Synthesis of Aspirin

The purpose of this lab is to synthesise acetylsalicylic acid (aspirin) by creating a reaction between acetic anhydride and salicylic acid. This was be accomplished through the use of recrystallization. Acetic anhydride and salicylic acid are mixed together, and then acidified by the addition of a few drops of concentrated sulfuric acid, which catalyzed the reaction. The percent yield is calculated to determine the effectiveness of the reaction in preparing the desired product (aspirin).

The limiting reactant of the equation was salicylic acid. After the limiting reactant was determined, the theoretical yield of aspirin was calculated at approximately 1.97g. The actual yield was only around 0.67g, producing a percent yield of 34.3%. These results show that the methods used were only partially successful at achieving the goal of the experiment (synthesising aspirin). The findings showed that acetylsalicylic acid can be produced through a reaction between salicylic acid and acetic anhydride, but that a much lower yield will be produced. A higher yield could surely be achieved if several sources of error were to be eliminated.

Introduction

Acetylsalicylic acid is commonly used to alleviate minor aches and pains (Wikipedia, Aspirin, 2013). The active metabolite ingredient in acetylsalicylic acid (aspirin) is salicylic acid (Wikipedia, Salicylic acid, 2013), which was first discovered by Edward Stone in 1763 (Wikipedia, Aspirin, 2013).

Salicylic acid is toxic in large quantities but in small doses can be useful for food preservatives and as an antiseptic. Other than being used in the production of aspirin, acetic anhydride is used to convert cellulose to cellulose acetate, a key component in photographic film and other coated materials (Wikipedia, Acetic anhydride, 2013). Sulfuric acid has many applications, such as pigments, explosives, lubricants, batteries, antifreeze, and detergents. In the synthesis of aspirin, sulfuric acid is also used as a catalyst to speed up the reaction (Wikipedia, Sulfuric acid, 2013).

Limiting reactants are important in chemical reactions because a reaction cannot proceed without all of the reactants. That is to say, a reaction can only occur until one reactant is used up (Kirk, 2013). Percent yields are related to limiting reactants because the formula to solve for percent yield includes theoretical and actual yield.

The theoretical yield is the amount of a product formed when the limiting reactant in completely consumed, and is the maximum amount that can be produced from the amount of reactants used in the reaction. The theoretical yield is rarely obtained because of sources of error, side reactions, or other complications. The percent yield is the actual yield of a product given as a percentage of the theoretical yield (Kirk, 2013).

How can one prepare aspirin through a reaction between salicylic acid and acetic anhydride?

Apparatus:

• (2) 250 mL beaker
• 10 mL graduated cylinder
• Filter paper
• Funnel support
• Hot plate
• 25 mL graduated cylinder
• 50 mL Erlenmeyer flask
• Eyedropper pipette
• Funnel
• 100 mL beaker
• Wash bottle

Materials:

• Boiling chips (calcium carbonate)
• Acetic anhydride
• 18M sulfuric acid
• Ethanol
• Salicylic acid
• Distilled water
• Ice

Method

• Prepared a water bath by half-filling a 250 mL beaker with water and heating it on a hot plate to until it was near boiling. Placed a few boiling chips in the beaker to prevent bumping if the water began to boil.
• Weighed out 1.5g of salicylic acid on a piece of filter paper. Recorded the weight on the data sheet. Transferred the salicylic acid to a 50 mL Erlenmeyer flask.
• Measured out 5.0 mL of acetic anhydride in a graduated cylinder from the fume hood. Recorded the volume of acetic anhydride used on the data sheet. Poured the acetic anhydride into the 50 mL Erlenmeyer flask containing the salicylic acid.
• Took the Erlenmeyer flask to the fume hood and added 5 drops of concentrated sulfuric acid to the mixture.
• Mixed the solution and placed the Erlenmeyer flask in the water bath for about 10 minutes, making sure the Erlenmeyer flask did not tip in the water bath.
• After the 10 minutes elapsed, added 2 mL of distilled water with an eyedropper carefully to avoid splatter. Waited 6 minutes, during which time an ice water mixture was prepared in another 250 mL beaker.
• After the 6 minutes elapsed, 10 mL of distilled water was added to the Erlenmeyer flask and placed in the ice water, avoiding getting any of the ice water in the Erlenmeyer flask. A precipitate formed as the solution cooled. Used the “scratching” method, in which the bottom of the Erlenmeyer flask is scraped, to speed up the process of precipitate formation.
• Prepared a piece of filter paper in a filter funnel and filtered off the precipitate from the Erlenmeyer flask solution. After the liquid drained through the filter paper, washed the filtrate with two 10 mL portions of cold distilled water. When all the water had drained through, the filter paper was removed, and the solid (impure aspirin) was scraped into a dry 100 mL beaker using a clean scoopula.

Recrystallization

• Measured out 5 mL of ethanol from the fume hood in a graduated cylinder, and added it to the 100 mL beaker containing the solid aspirin. Swirled the beaker to dissolve as much of the solid as possible, then placed the beaker on the hot plate until the solid completely dissolved.
• After the solid dissolved, added about 15 mL of distilled water. Prepared another ice water mixture in the 250 mL beaker and placed the 100 mL beaker in the ice water. Waited about 10 minutes.
• Weighed a piece of filter paper and recorded it on the data sheet. Prepared the filter paper in a funnel and filtered off the precipitate. Rinsed the dirty apparatus thoroughly with lots of water. After all of the water was filtered through, left the filter paper to dry until the next class.
• Weighed the piece of filter paper with the aspirin on it. Recorded the weight on the data table. Discarded the aspirin.

