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Synthesis of Aspirin lab

Purpose: The purpose of this experiment is to create aspirin by reacting salicylic acid with acetic anhydride. This product will be impure because of unreacted salicylic acid and visible spectroscopy can help determine the percentage of the impurity of the product. This product’s purity can be measured against that of pure aspirin by comparing the melting points of each. In the mentioned reaction, the Hydroxyl group reacts with acetic anhydride to form an ester functional group (esterification).

Method:

Salicylic acid and acetic anhydride needed an acid as a catalyst in order to form the ester. Thus, two drops of 85% Phosphoric acid were added to drive the reaction. To effectively react all of the salicylic acid, an excess of acetic anhydride was used. This helps increase the speed of the reaction as well and move more quickly towards the end point.

When the impurity of aspirin is measured, the salicylic acid can be removed and the specific amount can be determined by a reaction. Salicylic acid complexes with iron (III) to form a purple colored solution. By using spectroscopy, it is possible to determine the absorbance of the solution. Subsequently, Beer’s law will help determine the concentration of salicylic acid present. As one salicylic acid ion complexes per iron (III) ion, it is possible to calculate the percentage of salicylic acid in the product.

By comparing the melting points of the product created to commercial aspirin, it is possible to quantify the impurity. This is due to the fact that impurities lower the melting point proportionally to the impurity of a substance. This occurs because impurities disrupt intermolecular forces. Impure substances also have ranges of substances rather than a specific temperature.

Experimental procedure and observations:

The procedure on page 23 of the CHEM 165 course pack from Spring 2012 was followed. During step 3, the reaction was heated to a maximum of 70 degrees Celsius rather than 50 degrees Celsius.

This should not affect results as the melting and boiling points for the substances involved were above this temperature. During the filtration process, a minute amount of Aspirin remained in the funnel.

Procedure:

1. To synthesize Aspirin

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a. Clean the conical vial with Acetone, and the waste should be placed in the hazardous waste bucket.

b. The vial should not have any water in it.c. Set up a heating apparatus as diagrammed in the lab manual. d. Place an aluminum block on top of the heater/stirrer device and place a

thermometer in it. e. Then insert the air condenser into the vial and adjust the temperature to about 70

degrees Celsius. f. Weigh out .21 g of Salicylic Acid and place it into the conical vial. The add two

drops of (corrosive) 85% Phosphoric acid and .48 mL of acetic anhydride. g. Add a spin vane to the vial and place in aluminum blockh. Turn on magnetic stirrer to stir mixturei. After solvation has occurred, stir for an extra 5 minutesj. Remove the vial from the heating apparatus and cool in a beaker of ice for 5

minutesk. Remove the spin vane and the air condenser from the vial and then add 3 mL of

water and stir. l. After cooling of the solution has occurred, the precipitate will be isolated though

vacuum filtrationm. Set up a Hirsch funnel and vacuum. Be sure to clamp the small filter flaskn. Weigh the small circle filter paper for the funnel, and then moisten it and place it

in the funnel. o. Transfer the mixture to the Hirsch funnel and begin filtration. p. Add 1 mL of cold water to the vial and transfer it to the Hirsch funnelq. Once all of the precipitate has been collected, rinse with 0.5 mL portions of water

three timesr. Leave the product on the funnel for 10 minutes so it driess. Use a spatula to scrape precipitate onto weigh papert. Place under heat lamp for 5 minutesu. Scrape solid into the previously weighed vialv. Place liquid waste into liquid waste bottle

B. Determination of % Salicylic Acid:

1. Weigh out 0.023 g of Aspirin for salicylic acid analysis2. Record exact mass3. Transfer the Aspirin to a 10 mL flask and dissolve it in 1 mL of 95% ethanol4. Add 1 mL of 0.025 M Fe(NO3)3 in 0.5 M HCl solution deionized water to the 10-mL

mark5. Rinse out one cuvette with 1-2 mL of solution that was created in prior step6. Fill it to ¾ full with the solution from step I

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7. Measure the absorbance at 525 nm on the spectrophotometer8. The absorbance should be made within 5 minutes of the time the sample was

dissolved in ethanol9. Calculate the % salicylic acid in the sample using the extinction coefficient of 950 M-

1cm-1 for the iron(III)-salicylate complex at 525 nm

C. Melting Points:

1. The melting point apparatus is set up with a voltage control to regulate the rise in temperature

2. Place the open end of the capillary tube into the sample powder and drop it into the drop down tube to compact the powder

