Required practical 10 - Prepare a pure organic solid and test its purity

Why do we dissolve the impure compound in a minimum volume of hot solvent?

An appropriate solvent is one which will dissolve both compound and impurities when hot and one in which the compound itself does not dissolve well when cold. The minimum volume is used to obtain saturated solution and to enable crystallisation on cooling

Why do we hot filter the solution through the filter paper quickly?

This step will remove any insoluble impurities and heat will prevent crystals reforming during filtration

Why do we cool the filtered solution by inserting the beaker in ice?

Crystals will reform but soluble impurities will remain in solution form because they are present in small quantities so solution is not saturated. Ice will increase the yield of crystals

Why do we use a buchner flask when suctioning the filtrate with a buchner flask and funnel to seperate out crystals?

The water pump connected to the Buchner flask reduces the pressure and speeds up the filtration.

Why do we wash the crystals with distilled water?

To remove soluble impurities

What is the last step?

Dry the crystals between absorbent paper

How can yield be lost during this process?

• Crystals lost when filtering or washing

• Some product stays in solution

after

recrystallisation

• other side reactions occurring

How could the percentage yield be greater than 100?

If the crystals are not dried properly the mass will be larger than expected which can lead to a percentage yield >100%

What two reactants react together to produce asprin? What happens to the hydroxybenzoic acid?

2-hydrocybenzoic acid and ethanoic anhydride. Aspirin is made from 2-hydroxybenzoic acid which contains a phenol group. In the reaction the phenol group is turned into an ester by reacting it with the reactive ethanoic anhydride

Why do we use ethanoic anhydride instead of acid chloride?

Ethanoic anhydride is used instead of acid chlorides because it is cheaper, less corrosive, less vulnerable to hydrolysis, and less dangerous to use.

What do you add to 2-hydroxybenzoic acid and ethanoic anhydride?

Phosphoric acid

What condition does this practical need to be under?

Reflux, use a reflux condenser and heat the mixture in a boiling water bath.

What happens after the vigorous reaction has ended?

Pour the mixture into 40 cc of cold water, stir and rub the sides of the beaker with a stirring rod to induce crystallisation. Allow the mixture to stand in an ice bath to complete crystalisation. Collect the product by suction filtration.

What stage happens after this?

Recrystalisation.

Describe recrystalisation

Measure out 15 cc of ethanol, prepare a beaker half filled with hot water. Add a spatula of crude aspril to the boiling tube with ethanol and place the test tube in the beaker. Stir the contents until the asprin dissolves in the ethanol. Pour the hot solution through a warmed filter funnel and fluted filter paper. Then pour the filtrate into 40 cc of water in a conical flask and allow the conical flask to cool slowly, in an ice bath. Filter off the purified solif under reduced pressure and allow it to dry. Record the mass of the dry purified solid.

Why do you use a warmed filter funnel and fluted filter paper?

Remove any insoluble impirites and heat will prevent crystals reforming during filtration.

why is the solution not saturated?

Soluble impurities will remain in solution form because they are present in small quantities so solution is not saturated. Ice will increase the yield of crystals