R.P 10 (Preparation & purification of pure organic liquid/solid) Flashcards
Outline the 6 stages in the preparation of aspirin
(1) Add to a 50 cm3 pear-shaped flask 2.0 g of 2-hydroxybenzoic acid and 4 cm3 of ethanoic anhydride
(2) To this mixture add 5 drops of 85% phosphoric(v) acid and swirl to mix
(3) Fit the flask with a reflux condenser and heat the mixture on a boiling water bath for about 5 minutes
(4) Without cooling the mixture, carefully add 2 cm3 of water in one portion down the condenser.
(5) When the vigorous reaction has ended, pour the mixture into 40 cm3 of cold water in a 100 cm3 beaker, stir and rub the sides of the beaker
with a stirring rod necessary to induce crystallisation
(6) Finally, allow the mixture to stand in ice bath to complete crystallisation
Outline the 9 stages in the purification by recrystallisation of a crude sample of aspirin (OR ANY CRUDE ORGANIC COMPOUND)
(1) Using a measuring cylinder, measure out 15cm3 of ethanol into a boiling tube. (OF MINIMUM VOLUME)
(2) Prepare a beaker half-filled with hot water from a kettle at a temperature of approximately 75 °C.
(3) Use a spatula to add the crude aspirin to the boiling tube with ethanol and place the tube in the beaker of hot water.
(4) Stir the contents of the boiling tube until all of the aspirin dissolves into the ethanol.
(5) Pour the hot solution containing dissolved aspirin through a warmed filter funnel and fluted filter paper to hot filter, then pour filtrate into 40 cm3 of water in a conical flask
(6) Allow the conical flask to cool slowly and white needles of aspirin should separate.
(7) Cool the whole mixture in an ice bath.
(8) Filter off the purified solid under reduced pressure
(9) Wash and allow it to dry on filter paper.
(9) Record the mass of the dry purified solid
How do we make aspirin
salicylic acid + ethanoic anhydride —> aspirin + ethanoic acid
State 3 advantages of manufacturing aspirin from ethanoic anhydride over ethanoyl chloride
Cheaper
Safer by product (ethanoyl anhydride gives HCl gas whilst this method gives ethanoic acid which is less corrosive)
Doesn’t react violently with water
What are the 2 main reasons why we manufacture aspirin from ethanoic anhydride over ethanoyl chloride
Cheaper
Safer by product
State 3 advantages of manufacturing aspirin from ethanoyl chloride over ethanoic anhydride
Faster
Irreversible
Larger yield
How do we purify a sample of impure benzoic acid
Same steps as purification by recrystallisation of a crude sample of aspirin
What is the IUPAC name for salicylic acid
2-hydroxybenzoic acid
Why do we use a water bath and not a Bunsen burner to heat the ethanol used in the purification of a crude aspirin sample
Due to flammability of ethanol
What is the purpose of pouring the hot solution containing dissolved aspirin through a warmed
filter funnel and fluted filter paper in the purification of a crude aspirin sample
This step will remove any insoluble
impurities and heat will prevent crystals
reforming during filtration
How do increase the yield of aspirin crystals formed in the purification of a crude aspirin sample
Use a colder ice bath
How does melting point act as a way of testing purity ?
If the sample is very pure then the melting point will be a sharp one, at the same value as quoted in data books.
Describe the behaviour and characteristics of the melting point of a sample which is impure
If impurities are present (and this can include solvent from the recrystallisation process) the melting point will be lowered and the sample will melt over a range of several degrees Celsius
Give an example of an impurity which can arise in the purification by recrystallisation process
Solvent (ethanol)
What are the 2 ways of measuring melting point
In an electronic melting point machine or
by using a practical set up where the capillary tube is strapped to a thermometer immersed in some heating oil
What are the 2 conditions of the heating oil used in measuring melting point via a practical set up
Must have a higher boiling point than samples melting point
Low flamability
Why should pressure be noted when measuring boiling point
Pressure should be noted as changing pressure can change the boiling point of a liquid
Why is boiling point not the most accurate method of identifying a substance
Several substances may have the same boiling point
What is the purpose of anti bumping granules
Added to the prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles
What is the reason why we dissolve the impure compound in a minimum volume of hot solvent in purification by recrystallisation
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
What is the reason why we use a hot filter and fluted filter paper when purifying by recrystallisation
This step will remove any insoluble impurities and heat will prevent crystals reforming during filtration
What are the 2 methods of washing the product when purifying an organic liquid, stating the role of both
- Add sodium hydrogencarbonate solution , shaking and releasing the pressure from CO2 produced
- Sodium hydrogencarbonate will neutralise any remaining reactant acid
-Add Saturated sodium chloride solution
-Sodium chloride will help separate the
organic layer from the aqueous layer
Outline the 5 general stages in purification by recrystallisation (THIS IS WHAT THEY MEAN WHEN QUESTION ASKS HOW IT CAN BE PURIFIED)
Dissolve crude product in hot solvent/water (OF MINIMUM VOLUME)
Filter (hot to remove insoluble impurities)
Cool to recrystallise
Filter under reduced pressure with Buchner apparatus
Wash (with cold solvent) and dry using CaSO4
