In organic chemistry experiments, target compounds are synthesized, separated, and purified, and the results are evaluated using yield, melting point, boiling point, TLC, IR spectra, NMR spectra, and similar methods.
University organic chemistry laboratories may cover esterification, acetylation, oxidation, reduction, aromatic substitution reactions, Grignard reactions, extraction, recrystallization, distillation, chromatography, and other procedures.
In an organic chemistry laboratory report, it is not sufficient simply to write that “the target product was obtained” or “the yield was ○%.”
It is necessary to discuss why that yield was obtained, how the formation and purity of the target product can be judged from the melting point and spectra, and how incomplete reaction, side reactions, insufficient purification, insufficient drying, and similar factors affected the results.
This article clearly explains perspectives that are useful for discussions in organic chemistry laboratory reports, how to write about yield, melting point, and spectra, sources of error, points for improvement, and discussion examples that can be used in reports.
Note:
This article is a reference intended to assist with discussions of results obtained in chemistry experiments at universities and similar institutions.
For the actual synthesis procedures, handling of reagents, heating, cooling, extraction, distillation, recrystallization, waste-liquid disposal, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What to Discuss in an Organic Chemistry Laboratory Report
- Main Items to Include in the Results
- How to Calculate and Write the Yield
- Main Causes of Low Yield
- Discussion When the Yield Is Too High
- How to Write the Melting Point
- Causes of a Low Melting Point
- Causes of a Broad Melting-Point Range
- Discussion When the Melting Point Is Close to the Literature Value
- How to Write About an IR Spectrum
- Discussion of Confirming the Target Product Using an IR Spectrum
- How to Write About an NMR Spectrum
- Discussion of Confirming the Target Product Using NMR
- How to Write About TLC
- Discussion of an Incomplete Reaction
- Discussion of Side Reactions
- Discussion of Extraction Operations
- Discussion of Washing Operations
- Discussion of Drying Operations
- Discussion of Recrystallization
- Discussion of Distillation
- Discussion Relating Spectra and Yield
- When the Target Product Can Be Considered to Have Been Obtained
- Example Discussion When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing an Organic Chemistry Laboratory Report
- Summary
What to Discuss in an Organic Chemistry Laboratory Report
In the discussion section of an organic chemistry experiment, whether the target compound was obtained is judged from multiple results.
Yield indicates how much product was obtained, while melting point and boiling point provide clues about purity and identification of the substance.
Spectra such as IR and NMR are used to confirm functional groups and molecular structure.
In other words, in the discussion of an organic chemistry experiment, it is important not to judge from only one result, but to explain yield, physical-property values, spectra, appearance, TLC, and purification operations in relation to one another.
Example Discussion:
The yield of the obtained product was moderate, and the melting point was close to the literature value.
In addition, in the IR spectrum, an absorption characteristic of the starting material became weaker, and an absorption corresponding to a functional group in the target product was confirmed.
From these results, the target compound is considered to have been generally formed.
However, because the yield was lower than the theoretical value, incomplete reaction or losses during purification may have occurred.
Main Items to Include in the Results
In the results of an organic chemistry experiment, organize not only the mass and yield of the product but also its appearance, physical-property values, and analytical results.
The items measured differ depending on the experiment, but organizing the obtained data with the intention of using them for “confirmation of the target product” and “evaluation of purity” 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 boiling point
- TLC results
- Main absorptions in the IR spectrum
- Main signals in the NMR spectrum
- Comparison with literature values or standard samples
- Changes after purification
Example of How to Write the Results:
After the reaction, the product was extracted, washed, dried, and purified by recrystallization.
The obtained product was white crystals, and the mass after drying was 1.24 g.
Because the theoretical yield was 1.80 g, the yield was 68.9%.
The melting point was within a range close to the literature value, and an absorption corresponding to the target functional group was confirmed in the IR spectrum.
How to Calculate and Write the Yield
Yield is a value that indicates how much product was actually obtained relative to the maximum amount that could theoretically be obtained.
