Transcript Preparation of a haloalkane

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, 2chloro 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.
Part two: purifying the product
Drain off the lower, aqueous layer.
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!
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.
Purified
haloalkane
Thermometer
Water out
Water in
Sample
Heating mantle
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.