Aspirin Synthesis Discussion Examples | How to Think About Yield, Melting Point, and Impurities

What Is an Aspirin Synthesis Experiment?

The chemical name of aspirin is acetylsalicylic acid.
In an aspirin synthesis experiment, the phenolic hydroxyl group of salicylic acid is acetylated to obtain acetylsalicylic acid.
Because the functional group of the starting material, salicylic acid, changes in this reaction, melting point, IR spectra, color reactions, and similar methods are used to confirm the product.

If the reaction proceeds sufficiently, the properties of the phenolic hydroxyl group derived from salicylic acid become weaker and an acetylated product is obtained.
However, if the reaction is incomplete, unreacted salicylic acid remains and affects the melting point, color reaction, yield, and purity.

Main Items to Include in the Results

In the results of an aspirin synthesis experiment, organize not only the product mass and yield but also the appearance of the crystals, melting point, changes before and after recrystallization, and impurity confirmation reactions.
Organizing the obtained data from the two perspectives of “whether the target product was obtained” and “what the purity was” makes the discussion easier to write.

Main Items to Include in the Results

• Amount of starting material used
• Limiting reagent
• Appearance of the product
• Mass of the product
• Theoretical yield
• Actual yield
• Yield
• Melting point or melting-point range
• Differences in the crystals before and after recrystallization
• Results of confirmation reactions using iron(III) chloride reagent or similar methods
• Analytical results such as IR spectra
• Comparison with literature values

Calculation and Discussion of Yield

In aspirin synthesis, the theoretical yield is determined from the limiting reagent and compared with the mass of aspirin actually obtained to calculate the yield.
Yield is an important result indicating how much of the target product was obtained.

Yield (%) = Actual yield ÷ Theoretical yield × 100

However, the quality of the experiment cannot be judged from the yield alone.
Even if the yield is high, purity is low if many impurities are present, while even if the yield is low, a highly pure product may have been obtained if the melting point and spectrum agree well with those of the target compound.

Causes of Low Yield in Aspirin Synthesis

Causes of low yield in aspirin synthesis are easier to organize by separating cases in which the reaction did not proceed sufficiently from cases in which the product was lost during recovery.

Discussion When the Yield Is Too High

If the yield is too close to 100% or exceeds 100%, the measured mass may include components other than the target product.
In aspirin synthesis, possible causes include residual water or solvent due to insufficient drying, contamination by unreacted salicylic acid, contamination by by-products, and residual inorganic acids or inorganic salts.

A high yield may appear to be a good result at first glance, but if the melting point is low, the melting-point range is broad, or salicylic acid is detected by a color reaction, the purity may be low.

How to Interpret the Melting Point

In an aspirin synthesis experiment, the purity of the product can be evaluated by melting-point measurement.
Highly pure aspirin is expected to have a melting point close to the literature value and a relatively narrow melting-point range.
On the other hand, if impurities are mixed in, the melting point may decrease or the melting-point range may broaden.

In a report, it is useful to write the melting point as a range from “the temperature at which melting began to the temperature at which melting was complete.”
Discuss not only the melting-point value, but also how close it is to the literature value and whether the melting-point range is narrow or broad.

Causes of a Low Melting Point

Contamination by impurities is a possible cause of an aspirin melting point lower than the literature value.
When impurities are mixed into the crystals, the crystal lattice is disturbed and melting may begin at a lower temperature than for pure aspirin.
Residual water or solvent caused by insufficient drying may also lower the melting point.

Causes of a Broad Melting-Point Range

If the melting-point range is broad, multiple components may be mixed into the product.
Highly pure substances melt over a relatively narrow temperature range, but if impurities are present, the temperature difference between the beginning of melting and complete melting tends to become larger.

In aspirin synthesis, unreacted salicylic acid, decomposition products, residual solvent, and impurities not completely removed by recrystallization may broaden the melting-point range.

Effects of Unreacted Salicylic Acid

One of the easiest impurities to discuss in aspirin synthesis is unreacted salicylic acid.
If the reaction does not proceed completely, salicylic acid may remain in the product.
Contamination by salicylic acid affects the melting point, color reactions, IR spectrum, and similar results.

Because salicylic acid has a phenolic hydroxyl group, it may show a color reaction with iron(III) chloride reagent.
This reaction can be used to confirm the possibility that salicylic acid remains in the product.