In organic chemistry experiments, the theoretical yield is calculated from the amount of the limiting reagent and compared with the mass of product actually obtained.
Yield (%) = Actual yield ÷ Theoretical yield × 100
In a report, discuss not only the numerical value of the yield but also whether it is high or low and why that yield was obtained.
In particular, it becomes easier to write if losses during the reaction stage and losses during the purification stage are considered separately.
Example Discussion:
The yield in this experiment was lower than the theoretical value.
Possible causes include the reaction not proceeding completely, loss of some product during extraction or transfer, and some of the target compound remaining in the mother liquor during recrystallization.
Therefore, the decrease in yield is considered to have been affected not only by the reaction itself but also by losses during separation and purification.
Main Causes of Low Yield
Causes of low yield in organic chemistry experiments are easier to organize when divided into the reaction stage, separation stage, purification stage, and drying and weighing stage.
| Stage | Cause | Effect on Yield |
|---|---|---|
| Reaction | Incomplete reaction | Amount of target product formed decreases |
| Reaction | Side reaction | Starting material is consumed to form another product |
| Extraction | Target product remains in the aqueous layer | Recovered amount decreases |
| Washing | Target product dissolves in the washing solution | Actual yield becomes smaller |
| Recrystallization | Target product remains in the mother liquor | Yield decreases |
| Transfer | Adhesion to apparatus or spillage | Recovered amount decreases |
Example Discussion:
One possible reason the yield was low is that some of the target compound remained dissolved in the mother liquor during recrystallization.
Recrystallization is effective for increasing purity, but because the target compound also dissolves to some extent in the solvent, it is difficult to recover the entire amount.
Therefore, although purity improved through purification, the yield is considered to have decreased.
Discussion When the Yield Is Too High
If the yield exceeds 100% or is unnaturally high, the measured mass may include components other than the target product.
Representative causes include insufficient drying, residual solvent, contamination by unreacted starting materials, contamination by by-products, and contamination by inorganic salts or drying agents.
A high yield does not necessarily mean a good result.
If impurities are present, the yield may appear high while the purity is low.
Example Discussion:
One possible reason the yield exceeded 100% is that the product was insufficiently dried and solvent or moisture remained.
In addition, if unreacted starting material or by-products were mixed into the product, the measured mass would become larger than the true mass of the target product.
Therefore, even when the yield is high, it does not necessarily mean that the purity is high.
How to Write the Melting Point
Melting point is an important indicator for evaluating the purity of solid organic compounds.
Highly pure substances often melt over a relatively narrow temperature range and show a melting point close to the literature value.
If impurities are present, the melting point may decrease or the melting-point range may broaden.
In a report, the melting point may be written not as a single temperature but as the range from the beginning of melting to complete melting.
For example, it may be written as “123.5–125.0 °C.”
Example of How to Write the Results:
The melting point of the obtained product was 123.5–125.0 °C, which was close to the literature value of 124–126 °C.
Because the melting-point range was also relatively narrow, the purity of the product is considered to have been relatively high.
Causes of a Low Melting Point
When the measured melting point is lower than the literature value, contamination by impurities is commonly considered as a possible cause.
When impurities are mixed into the crystals, the crystal lattice becomes disturbed and melting may begin at a lower temperature than for the pure substance.
In addition, if the sample is not sufficiently dried or solvent remains, the melting point may also be measured lower.
Example Discussion:
One possible reason the measured melting point was lower than the literature value is that impurities were present in the product.
When impurities are mixed in, the crystal lattice becomes disturbed and melting begins at a lower temperature than for the pure substance.
In addition, if solvent or moisture remained because of insufficient drying, this could also lead to a lower melting point and a broader melting-point range.
Causes of a Broad Melting-Point Range
If the melting-point range is broad, the sample may not be pure.
In a highly pure sample, the difference between the temperature at which melting begins and the temperature at which melting is complete tends to be small.
On the other hand, if many impurities are present or the sample is wet, the temperature range becomes broader.