3. Prepare two identical tubes using experimentally obtained aspirin and one using normal aspirin

4. Insert the tubes into the apparatus three at a time and record the ranges over which melting occurs.

5. Dispose of the capillary tubes in the troughs next to the apparatus

Data:

Table of Observed Values

Type of measurement Obtained dataMass of dry filter paper (g) .005Mass of weighing paper (g) .518Mass of Precipitate (g) .126Mass of dry filter product + weighing paper (g) .646Mass of sample used in spectroscopy (g) .023Absorbance of iron(iii)-salicylate complex solution at 525 nm

1.08

Table of melting point values

Starting Melting point(Degrees Celsius)

Ending Melting Point (Degrees Celsius)

Sample 1 of product 126 133Sample 2 of product 126 132Pure aspirin 120 135

Calculations:

1. % yield assuming that the product is 100% pure aspirin: a. Mass product = (Mass dry paper + sample + weighing paper)-(mass dry paper)-

(mass weighing paper) = .646g-.518g-.005g= .126 g productb. Moles Salicylic acid used = .23 g x mol/138 g = .001666 mol salicylic acid

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c. Moles product obtained = .126 g x mol/180 g = .0007 mold. % yield = .0007 mol product/.001666 mol salicylic acid x 1 mol salicylic

acid/1mol product x 100 = 42.01%2. % Salicylic acid impurity in product:

a. Abs = eCli. 1.08 = (950/M*cm) *C*1cm

ii. C = .0011368 mol/L Salicylic acidiii. .0011368 mol/L Salicylic acid* .01 L*138 g/mol = .00156 giv. .00156 g Salicylic acid/.23 g product *100 = .68% salicylic acid by mass

3. Actual % yield of product: a. Moles salicylic acid in 10 ml flask = .00156 g* mol/138 g = 1.136 E-5 molb. Moles Aspirin in 10 ml flask = (.023 g - .00156 g)* mol/180 g = 1.19E-4 molc. 1.136 E-5 mol/1.19E-4 mol = .095373 mol salicylic acid unreacted/ 1 mol aspirin

producedd. Starting moles Salicylic acid= 1 mol + .095373 mol = 1.095373 mole. % yield = 1mol aspirin produced/ 1.095373 mol starting salicylic acid * 100 =

91.29 %4. Difference in melting point ranges between product and pure aspirin:

a. Melting point pure aspirin – melting point product = 120 degrees Celsius to 130 degrees Celsius – 126 degrees Celsius to 133 degrees Celsius = 6 degrees Celsius to 3 degrees Celsius

5. Excess of acetic anhydridea. Acetic anhydride unreacted = .00166 mol*60g/mol = .0996 g acetic anhydrideb. Acetic anhydride added = 0.48 mL * 1.082 g/ml = 0.519 g acetic anhydridec. .519 g - .0996 g = .4194 g excess acetic anhydride

Results:

Quantity measured Data% Yield of product assuming product was 100% aspirin

42.01%

% salicylic acid in product by mass .68%Actual % yield 91.29%Difference in melting points 6 degrees Celsius to 3 degrees CelsiusMass of excess acetic anhydride .4194 g

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Conclusion:

Using the assumption that the yield was 100% aspirin is incorrect. This is due to the fact that some of the product definitely had salicylic acid that was unreacted. The salicylic acid that was used either reacted to form aspirin or remained in the product as salicylic acid.

Therefore, the actual yield of the product must be calculated using the percentage of starting salicylic that eventually became aspirin. By using the 0.023 g sample of aspirin, it was possible to determine the molar ratio of unreacted salicylic acid to reacted salicylic acid can be determined. Since this was a ratio, a representative amount of the total amount of salicylic acid can be found by adding the values of the ratios. Since the 1.00 mol represents the aspirin, 1.00 can be divided by the total to find the % salicylic acid that ended as aspirin. This is much closer to the expected 100% value because the percent mass of salicylic acid in the product was low. The unreacted salicylic acid most probably explains the disparity between the two values. The reaction most probably did not go to 100% completion because the conditions for the reaction were non-ideal. Perhaps a more meticulous experiment can increase the yield of aspirin. Industrial methods may enable this.

The melting point range of the created product (126-132 degrees Celsius) was lower than that for pure aspirin (120-135degrees Celsius). The documented value for the melting point of pure aspirin is 135 degrees Celsius. This falls in the range of the pure aspirin, but not in the range of the synthesized product. An interesting fact is that the “pure” aspirin had a wide range. Perhaps it had such a wide range because it too had impurities in it.