There are number of ways to make a halogenoalkane, each of which has its own advantages and disadvantages.
- Reaction of an alkane with Cl2 or Br2 in the presence of uv light (radical substitution reaction). This is a fairly haphazard way of preparing a halogenoalkane as we always end up with a mixture of products that are difficult to separate. You can find out all about radical substitution reactions here.
- Reaction between an alkene and a hydrogen halide – this is an example of electrophilic addition.
- Halogenation of an alcohol 👇.
Halogenation of an alcohol
In order to persuade an OH group to ‘leave’ we need to activate it as hydroxide ions, OH–, make very poor leaving groups (the OH group is not keen on accepting another pair of electrons).
However, once activated, the alcohol will readily react with a halide ion to make a halogenoalkane via a nucleophilic substitution reaction.
In the reaction between an alcohol and a hydrogen halide such as HCl or HBr, the OH group is activated by protonation under acidic conditions.
We could carry out this reaction by heating the alcohol under reflux with sulphuric acid and a sodium halide, in which case the hydrogen halide, HX, is produced in situ (this is a little easier to manage practically than bubbling a gaseous hydrogen halide through the alcohol).
NaBr(s) + H2SO4(aq) ⇾ NaHSO4(aq) + HBr(aq)
Alternatively, we could react our alcohol with either phosphorus trichloride, PCl3, or phosphorus tribromide, PBr3.
If we are preparing a choroalkane, we also have the option of reacting the alcohol with thionyl chloride, SOCl2, in the presence of a base. The base needs to be strong enough to deprotonate the intermediate but not so strong as it would deprotonate the alcohol in step 1.
