RQP 10: Preparation of a pure organic solid and test of its purity

Question 1

Describe the steps in purifying an organic solid by recrystallisation

Answer 1

1. Dissolve the impure compound in a minimum volume of hot (near boiling) solvent
2. Hot filter solution through (fluted) filter paper quickly
3. Cool the filtered solution by inserting beaker in ice
4. Suction filtrate with a Buchner flask and funnel to separate out crystals
5. wash the crystals with distilled water
6. dry the crystals between absorbent paper

Question 2

Explain why the first step in purifying an organic solid is to dissolve the impure compound in a minimum volume of hot (near boiling) solvent

Answer 2

• An appropriate solvent is one which will dissolve both the 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 a saturated solution and to enable crystallisation on cooling.

Question 3

Explain why the second step in purifying an organic solid is to hot filter the solution through (fluted) filter paper quickly

Answer 3

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

Question 4

Explain why the third step in purifying an organic solid is to cool the filtered solution by inserting the beaker in ice

Answer 4

• Crystals of the organic solid will reform.
• Soluble impurities will remain in solution form because they are present in small quantities so the solution is not saturated
• Ice will increase the yeild of crystals.

Question 5

Explain why the fourth step in purifying an organic solid is to suction filtrate with a buchner flask and funnel to separate out crystals

Answer 5

The water pump connected to the buchner flask reduces the pressure asnd speeds up the filtration

Question 6

Explain why the fifth step in purifying an organic solid is to wash the crystals with distilled water

Answer 6

This removes soluble impurities

Question 7

What are the reasons for loss of yeild when purifying an orgaic solid by recrystallisation

Answer 7

• crytals may be lost when filtering or washing
• some product may remain in the solution after recrystallisation
• other side reactions may occur

Question 8

Why may the mass of the crystals formed when purifying an organic solid by recrystallisation be larger than expected and give a percentage yeild of >100

Answer 8

The crystals may not have been dried properly

Question 9

What is aspirin made from

Answer 9

2-hydroxybenzoic acid is turned into an ester by reacting it with ethanoic anhydride

Question 10

Why is ethanoic anhydride used in the preparation of aspirin instead of acid chlorides

Answer 10

• It is cheaper
• It is less corrosive
• It is less vulnerable to hydrolysis
• It is less dangerous to use

Question 10

Describe the detailed method for the preparation of aspirin

Answer 10

• Add to a 50cm^3 pear shaped flask 2.0g of 2-hydroxybenzoic acid and 4cm^3 of ethanoic anhydride
• To this mixture add 5 drops of 85% phosphoric (V) acid and swirl to mix.
• Fit the flask with a reflux condenser and heat the mixture in a boiling water bath for about 5 minutes
• Without cooling the mixture, carefully add 2 cm^3 of water in one portion down the condenser.
• When the vigorous reaction has ended, pour the mixture into 40cm^3 of cold water in a 100cm^3 beaker, stir and rub the sides of the beaker with a stirring rod necessary to induce crystallisation.
• finally, allow the mixture to stand in an ice bath to complete crystallisation
• collect the product by suction filtration and wash it with a little water

Question 10

What solvent is used for the purification of aspirin

Answer 10

ethanol

Question 10

Why must naked flames be avoided in the purification of aspirin

Answer 10

Ethanol is flammable

Question 11

What safetyl precaution must be taken when nitrating an orgabnic solid

Answer 11

• Wear gloves as concentrated acids are corrosive
• The acids react together to make the NO2+ ion and this is exothermic so acids are kept cool and the acid is added dropwise.

Question 11

Describe the detailed method for the nitration of an organic solid- in this case methyl benzoate

Answer 11

• Measure 2.5cm^3 of methyl benzoate into a small conical flask and then dissolve it in 5cm^3 of concentrated sulfuric acid
• When the liquid has dissolved, cool the mixture in ice.
• Prepare the nitrating mixture by adding drop by drop of 2cm^3 of concentrated sulfuric acid to 2cm^ concentrated nitric acid.
• Cool this mixture in ice as well.
• Add the nitrating mixture drop by drop to the solution of methyl benzoate
• stir with a thermometer and keep the temperature below 10 degrees celsius
• When the addition is complete, allow the mixture to stand at room temperature for 15 minutes
• Use ice to recrystallise then purify.

Question 11

Why is the temperature kept low nwhen nitrating an organic solid

Answer 11

To prevent multiple substitutions of nitro groups on the benzene ring

Question 12

What is the most common way of testing the purity of an organic sample

Answer 12

Determining the melting point/range of the sample

Question 13

If the organic sample is very pure, what will its melting point be like

Answer 13

If the sample is very pure, the melting point will be sharp and match the melting point given by data books

Question 14

If the organic solid contains impurities, what will its melting point be like

Answer 14

• The melting point will be lowered and the sample will melt over a range of temperatures

Question 15

Describe how the melting point of an organic sample can be accurately determined

Answer 15

• melting point can be measured with an electronic melting point machine
• Alternatively, it could be measured by using a practical set up where the capillary tube is strapped to a thermometer immersed in some heating oil
• In both cases a small amount of the sample is put into a capillary tube. The tube is heated up and is heated slowly near the melting point.