How to prepare a halogenoalkane

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There are number of ways to make a halogenoalkane, each of which has its own advantages and disadvantages.

  1. 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.
  2. Reaction between an alkene and a hydrogen halide – this is an example of electrophilic addition.
  3. 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 warm our alcohol with either phosphorus trichloride, PCl3, phosphorus tribromide, PBr3, or phosphorus triiodide, PI3 (PBr3 and PI3 can be made in situ using red phosphorus and either bromine or iodine).

If we are preparing a choroalkane, we also have a couple of other options …

  • Reacting the alcohol with thionyl chloride / sulfur dichloride oxide, 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.

The equation for the overall reaction would be:

CH3OH + SOCl2 ⇾ CH3Cl + SO2 + HCl

Both the by-products, sulfur dioxide and hydrogen chloride, are gases which means we do not need to distill off the halogenoalkane product as the gases will escape the reaction mixture.

  • Reacting the alcohol with solid phosphorus(V) chloride, PCl5, at room temperature.

CH3OH + PCl5 ⇾ CH3Cl + HCl + POCl3

The acidic HCl gas that is released from this reaction is observed as ‘steamy fumes’ and so this reaction can be used as a test for the hydroxyl group, OH, in alcohols.

Practice questions

  1. 3,4-dimethylcyclohexanol is refluxed with sodium bromide and sulfuric acid. Draw and name the product of the reaction.
  2. Tertiary alcohols undergo nucleophilic substitution when reacted with concentrated hydrochloric acid. A student wants to prepare a chloroalkane starting with a primary alcohol but the reaction is very slow. Suggest an alternative reagent.
  3. Chloroethane can be prepared in a reaction between ethanol and dry hydrogen chloride gas. The dry hydrogen chloride gas is produced in situ when sodium chloride reacts with concentrated sulphuric acid.

(a) Explain what ‘in situ’ means.

(b) Write an equation for the reaction of sodium chloride and concentrated sulphuric acid.

Answers

  1. 1-bromo-3,4-dimethylcyclohexane

2. PCl5, PCl3, SOCl2

3. (a) ‘in situ’ means that the HCl is generated in the reaction vessel during the reaction, rather than being produced in a preliminary step and then added to the ethanol

(b) NaCl + H2SO4 ⇾ NaHSO4 + HCl