6.2.5 (a) Practical skills

(a) the techniques and procedures used for the preparation and purification of organic solids involving use of a range of techniques (see also 4.2.3 a) including:

(i) organic preparation

• use of Quickfit apparatus

• distillation and heating under reflux

(ii) purification of an organic solid

• filtration under reduced pressure

• recrystallisation

• measurement of melting points

Although you'll need to practise these skills to pass the PAG questions on the written papers can also require your knowledge of the above.

Quickfit is simply equipment made with ground glass connections to avoid having to use cork or rubber connectors.

What if one of the reactants evaporates when heated?

Under reflux it should condense and fall back into the flask and have another chance to react.

What if one of the products evaporates when heated?

Under reflux it should condense and fall back into the flask.

Again, the water must go in at the bottom so that the cooling is as efficient as possible.

Attaching the apparatus to a running tap (or a hand-pump) pulls air out of the flask, which pulls liquid through the funnel.

This is much faster than simple filtration and helps dry the crystals a little by sucking air through them.

Even after filtration, there is likely to be so contamination of your product.

To remove this we gently warm a solvent in which our desired product is not very soluble.

We add the warm solvent to our impure crystals, stirring constantly until it just dissolves - the object being to dissolve in the least possible solvent.

The solution is now allowed to cool until crystals appear.

These crystals are then removed by filtering under reduced pressure.

The theory is that, although some of our desired product remains in the solution so do the impurities.

This works best if the impurities are a little more soluble than the desired product.

The problem with the method is that a lot of desired product is lost.

So, this method is good for purification but bad for yield.

A pure product will melt over a very narrow range of temperatures (a few degrees at most).

An impure product melts over a bigger range of temperatures because one part of the sample is chemically different from another.

The bigger the range the more impure.

So Melting Point Determination is a way of estimating purity of an unknown solid.

It is also a good way of checking if a "known" solid is what we suspect it to be.

However, checking the melting point against the book-value is of no use for new substances which (obviously) have no previously measured melting point.

The first thing we would need to do is to seal one end of a capillary, then tap the open end onto our sample until we have a decent size of sample.

We then tap on the closed end to drop the sample down to the sealed end.

THe sample could then be slowly heated in a water bath and watched to see the temperature fo the water as the crystals melt.

But placing the sample in an electronic melting point apparatus is more convenient and more likely to give you an accurate temperature.

Common sense suggests doing the melting point once quickly to establish an approximate value before repeating slowly.

The second time we could heat slowly to nearly 10^o of the value we're expecting and then turning the heater down to raise the temperature slowly so that we get a better value.