Example Discussion:
One possible reason the melting-point range was broad is that unreacted starting material or by-products were mixed into the product.
When multiple components are present, their melting behaviors overlap and the sample does not readily melt sharply at a constant temperature.
Therefore, the broad melting-point range provides a clue that the purity of the product was insufficient.
Discussion When the Melting Point Is Close to the Literature Value
If the melting point is close to the literature value and the melting-point range is also narrow, the target compound may have been obtained with relatively high purity.
However, the structure cannot be completely determined from the melting point alone.
Because other compounds with similar melting points may exist, the result should be judged together with spectra, TLC, and similar analyses.
Example Discussion:
The melting point of the product was close to the literature value, and the melting-point range was also narrow.
From this, the product is highly likely to be the target compound with relatively high purity.
However, because the structure cannot be completely identified from the melting point alone, the result must be judged together with the IR and NMR spectra.
How to Write About an IR Spectrum
IR spectra are used to confirm functional groups in molecules.
In organic chemistry experiments, the absorptions of specific functional groups before and after the reaction are compared to determine whether an absorption disappeared or a new absorption appeared.
For example, carbonyl groups, hydroxyl groups, amino groups, nitro groups, and ester bonds are functional groups that are relatively easy to discuss using IR spectra.
In a report, it is not necessary to write every peak.
Select and describe absorptions that are important for confirming the target product.
Example of How to Write the Results:
In the IR spectrum, a strong absorption was observed around ○○ cm−1, which is considered to correspond to the stretching vibration of a carbonyl group.
In addition, because the broad absorption derived from the hydroxyl group observed in the starting material had become weaker, the functional group may have changed through the reaction.
Discussion of Confirming the Target Product Using an IR Spectrum
In the discussion of an IR spectrum, check whether an absorption characteristic of the target product is present and whether an absorption characteristic of the starting material remains.
If an absorption of the target functional group is confirmed and an absorption derived from the starting material disappears or decreases, this provides evidence that the reaction progressed.
Example Discussion:
Because an absorption characteristic of the carbonyl group in the target product was observed in the IR spectrum, the target compound is highly likely to have formed.
In addition, because the broad hydroxyl-group absorption characteristic of the starting material had become weaker, the starting material is considered to have been consumed by the reaction.
However, because the structure cannot be completely determined from the IR spectrum alone, the result must be judged together with NMR and other analyses.
How to Write About an NMR Spectrum
NMR spectra are extremely important for confirming the structures of organic compounds.
In 1H NMR, the chemical environment, integration, splitting, and chemical shift of hydrogen atoms can be examined.
In 13C NMR, the types and chemical environments of carbon atoms can be examined.
In a report, discuss whether the main signals correspond to the target structure and whether extra signals derived from starting materials or by-products are present.
Even when it is not necessary to explain every peak in detail, it is useful to discuss peaks that are important for confirming the structure.
Example of How to Write the Results:
In the 1H NMR spectrum, signals corresponding to aromatic protons were observed around ○○ ppm.
In addition, signals corresponding to the substituents in the target product were confirmed, and the integration ratio generally agreed with the value expected from the structural formula.
Discussion of Confirming the Target Product Using NMR
In the discussion of an NMR spectrum, chemical shifts, integration ratios, and splitting patterns are related to the structural formula.
Check whether all signals required for the target product are present and whether extra signals derived from starting materials or by-products are present.
Example Discussion:
In the 1H NMR spectrum, major signals were observed at positions expected from the structure of the target product, and the integration ratios also generally agreed.
From this, the target compound is considered highly likely to have formed.
On the other hand, if signals derived from the starting material remain, the starting material may be present because of incomplete reaction or insufficient purification.
How to Write About TLC
TLC is commonly used to confirm reaction progress and product purity.
If the spot positions of the starting material and product differ, a different substance may have been formed through the reaction.
In addition, if multiple spots are observed in the product, impurities or by-products may be present.
In a report, it is useful to focus on the Rf values, number of spots, and presence or absence of the starting-material spot.
