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Objective: The purpose of this experiment is to gain a deeper understanding of Diels-Alder chemistry through the cracking of dicyclopentadiene and the synthesis of cic-norbornene-5,6-endo-dicarboxylic anhydride. The success of this experiment will be determined based on the experiment's yield with a goal of a yield greater than 80%. We will also continue to practice chemical analysis techniques such as IR. NMR, and, if we choose, mass spec.
Compounds of Study:
Cyclopentadiene
Cyclopentadiene
Formula: C5H6
Molar Mass: 66.1 g/mol
Density: 0.805 g/mL
MP: -85°C
BP: 42.5°C
Polarity: Non-Polar
Image Source: Chemeo
Dicyclopentadiene
Dicyclopentadiene
Formula: C10H12
Molar Mass: 138.2 g/mol
Density: 980 g/mL
MP: 32.5°C
BP: 170°C
Polarity: Non-Polar
Image Source: Sigma-Aldrich
Cis-Norbornene-5,6-Endo-Dicarboxylic Anhydride
Cis-Norbornene-5,6-Endo-Dicarboxylic Anhydride
Formula: C9H10O4.
Molar Mass: 182.17 g/mol
Density: 1.482 g/mL
MP: 165°C
BP: 419.8°C
Polarity: Polar
Image Source: Sigma-Aldrich
Pre-Lab:
1. Outline what you will be doing in the lab. This outline should be in the style of other outlines and written as first I will do…., second I will do…, third I will do… These are just brief notes to help you along in the lab. As you do the lab, you must write more extensive notes in the experimental section of your lab notebook. You will write out specific steps, specific volumes, specific masses and add observations.
1.) Distill one mole of dicyclopentadiene via short path distillation by gently boiling at around 42 oC, carefully monitoring the solution to ensure that the temperature of the reaction flask does not exceed 45 oC.
2.) Run distillation until 3-4 mL of dicyclopentadiene is obtained, measuring very accurately.
3.) Carefully measure out 3 g of maleic anhydride.
4.) Combine 15 mLs of ethyl acetate and 3 g of maleic anhydride in an Erlenmeyer flask.
5.) Swirl the contents of the flask to ensure thorough dissolution.
6.) Slowly add either petroleum ether, ligoin, or hexane by dropping 1-2 mL at a time, and add 15 mL to the reaction flask.
7.) Cool the solution in an ice bath until the temperature is approximately 0 oC.
Steps 3-7 should be performed while the distillation process is running.
8.) Immediately after obtaining 3-4 mL of dicyclopentadiene, add cold dicyclopentadiene to the reaction mixture and seal the top of the graduated cylinder.
9.) Swirl the reaction within the ice bath for about 30 seconds.
10.) Swirl for an additional 5 minutes outside of the ice bath (but in the hood).
11.) One reaction process appears to have stopped; heat the reaction mixture at a boil until all product dissolves.
12.) Allow the solution to cool to room temperature.
13.) Filter via vacuum filtration.
14.) Wash with cold hexane.
15.) Place the product in a beaker and cover with a Kimwipe.
16.) Clean lab space according to the procedure in lab activity #4.
2. In 2 sentences, describe the Diels-Alder reaction. Describe the reaction in such a way that it would make sense to an organic chemistry student at the end of their first semester of O-chem.
The Diels-Alder reaction is a reaction where a diene (with two double bonds) reacts with a dienophile (with one double bond) to form a six-membered ring. It’s useful in organic chemistry because it builds complex ring systems in a single step with good control over stereochemistry.
3. Which reagent, cyclopentadiene or maleic anhydride is the diene? Which is the dienophile? How did you come to this conclusion?
Cyclopentadine is the diene and maleic anhydride is the dienophile. Cyclopentadiene donates its electrons to the maleic anhydride, forming a new six-membered ring, leading to the formation of cis-Norbornene-5,6-endo-Dicarboxylic Anhydride.
4. What would be the problem if the temperature within the distillation flask were much greater than 45 oC?
The cyclopentadine will evaporate away as cyclopentadine has a very low boiling point (literature value of 42.5 degrees celsius).
5. Why is the receiving “flask” for the distillation of cyclopentadiene within ice?
To cool the product that is formed and prevent it from reacting with itself and forming dicyclopentadiene again.
6. What is a lachrymator? What special precautions must we make in the lab?
A lachrymator is a chemical that irritates the eyes and causes the eyes to tear up (lacrimation). This means we should be extra careful to work within our fume hood and wear proper PPE, such as googles and a lab coat, to prevent the chemical from getting onto our skin or in our eyes.
