Lab Report on Synthesis of trans-5-norbornene-2.3-dicarboxylic acid from fumaric acid and cyclopentadiene

The lab report below was submitted as part of the coursework for CM2121 Organic Chemistry. Please do not plagiarise from it as plagiarism might land you into trouble with your university. Do note that my report is well-circulated online and many of my juniors have received soft copies of it. Hence, please exercise prudence while referring to it and, if necessary, cite this webpage.

Aim

To synthesize trans-5-norbornene-2,3-dicarboxylic acid from fumaric acid and cyclopentadiene using the Diels-Alder reaction. Infrared (IR) spectroscopy is applied to analyze the reaction product. The eventual yield of the product was determined to be 57.4%.

Introduction

The Diels-Alder reaction is an important cycloaddition reaction for it requires very little energy to create a cyclohexene ring, which is useful in many other organic reactions. In this experiment, cyclopentadiene undergoes Diels-Alder reaction with fumaric acid to form trans-5-norbornene-2.3-dicarboxylic acid.

Infrared spectroscopy is then employed to determine the different function groups present in the product. This spectroscopic method exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the frequency of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling. Hence, the functional groups present in the product can be determined.

Results and calculation

Mass of fumaric acid: 1.160 g

Moles of fumaric acid:  = 0.00999 mol (3 sig. fig.)

Mass of cyclopentadiene: 0.7270 g

Moles of cyclopentadiene:  = 0.0110 mol (3 sig. fig.)

Because fumaric acid and cyclopentadiene react in the mole ratio of 1:1, therefore fumaric acid is the limiting reagent. Hence,

Moles of trans-5-norborene-2,3-dicarboxylic acid produced = 0.00999 mol

Theoretical yield of trans-5-norborene-2,3-dicarboxylic acid = 0.00999 x 182.2 = 1.82 g (3 sig. fig.)

Mass of empty plastic bag: 0.7556 g

Mass of plastic bag and purified product: 1.7899 g

Experimental yield of trans-5-norborene-2,3-dicarboxylic acid = 1.7899 – 0.7556 = 1.0044 g

Percentage yield of trans-5-norborene-2,3-dicarboxylic acid:  = 57.4 % (3 sig. fig.)

Discussion

Reaction mechanism

·         To produce cyclopentadiene

The cyclopentadiene needed for this experiment cannot be purchases since it readily dimerises in a Diels-Alder reaction with itself to form dicyclopentadiene.

To produce the monomer, the dimer is distilled. At the boiling point of the dimer, dicyclopentadiene, equilibration with the monomer, cyclopentadiene, is rapid.  The monomer, being more volatile, may be drawn off at the end of a distillation bridge.

It should be used immediately, since it dimerises again at room temperatue. If necessary, it can be stored in the freezer over night.

·         To produce trans-5-norborene-2,3-dicarboxylic acid

Cyclopentadiene undergoes the Diels-Alder reaction with fumaric acid to form trans-5-norbornene-2.3-dicarboxylic acid. This Diels-Alder reaction involves the cycloaddition of a conjugated diene with an dienophile to form a six-membered ring compound. It takes place in a single step, with a cyclic flow of electrons.

In this experiment, fumaric acid with 2 π-electrons  is the dienophile and cyclopentadiene with a conjugated π-system of 4 π-electrons is the diene. A rearranging of the 6 electrons occurs. Two new π-bonds and one π-bond are formed as three π-bonds are broken. The reaction is thermodynamically favourable due to the conversion of 2 π-bonds into 2 new stronger σ-bonds. The reaction takes place in a single step to form a cyclohexene.

The reaction is facilitated by electron withdrawing groups, carboxylic acid groups, on the dienophile and electron-donating groups such as alkyl on the diene.

The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile, thus a trans-dienophile gives trans-substituents in the product.

Handling of experiment

Cyclopentadiene used in the experiment should be prepared freshly and added into the three-neck flask immediately. This is because the dimerization of cyclopentadiene can proceed at room temperature through Diels-Alder reaction. With dimerisation, less amount of cyclopentadiene is present, leading to a lower experimental yield.