Rf value = Migration distance of spot ÷ Migration distance of solvent front
Example Discussion:
In TLC, the product spot showed an Rf value different from that of the starting material.
From this, a substance different from the starting material is considered to have been produced by the reaction.
In addition, if only one product spot was observed, the sample can be judged to be relatively close to a single component under the TLC conditions.
However, because TLC alone cannot completely evaluate purity, it must be judged together with the melting point and spectra.
Discussion of an Incomplete Reaction
An incomplete reaction means that the starting material is not sufficiently converted into the target product and some remains unreacted.
Possible causes include insufficient reaction time, low temperature, insufficient catalyst or reagent amount, and inadequate mixing.
When the reaction is incomplete, the yield decreases and peaks or spots derived from the starting material may remain in TLC or NMR.
Example Discussion:
Because the yield was low and a spot corresponding to the starting material remained on TLC, the reaction may not have proceeded completely.
If the reaction time was insufficient or the temperature conditions were inappropriate, part of the starting material would remain unreacted.
As a result, the amount of target product formed would decrease and the yield is considered to have been reduced.
Discussion of Side Reactions
In organic reactions, side reactions other than the target reaction may occur.
When a side reaction occurs, starting material is consumed to form substances other than the target product, reducing the yield.
In addition, if by-products contaminate the product, the melting-point range may broaden, multiple spots may appear on TLC, or extra signals may be observed in NMR.
Example Discussion:
Because multiple spots were observed in the TLC of the product, by-products may have been present.
When a side reaction occurs, part of the starting material is converted into substances other than the target product, so the yield of the target product decreases.
In addition, if by-products are not sufficiently removed, they may appear as a broadened melting-point range or extra signals in the NMR spectrum.
Discussion of Extraction Operations
Extraction is an operation in which the target product is distributed into the aqueous or organic layer for separation.
Which layer the target product partitions into is affected by solubility, polarity, acid-base properties, and pH conditions.
If extraction is insufficient, some of the target product remains in the unwanted layer and the yield decreases.
Example Discussion:
One possible cause of the reduced yield is that some of the target product remained in the aqueous layer during extraction.
If the target product did not completely move into the organic layer, the amount recovered in subsequent operations would become smaller.
In addition, for compounds with acidic or basic properties, the ionization state changes depending on pH conditions, changing the layer into which the compound partitions.
Therefore, the extraction conditions are considered to have affected the yield.
Discussion of Washing Operations
Washing the organic layer is performed to remove acids, bases, salts, unreacted substances, water-soluble impurities, and similar components.
However, if the target product dissolves in the washing solution, some target product may be lost during washing.
If washing is insufficient, impurities remain, while excessive washing may reduce the yield.
Example Discussion:
Water-soluble impurities are considered to have been removed by the washing operation, but part of the target product may also have moved into the washing solution.
In particular, if the target product has some solubility in water or becomes ionized under the acid-base conditions, washing may reduce the yield.
Therefore, washing must be performed under conditions that remove impurities while minimizing loss of the target product.
Discussion of Drying Operations
If moisture remains in the organic layer, a drying agent may be used to remove the water.
If drying is insufficient, moisture remains in the product, causing the mass to be overestimated and affecting the melting point and spectra.
On the other hand, if the drying agent is not completely removed and contaminates the product, the mass and purity of the product are also affected.
Example Discussion:
One possible reason the yield of the product appeared high is that drying was insufficient and moisture or solvent remained.
In addition, if drying agent contaminated the product, the measured mass would become larger than the true mass of the target product.
Therefore, the drying operation is an important operation that affects both yield and purity.
Discussion of Recrystallization
Recrystallization is a representative method for purifying solid organic compounds.
The target product is dissolved at high temperature and precipitated as crystals upon cooling, leaving impurities in the mother liquor.
Recrystallization tends to increase purity, but because some of the target product remains in the mother liquor, the yield tends to decrease.
Example Discussion:
If the melting-point range became narrower after recrystallization, impurities are considered to have been removed and the purity of the product improved.
