PAG 6 - Synthesis of Aspirin Flashcards
What two reactants are used to synthesis Aspirin?
sodium hydroxide
hydrochloric acid
What is the purpose of concentrated
sulfuric acid in the synthesis Aspirin?
Catalyst (speeds up the rate of reaction)
Describe the process of filtration under
reduced pressure
- Place a piece of filter paper in a Büchner funnel.
- Place the funnel in a Büchner flask. Connect the side arm
of the flask to a water pump. - Connect the water pump to a tap and turn the tap on to
create the vacuum. Pour the solution into the funnel. - Once the solution has drained through, rinse the solid on
the filter paper with a small volume of solvent.
Describe the process of recrystallisation
- Dissolve the solid in the minimum volume of hot
solvent - Filter to remove insoluble impurities
- Cool the filtrate
- Filter to remove soluble impurities (the product
remains on the filter paper)
Why is ethanoic anhydride used for the recrystallisation of
aspirin?
- aspirin is soluble in ethanoic anhydride but not water
- ethanoic anhydride is a cheap and relatively safe
solvent
During recrystallisation, why is the
product dissolved in a minimum amount
of solvent?
If there is a large volume of solvent when
the mixture is cooled (and solubility
decreased), some of the product may
remain in solution.
How does recrystallisation improve the
purity of a product?
The desired product is dissolved in an
appropriate solvent that the impurities are
insoluble in. Subsequent filtration leaves the
insoluble impurities on the filter paper while the
solution flows through.
Why is the solution cooled in ice
immediately after recrystallisation?
To decrease the solubility of the product
so that it crystallises. Lower
temperatures increase the size of
crystals formed.
Why must the crystals be completely dry
before measuring their mass to calculate
the percentage yield?
Excess solvent on the crystals could
falsely increase the yield.
How can percentage yield be calculated?
(Actual yield ÷ maximum theoretical yield) x 100
Give 3 reasons why the yield may be
lower than 100%
- Product may be lost when filtering/ transferring
between apparatus - Some product may stay in solution after
recrystallisation - Other side reactions may occur
What technique could be used to investigate the
purity of the product before and after
recrystallisation?
Thin layer chromatography - comparison
of Rf values from the product before and
after recrystallisation to a pure lab
sample.
What piece of apparatus is used to
transfer a small amount of each liquid
onto the chromatography plate?
Capillary tube
During chromatography, why should the
beaker containing the solvent and the
TLC plate be covered with a clear watch
glass?
To prevent the evaporation of the solvent
The watch glass should be clear to allow
the plate to be monitored
In chromatography, what is the solvent
front?
The furthest distance travelled by the
solvent up the TLC plate
Why should the baseline be drawn in
pencil on a TLC plate?
Pencil is insoluble in solvent so it won’t
travel up the plate and obscure the
results
When using chromatography to assess
the purity of a sample of aspirin, what is used to make
the spots visible?
UV light
or
place in beaker containing Iodine crystals and cover wit cling film
How do you calculate an Rf value?
Rf = distance travelled by the substance
. ————————————————–
. distance travelled by the solvent
Without using chromatography, how can
the purity of a sample be assessed?
Use melting point apparatus
Pure substances have a specific melting point
whereas impure substances will melt over a
larger range of temperatures. The smaller the
range, the purer the sample.
What safety precautions should be taken
during this practical?
- using irritant so wear safety goggles and
avoid contact with skin - Iodine crystals are harmful if in contact with skin or when
inhaled so avoid contact with skin and keep away from your
face - Ethanoic anhydride is highly flammable so keep away from naked flames
