Halogenoalkanes (2) Some physical properties

3. What are the Properties of Halogenalkanes?

A: Physical properties

i) Why are halogenoalkanes polar molecules?

The diagram shows the halogen atom (X) is slightly more negative than the carbon atom.

The reason for this is the difference in electronegativity between the halogen and the carbon atom.

This difference occurs in all haloalkanes except the iodo– and astato– versions.

The iodine and astatine atom have electronegativities similar or lower than that of carbon.

ii) How do we know haloalkanes reflect the isotopic abundance of chlorine and other halogens?

The relative abundance of these elements is reflected in the mass spectra of the compounds:

For example:

Chloroethane                                                                      

CH₃CH₂³⁵Cl      :     CH₃CH₂³⁷Cl                            

            3          :           1                                             

Note the relative heights of the peaks at 64 and 66 at 3:1 reflecting the relative abundance of naturally occurring chlorine.

Bromomethane

CH₃⁷⁹Br          :           CH₃⁸¹Br

            1          :           1

The two molecular ion peaks are at 94 and 96 amu of fairly equal abundance because that pattern is true of naturally occurring bromine. 

iii) How do we know haloalkanes have molecular structures?

We see that all haloalkanes have relatively low melting and boiling points which point to their molecular structure. 

These bps are the result of weak intermolecular forces. 

The intermolecular forces here are Van der Waals forces and dipole – dipole interactions.