7. The reaction first calls for dissolving the maleic anhydride in ethyl acetate and this is followed by the addition of hexane.
A. Why don’t you just mix the ethyl acetate and hexane first and then dissolve the maleic anhydride?
Maleic anhydride is a polar molecule and dissolves well in ethyl acetate, which is a relatively polar solvent. Hexane is a nonpolar solvent making it a poor solvent for maleic anhydride.
If ethyl acetate and hexane were mixed first, the solvent mixture would be less polar than pure ethyl acetate, making it harder to dissolve maleic anhydride completely. If you first dissolve the maleic anhydride in ethyl acetate, you can the polarity of the reaction mixture with hexane.
B. Why are two solvents used in the reaction?
Using two solvents balances solubility and reaction conditions, ensuring proper mixing and precipitation control.
8. What is the limiting reagent for this reaction, the amount of cyclopentadiene or the amount of maleic anhydride?
Maleic anhydride is the limiting reagent because it is present in a lower molar amount than cyclopentadiene
9. How many grams of the product should you obtain if you have 100% yield? You should use this number to help you approximate your yield after conducting recrystallization.
If 100% yield, I should obtain 5.02 g of cis-Norbornene-5,6-endo-Dicarboxylic Anhydride.
Methods:
Activity 1: Cracking of Dicyclopentadiene
Set up a long-range distillation apparatus with a condenser, as shown in the image below.
Heated dicyclopentadiene at low heat, carefully monitoring the distillation apparatus to ensure that the reaction temperature did not exceed 45 degrees Celsius. Ran distillation apparatus until approximately 4 mL of clear, foul-smelling distillate was obtained.
Activity 2: Synthesis of cis-Norbornene-5,6-endo-Dicarboxylic Anhydride
While the distillation apparatus was running, 3 g of maleic acid was obtained and ground into a fine white powder that resembled powder sugar in texture and appearance.
Combined 15 mL of ethyl acetate and maleic acid in an Erlenmeyer flask, swirling the flask to dissolve all of the solid maleic acid.
The reaction mixture was placed on a hot plate, and a stir bar was added. The mixture was heated at a low setting and covered with a watch-glass. Approximately 7 mL of hexane was added to the flask, adding 1-2 mL at a time, allowing the mixture to swirl continuously.
Retrieved the product from the distillation reaction and added the 4 mL cyclopentadiene solution to the maleic acid anhydride solution and swirled the reaction for ~30 sec within an ice bath.
Swirled the solution for an additional 5 mins outside of the ice bath.
Added additional hexane to the solution and continued to swirl. A white, powdery precipitate began to form.
Continued to swirl the solution until no further precipitate formed.
Activity 3: Purification and Analysis of cis-Norbornene-5,6-endo-Dicarboxylic Anhydride
Recrystallized solvent on low heat and allowed product to cool.
Extracted pure cis-Norbornene-5,6-endo-Dicarboxylic anhydride via vacuum filtration.
Covered product with a Kimwipe and placed in laboratory drawer for analysis after the break.
Activity 4: Clean up
Turned off distillation apparatus and allowed water to drain after all laboratory groups had obtained their 4 mLs of cyclopentadiene.
Scrubbed all dishes with warm soapy water.
Wiped down both the bench top and the lab hood.
Wiped down the scales and assisted other lab groups with cleanup.
Activity 5: Analysis of Diels-Alder Product
Removed the final product from the laboratory drawer for final analysis. There were two obvious crystal types present within the product: a white, fine powder-like, solid, and another much more "clumpy" product (see image below titled "Final Product" for reference.
Weighed product and calculated % yield.
Prepared a sample in deuterated methanol for NMR analysis.
Analyzed the sample by IR and recorded all data within the laboratory notebook.
Final Product
Final Product
The final product after being allowed to dry for one week.
The two obvious solids were separated based on their consistency, with the larger, "clumpy" product on the left and the finer, more crystaline powder product on the right.
Results:
Final Mass: 4.13 g
% yield: 4.13 g / 5.03 g = 82% yield
Calculation of theoretical yield:
Cyclopentadiene: C₅H₆, MW ≈ 66.10 g/mol
Maleic anhydride: C₄H₂O₃, MW ≈ 98.06 g/mol
Product (cis-Norbornene-5,6-endo-dicarboxylic anhydride): C₉H₈O₃, MW ≈ 164.16 g/mol
Moles of cyclopentadiene: 3.208 g / 66.10 g/mol = 0.0485 mol
Moles of maleic anhydride: 3.00 g / 98.06 g/mol = 0.0306 mol
1:1 molar ratio
Theoretical mass = 0.0306 mol × 164.16 g/mol = 5.03 g
Melting Point:
First Attempt with fine powder product: 118.2-126.4°C
Second Attempt with larger, "clumpy" product: 148.2-155.5°C
IR: A complete IR analysis could not be performed.
