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.

1. Pear-shaped flask (round-bottom flasks also used)
2. Still-head
3. (Liebig) Condenser
4. (Screwcap) Adapter
5. Receiver Adapter
6. Inlet tube (no one really needs to know that)
7. Dropping Funnel
8. Stopper
9. 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 100^oC 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