Preparation of a haloalkane

Haloalkanes can be made by a substitution reaction with an alcohol.

Tertiary alcohols are the most reactive, and therefore the easiest to convert into haloalkanes.

We will react 2-methyl propan-2-ol (tertiary butyl alcohol) with concentrated hydrochloric acid to form 2-methyl, 2-chloro propane.

Part one: making the haloalkane

Measure 20 mL of concentrated hydrochloric acid in a measuring cylinder.

Also measure out 10 mL of 2-methyl propan-2-ol.

Pour both these reagents into a separating funnel.

Make sure the tap is closed before you start filling the funnel!

Make sure you are wearing safety glasses

while doing this:

With your thumb holding the stopper in place, turn the funnel upside down and immediately open the tap to release any gas.

Close the tap again, then turn the funnel right way up again.

Repeat several times to thoroughly mix the solutions.

Continue mixing for 5-10 minutes then leave to separate into two layers.

This is a nucleophilic substitution reaction. Cl– is a nucleophile (species which loves nuclei) because it has a negative charge.

The stopper must be removed before the liquid will flow.

It’s safer to drain the liquid into a beaker – that way if you make a mistake you can pour it back!

Part two: purifying the product

Drain off the lower, aqueous layer.

Add about 10 mL of sodium carbonate solution.

The carbonate reacts with any remaining acid, producing bubbles of carbon dioxide.

We’ll know the acid has been removed when it stops bubbling.

Mix well, remembering to release the pressure frequently, until the reaction has stopped.

Drain off the lower, aqueous layer.

Remember to remove the stopper first.

Transfer the organic layer to a test tube.

Add a little anhydrous sodium sulfate.

Stopper and shake well.

Anhydrous sodium sulfate is a drying agent. It combines with any remaining water in the liquid.

Part three: distillation

The purification steps already done have removed the water and acid from the organic material, but they do not separate one organic substance from another.

It is likely that there is unreacted alcohol still present, dissolved in the haloalkane. Organic reactions are usually equilibrium reactions. They seldom go to completion.

2-methyl propan-2-ol has a boiling point of 82.6 °C, while 2-chloro 2-methyl propane boils at 50.7 °C. We can separate these two substances by distillation.

Transfer the haloalkane to a distillation flask and collect the fraction which boils between 50 °C and 53°C.

Thermometer

Purified haloalkane

Sample

Heating mantle

Water in

Water out

This fraction has a boiling point within the required range.

In this preparation students did parts one and two in pairs, but then combined all their products for the final distillation phase.

If each pair of students had done their own distillation, they could have weighed the haloalkane finally collected and calculated their percentage yield.

Points to note:

• Tertiary alcohols are the most easily substituted

• Excess acid is removed with sodium carbonate solution

• Anhydrous (without water) sodium sulfate is used as a drying agent, to remove water from an organic solution.

• Organic reactions seldom go to completion: separate organic mixtures by distillation and collect that fraction which boils at the required temperature.