Attempted to perform IR analysis during week 1.
The first pellet formed by Morgan Madsen, unable to compress, resulting in only a powdery film.
Second pellet formed in collaboration with TA, Jack Cardwell, with similar results as before.
Attempted to repeat IR analysis during week 2.
Pellet formed by Morgan Madsen, unable to compress, resulting only in a powdery film.
No IR data successfully collected for Diels Alder Lab Analysis.
NMR: Dissolved sample in deuterated methanol spiked with TMS.
NMR had an obvious spike at 0, indicating successful spiking of the sample with TMS.
Singlet noted between 1.0-2.0 ppm - consistent with alkane or alkene character within the molecule.
Multiplet between 3.0-3.5 ppm - indicative of several molecules that are neighbors to an electronegative atom.
Very tall peak noted just below 5.0 ppm - indicative of a high concentration of a molecule bonded to an electronegative atom.
Multiplet noted between 6.0-6.5 ppm - indicative of several molecules that are bonded to an electronegative atom.
Discussion:
% yield: The % yield for this sample was 82%, consistent with the typical high yield that we would expect for Diels-Alder chemistry and the expected standard for this laboratory from the handout prepared by the laboratory instructor.
Melting Point: Melting point analysis revealed data that was inconsistent with pure cis-Norbornene-5,6-endo-dicarboxylic anhydride. The first melting point run had a range of 118.2-126.4°C, a result that is very far outside of the accepted margin of error for the literature reported melting point value for cis-Norbornene-5,6-endo-dicarboxylic anhydride, of 165°C. The first melting point test also gave a very broad melting point range, suggesting the sample is very likely impure.
Due to uncertainty in the first melting point analysis, a second melting point test was run, which gave a range of 148.2-155.5°C, which was much closer to the expected literature value, but still quite far from the accepted margin of error. The two melting points differed by approximately 30°C on both sides of the melting point range. This difference in melting point could be explained by the presence of unreacted starting material in part of the sample. The literature value for the melting point of maleic acid is 135°C. This melting point fits between both of the melting point tests run in the lab and is consistent with the chemicals used in product formation, meaning some unreacted maleic acid may still be present within the final product.
NMR: The sample had a clear, obvious spike at 0, indicating successful spiking of the sample with TMS. This peak was used for standard comparison of the absorbance for all samples. Several prominent peaks were noted on the NMR spectra. Peaks noted on NMR as consistent with the high amount of polar character present in cis-Norbornene-5,6-Endo-Dicarboxylic Anhydride, caused by the four oxygen molecules within the molecule. Oxygen is a highly electronegative atom, which would allow each of the oxygen molecules and their respective bonded atoms to be seen easily on NMR. Interestingly, a prominent peak was not noted within the expected ranges, indicative of a carboxylic acid or aldehyde functional group (from 9-10 ppm for straight or branched chains). This group would be expected for this molecule based on its chemical name and known structure, indicating that the final product may contain little to no Cis-Norbornene-5,6-Endo-Dicarboxylic Anhydride.
IR: IR analysis could not be performed for this sample, preventing further identification and analysis of the chemical makeup of the final product. With other testing indicating that there is likely a mix of chemicals within the final product, it is highly possible that the differing densities prevented the compound from compressing uniformly into a pellet for IR analysis.
Conclusion: While a high yield was obtained, the final, pure cis-Norbornene-5,6-endo-dicarboxylic anhydride, was not synthesized with great success, resulting in a product with several impurities indicative of unreacted starting material when analyzed by melting point and NMR testing. The inability to for a stable IR pellet for solid IR analysis may be another indicator fo the impure nature of the final product.
Reflection: In this lab, I practiced performing Diels-Alder chemistry for the first time by attempting to synthesize cis-Norbornene-5,6-endo-dicarboxylic anhydride. I continued to practice essential organic chemistry lab skills such as distillation, recrystallization, and purification techniques while also performing analysis skills such as NMR, melting point, and IR. If I were to repeat this lab, I would allow my final product more time to recrystalize, ensuring proper swirling of the flask to help ensure more unreacted starting material is converted into the desired product.
Post-Lab: None.
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