PAG 6 - Synthesis of Aspirin

Question 1

What two reactants are used to synthesis Aspirin?

Answer 1

sodium hydroxide

hydrochloric acid

Question 2

What is the purpose of concentrated

sulfuric acid in the synthesis Aspirin?

Answer 2

Catalyst (speeds up the rate of reaction)

Question 3

Describe the process of filtration under

reduced pressure

Answer 3

• 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.

Question 4

Describe the process of recrystallisation

Answer 4

• 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)

Question 5

Why is ethanoic anhydride used for the recrystallisation of

aspirin?

Answer 5

• aspirin is soluble in ethanoic anhydride but not water
• ethanoic anhydride is a cheap and relatively safe
  solvent

Question 6

During recrystallisation, why is the
product dissolved in a minimum amount
of solvent?

Answer 6

If there is a large volume of solvent when
the mixture is cooled (and solubility
decreased), some of the product may
remain in solution.

Question 7

How does recrystallisation improve the

purity of a product?

Answer 7

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.

Question 8

Why is the solution cooled in ice

immediately after recrystallisation?

Answer 8

To decrease the solubility of the product
so that it crystallises. Lower
temperatures increase the size of
crystals formed.

Question 9

Why must the crystals be completely dry
before measuring their mass to calculate
the percentage yield?

Answer 9

Excess solvent on the crystals could

falsely increase the yield.

Question 10

How can percentage yield be calculated?

Answer 10

(Actual yield ÷ maximum theoretical yield) x 100

Question 11

Give 3 reasons why the yield may be

lower than 100%

Answer 11

• Product may be lost when filtering/ transferring
  between apparatus
• Some product may stay in solution after
  recrystallisation
• Other side reactions may occur

Question 12

What technique could be used to investigate the
purity of the product before and after
recrystallisation?

Answer 12

Thin layer chromatography - comparison
of Rf values from the product before and
after recrystallisation to a pure lab
sample.

Question 13

What piece of apparatus is used to
transfer a small amount of each liquid
onto the chromatography plate?

Answer 13

Capillary tube

Question 14

During chromatography, why should the
beaker containing the solvent and the
TLC plate be covered with a clear watch
glass?

Answer 14

To prevent the evaporation of the solvent

The watch glass should be clear to allow
the plate to be monitored

Question 15

In chromatography, what is the solvent

front?

Answer 15

The furthest distance travelled by the

solvent up the TLC plate

Question 16

Why should the baseline be drawn in

pencil on a TLC plate?

Answer 16

Pencil is insoluble in solvent so it won’t
travel up the plate and obscure the
results

Question 17

When using chromatography to assess
the purity of a sample of aspirin, what is used to make
the spots visible?

Answer 17

UV light
or
place in beaker containing Iodine crystals and cover wit cling film

Question 18

How do you calculate an Rf value?

Answer 18

Rf = distance travelled by the substance
. ————————————————–
. distance travelled by the solvent

Question 19

Without using chromatography, how can

the purity of a sample be assessed?

Answer 19

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.

Question 20

What safety precautions should be taken

during this practical?

Answer 20

• 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