Halogenoalkanes are very important in industrial chemical processes. Though halogens themselves are very reactive, many halogenoalkanes are relatively unreactive under normal conditions and are therefore used in flame retardants and anaesthetics.
Formation of halogenoalkanes
Alkanes do not react with halogens in the dark at room temperature. However, they will react with halogens in ultraviolet light to form halogenoalkanes.
For example, a mixture of hexane and a little liquid bromine in a test tube will stay red-brown (the colour of bromine) in the dark. However, if you shine ultraviolet light onto the mixture, it becomes colourless (bromohexane) and misty fumes of hydrogen bromide appear.
The main reaction here is:
hexane + bromine → bromohexane + hydrogen bromide
C6H14(g) + Br2(l) → C6H13Br(l) + HBr(g)
Bromohexane is a halogenoalkane.
When naming halogenoalkanes:
- prefixes fluoro-, chloro-, bromo-, and iodo- are used tell us which halogen is present.
- numbers are used, if needed, to show on which carbon the halogen is bonded.
- prefixes di-, tri-, tetra-, and so on, are used to show how many atoms of each halogen are present.
- different halogens in a compound are listed in alphabetical order, not in order of the number of the carbon atom to which they are bonded.
For example, 2-bromo-2-methylpropane means there is a :
- bromo functional group on carbon atom number 2
- and a methyl functional group attached to carbon atom number 2 of the propane chain, as follows:
Physical properties of halogenoalkanes
Solubility of halogenoalkanes
The Cδ+-Xδ- bonds are not polar enough to make the halogenoalkanes soluble in water. In halogenoalkanes the main intermolecular forces of attraction are dipole-dipole attractions and van der Waal’s forces. This makes halogenoalkanes soluble in hydrocarbons and are therefore used as dry-cleaning fluids and to remove oily stains.
Just like with alkanes and alkenes:
- the boiling points of halogenoalkanes increase with the length of the carbon chain length.
- increased branching of the carbon chain reduces their melting points.
Halogenoalkanes have higher boiling points than alkanes with similar chain lengths because they:
- have higher relative molecular masses.
- are more polar.
Boiling points also increases down the halogen group, and this is due to increased van der Waals forces because the larger the molecules, the greater the number of electrons, and therefore the larger the van der Waals forces.
Bond polarity of halogenoalkanes
Halogenoalkanes have a C-X bond which is polar, Cδ+-Xδ-, because halogens are more electronegative than carbon. This means that the carbon bonded to the halogen has a partial positive charge (electron deficient). The carbon can therefore be attacked by nucleophiles (reagents that are electron rich or have electron-rich areas). A nucleophile is an electron pair donor.
Using the polarities of the C-X bonds you can predict that the C-F bond is the most reactive, because it is the most polar.
The table below shows the electronegativities of carbon and the halogens.
Halogens become less and less electronegative as you go down the group and the bonds become less polar.