6.2.5 (a) Practical skills
Syllabus
(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
What does this mean?
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.
Distillation
You'd be using Quickfit kit and would likely have a thermometer at the top of the flask but, other than those differences, this is how to set up a distillation.
Not that the cold water goes in at the bottom of the condenser so that we don't get an air-gap at the top of the condenser.
Providing that the impure liquid contains a solid solute or a liquid with a significantly different boiling point then pure(r) liquid will be obtained.
Make sure that you can sketch this, and don't forget to label water in at the bottom.
Make sure the condensor is closed at the top the way it looks on the left, preferably with a thermometer.
Heating under reflux
A pear-shaped flask is used, although any flask would work.
The anti-bumping granules are glass beads that simply allow the boil to happen more smoothly.
This time the condenser is vertical because we don't want to separate the substances.
We want them to react but the reaction needs heating to have a good rate.
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.
Filtering under reduced pressure
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.
A Buchner funnel is generally used and filter paper is used to cover the holes.
before adding your mixture- this makes sure the paper is pulled into the holes so that fewer crystals are lost.
Hirsch funnels are essentially the same thing useful for recovering smaller amounts of crystals
To maximise yield, you should wash your reaction vessel into the funnel to avoid leaving crystals behind.
To maximise purity, you should wash your crystals on the filter paper with plenty of solvent - this removes soluble impurities.
Recrystalisation
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.
Melting Point Determination
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 10o of the value we're expecting and then turning the heater down to raise the temperature slowly so that we get a better value.
Separating and drying
If your synthesis produces a mixture of an organic substance and an aqueous solution that don't mix...
Then usually the organic phase floats on the water - but not always.
If the question gives their densities then make sure you put the densest layer on the bottom.
Leaving the mixture in a separating funnel for a while allows the two phases to separate.
Slowly run out most of the heavy phase.
Throw away the liquid on either side of the interface.
If your product is the organic phase then you may think that it now contains no water.
But that's very unlikely.
So add a drying agent (like Calcium Chloride crystals etc) and leave for a few hours.
The crystals should absorb any water and can be filtered off, leaving the organic product dry.