Results

Overall Findings:

When the salicylic acid and acetic anhydride were mixed, a white, powdery solution formed. When the sulfuric acid was added, a clear solution formed that produced heat. After heating, then cooling and scratching the solution, a white precipitate formed. The moisture in the precipitate was filtered overnight and what was left over was the desired product, aspirin.

Qualitative Data:

The boiling chips (calcium carbonate) were white, opaque crystals. The acetic anhydride was a clear solution with a vinegar-like odour. The salicylic acid was a find, white solid powder. The ethanol was a clear solution with an odour similar to strong alcohol. The sulfuric acid was a clear solution with a strong odour when heated. The aspirin (acetylsalicylic acid) was a white, solid powder.

When the acetic anhydride and salicylic acid were mixed, they produced a white, powdery solution. When the sulfuric acid was added to this solution, it turned clear and was warm. Upon heating the solution and adding water, puffs of smoke were produced. When the solution cooled and the “scratching” method was used, a white precipitate formed.

Quantitative Data:

Density (Table #1)

Substance/Density

1. Acetic anhydride 1.08 m/L

Volume (Table #2)

Substance/Volume

• Acetic anhydride
  5.0 mL

Molar weight (Table #3)

Substance/ Molar weight

• Salicylic acid
  138.12 g/mol
• Acetic anhydride
  102.09 g/mol
• Acetylsalicylic acid
  180.16 g/mol
• Weight (Table #4)

Substance/Weight

• Salicylic acid
  1.51 g
• Acetic anhydride
  5.4 g
• Empty filter paper
  1.27 g
• Filter paper and aspirin
  1.946 g
• Aspirin
  0.676 g

Percent Yield (Table #5)

Substance/Percent Yield

• Aspirin
  34.3%

Analysis:

Determine whether the limiting reactant is the salicylic acid or the acetic anhydride. First, convert both masses to moles. 1 mole is equal to the molar mass of a substance; therefore, the grams of acetic anhydride and grams of salicylic acid must be divided by their molar masses respectively.

• Molar mass of acetic anhydride = 4(12.01) + 6(1.008) + 3(16) = 102.088g Molar mass of salicylic acid = 7(12.01) + 6(1.008) + 3(16) = 138.118 g
• Acetic anhydride: 5.4g C4H6O3 x 1 mole C4H6O3 l= 0.052895541 moles C4H6O3
  102.088g C4H6O
• Salicylic acid:1.51g C7H6O3 x 1 mole C7H6O3 = 0.01093268 moles C7H6O3138.118 g C7H6O3

Since both acetic anhydride and salicylic acid have a coefficient of 1, the smaller number is the limiting reactant. Therefore, salicylic acid is the limiting reactant of the reaction. Next, calculate the theoretical yield of aspirin by converting the mass of the limiting reactant to grams of aspirin. 1.51g C7H6O3 x 1 mole C7H6O3 x 1 mole C9H8O4 x 180.154 g C9H8O4 138.118 g C7H6O3 1 mol C7H6O31 mol C9H8O4
= 1.969566168g Aspirin

Using the theoretical yield and the actual yield (from the data table), calculate the percent yield.

Percent yield = Actual yield x 100 = l 0.676g l x 100 = 34.3%
Theoretical yield1.969566168g

Discussion

Sources of error that were possibly present in the synthesis of aspirin lab are an improperly calibrated balance, an inaccurate hot plate, contamination of the substances, age of the substances, or contamination of the glassware. Inaccurate calibrations of the hot plate or balance could have shown higher or lower quantitative data which would have affected the results by producing either a higher or lower percent yield. A contamination of the substances used or the glassware could have caused the substances to react differently with each other, again causing the percent yield to change depending on how the contamination affected the substances by producing a higher or lower percent yield.

Another source of error could have been due to the transport of the aspirin from the Erlenmeyer flask after cooling to the funnel to be filtered. Some of the aspirin may have been lost or left behind and that would have showed a lower percent yield. The accuracy of this lab could be improved with more precise equipment, allowing the experimenter to be more confident in the accuracy of the measurements obtained. While the aspirin was left overnight to allow time for the moisture to be filtered out, the air could have been a contaminant.

A way to get rid of the source of error would be to keep the samples in a more isolated area where there is a smaller chance of contamination. Aspirin has many real-life applications. It is a pain reliever and a non-steroidal anti-inflammatory (NSAID). It has become very common as an antiplatelet to prevent blood clot formation and is used to prevent heart attacks and strokes. It has been present for over one hundred years and is one of the most widely used medications in the world.

Conclusion

The main objective of the synthesis of aspirin lab was so produce aspirin (acetylsalicylic acid) through the reaction of salicylic acid and acetic anhydride. The methods used included recrystallization and scratching to produce a precipitate, which was then filtered to remove any excess moisture. The results displayed a percent yield of 34.3%, from a theoretical yield of about 1.97g of aspirin and an actual yield of approximately 0.68g of aspirin. Upon completion of the lab, analysis, and calculations, it is evident that the synthesis of aspirin is possible using these methods but that the yield will be relatively low.