Discussion of the Confirmation Reaction With Iron(III) Chloride Reagent

Iron(III) chloride reagent may be used to confirm compounds containing phenolic hydroxyl groups.
Because salicylic acid has a phenolic hydroxyl group, it may show a color reaction with iron(III) chloride reagent.
On the other hand, in aspirin the hydroxyl group has been acetylated, so it is less likely to show as strong a color reaction as salicylic acid.

Therefore, if a strong color reaction is observed when iron(III) chloride reagent is added to the aspirin product, residual unreacted salicylic acid can be considered.

Discussion of Improved Purity Through Recrystallization

In aspirin synthesis, the product may be purified by recrystallization.
Recrystallization uses differences in solubility between the target product and impurities to recover the target product as crystals while leaving impurities in the mother liquor.
As a result, the purity of the product tends to increase.

However, because some aspirin remains dissolved in the mother liquor, the yield after recrystallization tends to decrease.
In other words, recrystallization is an operation in which “improved purity” and “reduced yield” tend to occur at the same time.

When Purity Remains Low Even After Recrystallization

If the melting point remains low, the melting-point range remains broad, or coloration occurs with iron(III) chloride reagent even after recrystallization, impurities may not have been sufficiently removed.
Possible causes include inappropriate selection of the recrystallization solvent, use of too much solvent, rapid crystallization, and insufficient washing or drying.

Effects of Insufficient Drying

If aspirin crystals are not sufficiently dried, water or solvent remains and affects both the yield and melting point.
If the product is weighed before sufficient drying, its mass becomes larger than the actual value and the yield is overestimated.
Residual solvent or moisture may also lower the melting point or broaden the melting-point range.

Discussion of Aspirin Decomposition and Hydrolysis

Depending on the conditions, aspirin may undergo hydrolysis to produce salicylic acid and acetic acid.
In particular, depending on moisture, heating conditions, and storage conditions, some of the aspirin may decompose after formation.
If decomposition occurs, reactions derived from salicylic acid or changes in melting point may be observed.

In a report, the reason salicylic acid remained in the product can be discussed not only as incomplete reaction but also as possible hydrolysis of the product.

Discussion Using an IR Spectrum

If an IR spectrum is measured, absorptions characteristic of the functional groups in aspirin can be confirmed.
In aspirin, absorptions derived from the carboxylic acid, ester, aromatic ring, and similar structures are targets for discussion.
When salicylic acid changes to aspirin, changes occur in absorptions related to the state of the phenolic hydroxyl group and the ester bond.

In a report, discuss whether absorptions characteristic of the target product were observed and whether absorptions derived from the starting material salicylic acid remained.

Discussion Using TLC

If TLC is performed, the spots of the starting material salicylic acid and the product aspirin can be compared.
If the starting-material spot remains in the sample after the reaction, incomplete reaction or insufficient purification may be involved.
If multiple spots are observed in the product, contamination by impurities or by-products should be considered.

Rf value = Migration distance of spot ÷ Migration distance of solvent front

Discussion Based on Crystal Appearance

Aspirin products are often obtained as white crystals.
However, even if the crystals are white, they are not necessarily highly pure.
If the crystals are wet, uneven in color, fine and powdery, or retain mother liquor, impurities or insufficient drying should be considered.

Losses During Filtration and Washing

When aspirin crystals are filtered and washed, the yield decreases if some crystals remain on the filter paper or apparatus or dissolve in the washing solution.
Particularly when the crystals are fine, they are more likely to pass through during filtration or adhere to the apparatus.

Impurity Contamination Caused by Insufficient Washing

If washing is insufficient, mother liquor, acids, acetic acid, unreacted substances, and similar components may remain on the crystal surface.
As a result, the mass of the product may appear larger, the melting point may decrease, and the melting-point range may broaden.
However, because excessive washing can also cause some aspirin to be lost, both insufficient and excessive washing can be discussed.

When the Result Can Be Considered Good

A good result in aspirin synthesis is indicated when the product is obtained as white crystals, the melting point is close to the literature value, the melting-point range is narrow, and reactions indicating residual salicylic acid are weak or absent.
If IR or TLC was performed, results corresponding to the target product and only small amounts of peaks or spots derived from the starting material also provide evidence.