On the other hand, because some of the target product remains dissolved in the mother liquor during recrystallization, the recovered amount decreases.
Therefore, recrystallization is an operation that may increase purity while decreasing yield.
Discussion of Distillation
Distillation is an operation used to separate liquid mixtures based on differences in boiling point.
In organic chemistry experiments, it is used to remove solvents, purify liquid products, and separate reaction mixtures.
In distillation, the boiling point, temperature range of the fraction, recovered amount, azeotropy, and contamination by impurities are discussed.
Example Discussion:
Because the boiling-point range of the obtained fraction was close to the literature value, the target product is highly likely to have been recovered as the main component.
On the other hand, if the boiling-point range was broad, multiple components may have been present or the separation may have been insufficient because the distillation rate was too fast.
Therefore, the distillation result must be evaluated by considering both the temperature range of the fraction and the recovered amount.
Discussion Relating Spectra and Yield
In organic chemistry experiments, impurity-derived peaks may be observed in spectra even when the yield is high.
Conversely, even if the yield is low, a highly pure product may have been obtained if the spectrum agrees well with that of the target compound.
It is important to consider yield and purity as separate evaluation criteria.
Example Discussion:
The yield of the product was relatively high, but signals other than those of the target product were observed in the NMR spectrum.
From this, unreacted starting material or by-products may have been present in the product.
Therefore, a high yield does not necessarily mean high purity, and the yield and spectral results must be evaluated together.
When the Target Product Can Be Considered to Have Been Obtained
It is relatively easy to judge that the target product was obtained when the yield is reasonable, the melting point or boiling point is close to the literature value, the starting material disappears on TLC, and IR and NMR give results corresponding to the target structure.
However, because any one result alone is often insufficient, multiple pieces of evidence should be combined to make the judgment.
Example Discussion:
The obtained product was white crystals, and the melting point was within a range close to the literature value.
In addition, an absorption characteristic of the target functional group was confirmed in the IR spectrum, and signals expected from the structure were also observed in the NMR spectrum.
From these results, the target compound is considered to have been generally synthesized successfully.
Example Discussion When the Experiment Did Not Go Well
When an organic chemistry experiment does not go well, possible causes are considered from results such as low yield, low melting point, broad melting-point range, multiple TLC spots, or starting-material peaks remaining in the spectrum.
It is easier to organize the discussion by separately considering incomplete reaction, side reactions, extraction failure, insufficient purification, insufficient drying, and measurement errors.
Example Discussion:
In this experiment, the yield was low and the melting-point range was broader than the literature value.
Possible causes include the reaction not proceeding completely and starting material remaining, as well as by-products being mixed into the product.
In addition, if some of the target product was lost during recrystallization or extraction, the yield would decrease.
Therefore, both the reaction conditions and the purification operations may have affected the results.
How to Write Points for Improvement
In the discussion of an organic chemistry experiment, including points for improvement as well as sources of error makes the report easier to organize.
Improvements are easier to organize when divided into reaction conditions, separation operations, purification operations, and analytical operations.
Improvements to Reaction Conditions
- Ensure sufficient reaction time
- Maintain appropriate temperature conditions
- Measure reagent amounts accurately
- Stir the reaction mixture sufficiently
- Strictly control conditions for reactions that are sensitive to moisture or oxygen
- Confirm reaction progress using TLC or similar methods
Improvements to Separation and Purification
- Confirm which layer contains the target product during extraction
- Use an appropriate type and amount of washing solution
- Use an appropriate amount of drying agent
- Select an appropriate recrystallization solvent
- Avoid losing the product during filtration and transfer
- Dry sufficiently before weighing
Improvements to Analytical Operations
- Do not pack too much sample during melting-point measurement
- Use an appropriate heating rate during melting-point measurement
- Do not make TLC spots too concentrated
- Be careful about contamination of the sample or measurement surface during IR measurement
- Distinguish solvent peaks and impurity peaks in NMR
- Check measurement conditions when comparing with literature values
Example of How to Write Points for Improvement:
To improve the yield, it is necessary to appropriately control the reaction time and temperature and confirm using TLC or a similar method that the reaction has proceeded sufficiently.
