4.2.3 (a) Practical Skills
Syllabus
(a) the techniques and procedures for:
(i) use of Quickfit apparatus including for distillation and heating under reflux
(ii) preparation and purification of an organic liquid including:
• use of a separating funnel to remove an organic layer from an aqueous layer
• drying with an anhydrous salt (e.g. MgSO4, CaCl2)
• re-distillation
What does this mean?
Quickfit equipment is designed to fit together through ground glass joints without the need for the corks and cork-borers of previous generations.
It seems unlikely that anyone will ask you written questions about how to fit it together but you will have to be able to demonstrate these skills for the practical endorsement.
And you may well be asked questions where you have to pick the correct technique for a particular aim.
So knowing the correct terms for some of the pieces of glassware in a Quickfit kit could prove useful.
- Pear-shaped flask (round-bottom flasks also used)
- Still-head
- (Liebig) Condenser
- (Screwcap) Adapter
- Receiver Adapter
- Inlet tube (no one really needs to know that)
- Dropping Funnel
- Stopper
- Termometer
Reflux
The majority of reactions are very slow unless heated.
But most Organic substances are volatile - evaporate easily.
So how can you heat a reaction mixture without evaporating the reactants or products or both?
With a Condenser set-up vertically anything which evaporates is returned to the flask.
This means un-reacted reactants have multiple chances to react.
And products do not escape.
Things to note:
Anti-bumping granules: small glass beads that stops the mixture suddenly jumping up into the condenser. Not strictly necessary but certainly good practice.
Water inlets: cold water in at the bottom, warmed water out at the top. This is the most efficient way to cool the condenser and avoid losing product/reactant.
Open top: you'd be surprised how many students put a bung in the top and are then surprised when it later pops off!
Grease: ideally you should grease the ground glass joints. It helps to make a water-tight seal and (importantly) the equipment will come apart better at the end!
Heat Source:
a) If what you are heating is very flammable then a heating-mantle is the best choice but schools rarely have many.
Heating mantles are electrical and so have no naked flames.
Their thermostats aren't entirely reliable but they are usually easier to control temperatures with than a Bunsen.
b) If the operating temperature is below 100oC then a Bunsen is a very poor choice and a water-bath is generally used.
If the flask is placed in a large beaker on top of an electric heater/stirrer then it is possible to put a magnetic-flea in a round bottom flask and stir the mixture.
Generally, this is not necessary, however, as the action of heating the flask is enough to mix the reactants by convection.
c) In some cases a simple Bunsen and a great deal of care will suffice.
But overheating a reflux can destroy the product even if it doesn't escape.
So it is important to heat with care and not assume that you can simply leave the
set-up unattended.
This will result in a flask full of tarry soot and glassware that is welded together irrevocably
Distillation
Setting-up a distillation involves slightly more equipment.
A still-head is used to connect the flask to the condenser.
The thermometer blocks the top of the still-head by sitting in a screw-cap adapter.
The water-hoses should be connected the same way round even though this means that the distillate
first encounters warmed water.
This is to avoid an air-gap forming at the top of the condenser.
The receiver adapter channels distillate into the collection vessel.
Without a clip it is highly likely to fall off and smash!.
Distillation may be used if there is a danger of a desired product re-reacting under reflux, as when trying to make an Aldehyde by Oxidising a Primary Alcohol.
More usually distillation is used to separate your product from excess reactants and other impurities after a reflux has been completed.
To do this well, its important to consider the boiling points of the desired product and any other likely impurities.
Separating Funnel
Many Organic reactions produce water as a by-product.
And the vast majority of Organic compounds are not water soluble.
So pouring the mixture into a separating funnel and waiting will result in two layers.
The denser liquid will always sink to the bottom - this is almost always the water.
But to make sure you could pour some distilled water in the top and see which layer it joins.
Once enough time has elapsed for the separation to be complete you can open the tap to collect the two different layers in separate flasks.
If percentage yield is less important than purity then you may want to use a third container to collect the small part where the two layers meet.
Drying products.
However careful you are there's a chance that water remains in the Organic product.
Mixing this with an anhydrous salt for a few minutes will allow the water to be absorbed.
The hydrated salt won't dissolve in the Organic product.
Clumps of salt suggest that the crystals are fully hydrated and being held together by unabsorbed water.
So adding more salt and stirring for a few minutes more should result in individual, fully hydrated crystals which can be removed by filtering or by decanting.
Suitable salts for the process are Magnesium Sulphate, or Calcium Chloride (although Calcium Sulphate would be just as good.)
Redistillation
Even after distilling your reaction mixture there is a chance that it is not pure.
This is because many substances have similar boiling points.
If this is likely to be the case then you have two (or more) Organic substances that will mix together well.
There is little choice but to distill again - this time making sure that the distillate you collect comes over at exactly the boiling point of the expected product.
This can be much harder than it sounds!
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