Pre-lab 5: Synthesis and TLC of Aspirin

Name:______________
Pre-lab 5: Synthesis and TLC of Aspirin
Section:_____________
Read the background information and answer the following question before lab. Today you will
need to calculate the % yield of your aspirin, so the pre-lab will set you up for this.
1. What are the molar masses of salicylic acid (C 7 O 3 H 6 ), acetic anhydride (C 4 O 3 H 6 ), and
acetylsalicylic acid (C 9 O 4 H 8 )? (Hint: Chapter 6.5 page 226)
2. If 1g of salicylic acid is used to make acetylsalicylic acid, how much acetylsalicylic acid, in
grams, is made? The reaction of salicylic acid to make acetylsalicylic acid is 1mol to 1mol.
(Hint: use molar masses from #1 to go from grams to mol and Chapter 6.6 -6.7, page 230)
3. If 2mL of acetic anhydride is used to make acetylsalicylic acid, how much acetylsalicylic
acid, in grams, is made? The reaction of acetic anhydride to make acetylsalicylic acid is
1mol to 1mol. (Hint: use the density of acetic acid (density =1.082 g/mL) to find grams of
acetic acid.)
~ 62 ~

4. Which of the answers for #2 and #3 is a lower value? The reactant that produced this value
is called the limiting reagent. Is the limiting reagent salicylic acid or acetic anhydride?
5. The lower value in #4 is also called the theoretical yield of the reaction. If your experimental
yield was 0.98g, what is the % yield for your experiment? (Hint: Chapter 6.8, page 236,
%yield = (actual yield/theoretical yield) x 100%)
~ 63 ~

Lab 5: Synthesis and TLC of Aspirin
Objective: The objectives of this experiment are to make aspirin and then use TLC to determine
the purity of your aspirin.
Background Information:
Synthesis of Aspirin
Salicylic acid was once used as a pain killer, but it caused irritation to the stomach lining.
Scientist found that by adding a new functional group to salicylic acid and forming
acetylsalicylic acid, the stomach irritation was reduced. The reaction for this is shown in Figure
1.
salicylic acid acetic anhydride acetylsalicylic acid acetic acid
Figure 1.
Over time Acetylsalicylic acid (aspirin) will breakdown into salicylic acid and acetic acid. In
order to slow this breakdown aspirin should be stored in a dry place. Today you will look at
aspirin that has been left in water for a little over a week and has broken down. You can tell
when aspirin has broken down by the vinegar smell of acetic acid. Since acetic acid is also
produced by the aspirin reaction, if the aspirin made today is not rinsed well, acetic acid will also
show up in the aspirin sample.
TLC Analysis
TLC stands for Thin Layer Chromatography. This is a technique used to look at samples that
may contain multiple compounds in them or determine an unknown when you have multiple
known samples to compare it to. You will be using TLC to compare the aspirin that you make to
a commercial aspirin, the starting material (salicylic acid), and a decomposed sample of aspirin.
~ 64 ~

Procedure:
Data:
Synthesis of Aspirin
Equipment list:
Small Vial with Lid
Vial Holder
Thermometer
Ring Stand
Thermometer Clamp
Ice Bath
A) Synthesis of Aspirin
1) Place the small vial on the balance and tare it.
2) Weigh out about 1g of salicylic acid into the vial.
3) Record the exact weight on the blank line. Include units. __________________
4) Record this value on the report sheet also.
5) Add 2 mL of acetic anhydride to the vial.
6) Add 0.4 mL of sulfuric acid (catalyst) to the vial.
7) Mix the contents of the vial by gently swirling.
8) Place a thermometer in the vial.
9) Place the vial in the vial holder. Support the thermometer.
10) Monitor the temperature of the reaction.
11) When the temperature begins to decrease, make an ice bath.
B) Set up an Ice Bath
1) Place the vial in a 250 mL beaker.
3) Pack enough ice around the vial to cover the liquid in the vial.
4) The acetylsalicylic acid should begin to form as a solid in the vial.
5) Add 2 mL of water to the vial.
6) Filter and dry the aspirin using a Buchner funnel.
C) Recrystalize the Aspirin (if time permits)
1) Dissolve your aspirin in hot toluene.
2) Allow to cool in an ice bath.
3) Filter and dry the aspirin again using a Buchner funnel.
7) Weigh the aspirin.
8) Record the weight on the blank line. Include units. __________________
9) Record the weight on your report sheet also.
~ 65 ~

10) a. What is the theoretical yield for the acetylsalicylic acid? Record the final value on the
report sheet.
b. What is the % yield for your acetylsalicylic acid? Record the final value on the report
sheet.
TLC Analysis of Aspirin
Equipment list:
Three- 50 mL Erlenmeyer Flasks
Small Tubes
TLC Chamber
TLC Plate
Ruler
Pencil
D) Set-up
1) Place a small amount of salicylic acid, commercial aspirin,
and synthesis aspirin in three separate, labeled 50 mL
Erlenmeyer flasks.
2) Add 5 mL of ethanol and mix by gently swirling.
3) Wait until the contents of all of the flasks are dissolved.
4) Obtain a TLC plate by handling the edges and not the front.
5) With a pencil, mark 1cm and 9cm up the plate and draw a line.
6) Make 4 marks on the 1 cm line, 1 for each sample.
~ 66 ~

7) Step 5 and 6 are shown in Figure 2.
8) Label on Figure 2, where each sample will go on the plate.
Step 5 Step 6
9cm
1cm
Figure 2.
9) Place each sample you labeled on the Figure on the real TLC
plate on the marks, using the small tubes.
10) The line 9 cm up the plate will be the stop point.
11) Use the UV light to check that your dots works.
E) Running the TLC Platess
1) Put 15 mL of the TLC solution in the TLC chamber.
2) Carefully place the TLC plate in the chamber.
3) Put the lid on the chamber.
4) Watch the solution move up the TLC plate, until it reaches
the 9cm line.
5) Remove the TLC plate from the chamber.
6) Allow the plate to dry completely.
7) Use the UV light to look at the spots on the TLC plate.
8) Draw the location and label the color of the dots on the TLC plate drawing below.
9) What did your aspirin look like? Compare it to the other
samples on the TLC plate.
~ 67 ~

Name:_______________________
Lab 5: Synthesis and TLC of Aspirin
Report Sheet
Partner(s):____________________
_____________________________
Section:______________________
Synthesis of Aspirin
A.3. Weight of the salicylic acid._____________________
C.8. Weight of the aspirin.__________________
C.10.a. Theoretical yield for the acetylsalicylic acid.________________
C.10.b. % yield for your acetylsalicylic acid._________________
TLC Analysis of Aspirin
Redraw the dots and label the colors from E.8.
~ 68 ~

This page was intentionally left blank!!
~ 69 ~