In addition, to avoid losing the target product during extraction and recrystallization, it is important to select solvents and washing conditions while considering the solubility of the target compound.
To increase purity, recrystallization and drying should be performed appropriately, and the condition after purification should be confirmed using the melting point and spectra.
Difference Between a Superficial Discussion and a Good Discussion
In the discussion of an organic chemistry experiment, simply writing that “the yield was low,” “the melting point was close,” or “a spectrum was obtained” results in a superficial discussion.
A more persuasive discussion can be produced by connecting the yield, melting point, spectra, reaction mechanism, and purification operations.
| Superficial Discussion | Good Discussion |
|---|---|
| The yield was low. | Possible causes of the low yield include incomplete reaction, side reactions, and loss of the target product during extraction, washing, and recrystallization. In particular, during recrystallization, some of the target product remains in the mother liquor, so the yield may decrease in exchange for improved purity. |
| The melting point was close to the literature value. | Because the melting point was close to the literature value and the melting-point range was also narrow, the product is highly likely to be the target compound with relatively high purity. However, because the structure cannot be completely confirmed from the melting point alone, the result must be judged together with spectral data. |
| There was a peak in the IR spectrum. | Because an absorption characteristic of the functional group in the target product was observed in the IR spectrum and an absorption derived from the starting material had become weaker, the functional-group conversion is considered to have proceeded through the reaction. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of organic chemistry experiments.
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 purification, and side reactions.
- Although impurities are considered to have been removed by recrystallization, some of the target product remained in the mother liquor, so the yield may have decreased.
- 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 range was broad include contamination by impurities or residual solvent.
- In the IR spectrum, an absorption corresponding to the target functional group was confirmed.
- In the NMR spectrum, signals expected from the structure of the target product were observed.
- Because the starting-material spot remained on TLC, the reaction may not have proceeded completely.
- A high yield does not necessarily indicate high purity, and the mass may be overestimated because of residual starting material or solvent.
- Confirmation of the target product requires a comprehensive judgment based on multiple results such as yield, melting point, TLC, IR, and NMR.
Points to Check When Discussing an Organic Chemistry Laboratory Report
Checking the following points before writing the report makes the discussion easier to write.
- Have you confirmed the limiting reagent and theoretical yield?
- Is the yield calculation correct?
- Have you considered the causes of low yield separately for the reaction and purification stages?
- If the yield is too high, have you considered insufficient drying or impurity contamination?
- Have you compared the melting point with the literature value?
- Have you related a broadened melting-point range to purity?
- Have you compared the starting material and product by TLC?
- Have you explained changes in functional groups using IR?
- Have you explained signals corresponding to the structure using NMR?
- Have you considered contamination by starting materials or by-products?
- Have you evaluated yield and purity separately?
- Do the points for improvement correspond to the sources of error?
Summary
In an organic chemistry laboratory report, yield, melting point, TLC, IR spectra, NMR spectra, and similar results are combined to discuss whether the target compound was obtained and what its purity was.
Yield indicates the amount of product, but a high yield does not necessarily mean high purity.
Insufficient drying or contamination by impurities may cause the yield to appear high.
Melting point is useful for evaluating the purity of solid products.
If the melting point is close to the literature value and the melting-point range is narrow, the purity is considered relatively high.
On the other hand, if the melting point is low or the melting-point range is broad, the effects of impurities or residual solvent must be considered.
In spectra, check whether functional groups or signals characteristic of the target product are confirmed and whether peaks derived from the starting material remain.
In a report, rather than writing about “yield,” “melting point,” and “spectra” separately, relate them to one another and comprehensively judge the formation and purity of the target product.
A persuasive organic chemistry laboratory report can be produced by also discussing the effects of incomplete reaction, side reactions, extraction, recrystallization, and drying.