Example Discussion When the Experiment Did Not Go Well

When aspirin synthesis does not go well, possible causes are considered from results such as low yield, low melting point, broad melting-point range, strong coloration with iron(III) chloride reagent, wet crystals, or a remaining starting-material spot on TLC.
Organizing incomplete reaction, hydrolysis, losses during recrystallization, insufficient drying, and impurity contamination separately makes the discussion easier to write.

How to Write Points for Improvement

In a discussion of aspirin synthesis, including points for improvement as well as sources of error makes the report easier to organize.
Improvements are easier to write when divided into those for allowing the reaction to proceed sufficiently, those for increasing purity, and those for improving yield.

Improvements for Allowing the Reaction to Proceed Sufficiently

• Ensure an appropriate reaction time
• Maintain the temperature conditions specified in the laboratory manual
• Measure reagent amounts accurately
• Mix the reaction mixture sufficiently
• Avoid moisture contamination as much as possible

Improvements for Increasing Purity

• Use appropriate recrystallization conditions
• Avoid rapid crystallization
• Remove mother liquor and acidic components appropriately
• Avoid insufficient washing
• Dry the product sufficiently
• Evaluate purity using melting point and confirmation reactions

Improvements for Increasing Yield

• Allow crystallization to proceed sufficiently
• Avoid losing crystals during filtration and transfer
• Do not use more washing solution than necessary
• Reduce losses into the mother liquor during recrystallization
• Recover the crystals sufficiently before drying

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of aspirin synthesis, simply writing that “the yield was low” or “the melting point was low” results in a superficial discussion.
A more persuasive discussion can be produced by relating the results to incomplete reaction, unreacted salicylic acid, impurities, recrystallization, and insufficient drying.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of an aspirin synthesis experiment.
Adjust the necessary parts according to your own experimental results.

• The yield of the obtained product was ○%, which was lower than the theoretical yield.
• Possible causes of reduced yield include incomplete reaction, losses during recrystallization, and losses during filtration and transfer.
• Although recrystallization removes impurities, some aspirin remains in the mother liquor, so the yield may decrease.
• Because the melting point was close to the literature value and the melting-point range was also narrow, the purity of the product is considered relatively high.
• Possible reasons the melting point was low and the melting-point range was broad include contamination by unreacted salicylic acid or residual solvent.
• If coloration was observed with iron(III) chloride reagent, salicylic acid may remain in the product.
• If drying is insufficient, water or solvent remains and causes the yield to be overestimated.
• Hydrolysis of aspirin may produce salicylic acid and lead to reduced purity.
• Even if white crystals are obtained, purity cannot be determined from appearance alone.
• Formation and purity of the target product must be judged comprehensively from yield, melting point, color reactions, IR spectra, and similar results.

Points to Check When Discussing Aspirin Synthesis

Checking the following points before writing the report makes the discussion easier to write.

• Is the yield calculation correct?
• Was the theoretical yield determined from the limiting reagent?
• Have you considered the causes of reduced yield separately for the reaction and recovery operations?
• If the yield is too high, have you considered insufficient drying and impurity contamination?
• Have you compared the melting point with the literature value?
• Have you related a broadened melting-point range to impurities?
• Have you considered the possibility of unreacted salicylic acid?
• Have you explained the color-reaction result with iron(III) chloride reagent?
• Have you explained improved purity and reduced yield caused by recrystallization?
• Have you considered the effects of insufficient drying and residual solvent?
• Have you considered the possibility of aspirin hydrolysis?
• Have you avoided determining purity from appearance alone?

Summary

In an aspirin synthesis experiment, salicylic acid is acetylated to obtain acetylsalicylic acid.
In the report, yield, melting point, crystal appearance, impurity confirmation reactions, IR, TLC, and similar results are considered together to evaluate whether the target product was obtained and what its purity was.

If the yield is low, possible causes include incomplete reaction, loss into the mother liquor during recrystallization, and losses during filtration, washing, and transfer.
If the yield is too high, the mass may have been overestimated because of insufficient drying or contamination by unreacted substances or by-products.

If the melting point is lower than the literature value or the melting-point range is broad, impurities such as unreacted salicylic acid, residual solvent, or hydrolysis products may be present.
If coloration is observed with iron(III) chloride reagent, residual salicylic acid provides important evidence for discussion.
Organizing and explaining the relationships among yield, melting point, and impurities makes it possible to produce a persuasive aspirin synthesis report.