therefore its London forces are stronger.
  2. Chloroethene likely is more soluble than ethene because it is polar (recall that water is a polar solvent).

Page 423 Practice Question 10

Both molecules have the same number of electrons and both exhibit dipole-dipole forces because of their polarity. In this case, molecular shape is the determining factor. The 2-bromopropane molecule is more compact and does not make as much surface contact with neighbouring molecules compared to the more "stretched out" 1-bromopropane molecules.