A magnetic stirrer is added to ensure homogeneous mixing of the reagents, thereby speeding up the reaction. Water bath is placed under the three-neck flask to heat up the three-neck flask and the reagents inside it. Reflux condenser is attached on top of the three-neck flask to condense any vapour formed in the reaction and to maintain the reaction at a constant temperature as a compound will always boil at a certain temperature. The internal thermometer is used to monitor the temperature in the three-neck flask.

One of the reagent, cyclopentadiene, is volatile as its boiling point is 40^oC. Hence , the three-neck flask should be stoppered to minimise loss of reagents and product.

After the reaction is completed, the reaction solution is cooled to 0⁰C to crystallise the product. When extracting the precipitated product into the Büchner funnel, only ice-cold water should be used. This decreases the solubility of the product and ensures that the loss of the product is minimized.

Precautions

Dicyclopentadiene is extremely flammable and toxic. It also has a very unpleasant odour. Hence, the experiment should be carried out in the fumehood as much as possible. Also, wear gloves and protective clothing throughout the lab.

Yield of product

The percentage yield of the product is low at 57.4%. This may be due to product lost during the transferring of product, such as from the three-neck flask to the Büchner funnel.

Another possible reason is that some cyclopentadiene may have dimerised before the Diels-Alder reaction with fumaric acid can take place. The occurrence of this side reaction reduces the amount of cyclopentadiene present for the desired reaction with fumaric acid, thus reducing the experimental yield of trans-5-norbornene-2.3-dicarboxylic acid. 

Infrared Spectroscopy

Different functional groups have characteristic absorptions in infrared spectroscopy (IR). The strength of an IR absorption varies with the change of dipole moment when the bond is stretched or bent.  

The complexity of infrared spectra in the 1450 to 600 cm^-1 region makes it difficult to assign all the absorption bands, and because of the unique patterns found there, it is often called the fingerprint region. Absorption bands in the 4000 to 1450 cm^-1 region are usually due to stretching vibrations of diatomic units, and this is also called the group frequency region.

The broad peak at 1675.84 cm^-1 shows the presence of C=O stretching. The presence of the strong and broad peaks between 2500 cm^-1 to 3300 cm^-1 confirm the presence of O-H stretching, distinctive to carboxylic acids.

Wavenumber / cm-1	Description of absorption peaks	Assignment
3084.09	Moderate peak that is only slightly visible due to the overlap with O-H absorption band	=C-H stretching
3300 – 2500	Very broad absorption band that is characteristic of O-H groups which experience hydrogen bonding and it overlaps the C-H absorptions	O-H in carboxylic acid stretching
1675.84	Broad band, typical of C=O stretch of carboxylic acids. Conjugation moves the absorption to a lower frequency.	C=O  stretching
1660 – 1600	This peak is not visible due to overlap with the absorption band of C=O stretching of carboxylic acid groups	C=C stretching
1426.46	Moderate peak that corresponds to the bending of methylene group of the bridging C.	CH2 deformation
1275.49, 1231.45	Moderate peak that corresponds to the C-O stretch in the range of 1320 – 1210 cm-1.	C-O stretching

Conclusion

Trans-5-norbornene-2,3-dicarboxylic acid is synthesized from fumaric acid and cyclopentadiene through a Diels-Alder reaction.

The product is verified through infrared spectroscopy with the distinctive broad absorption of the hydroxyl group observed from about 2400 to 3300 cm^-1 and the C=O absorption of the carboxylic acid groups seen at 1675.84 cm^-1.

1.00 g of the off-white product is obtained and this translated into a percentage yield of 57.4 %.

Questions

The specific reaction, chosen as an example, is drawn below. Ethene is the dienophile. 1,3 -butadiene is the diene. Cyclohexene is the cyclic product.

2a) Explain the term “cycloaddition”.

Cycloaddition is a pericyclic chemical reaction, in which two or more unsaturated molecules (or parts of a molecule) combine with the formation of a cyclic adduct. There is an overall reduction of the bond multiplicity. Electrons move round a circle and there are no positive or negative charges on any intermediates – indeed, there are no intermediates at all. The reaction mechanism occurs in a single step. These characteristics are observed in the reaction stated above. The most well-known example of a cycloaddition is Diels-Alder reaction.

2b) Explain the term “Diels-Alder reaction”.

Diels-Alder reaction occurs between a conjugated diene and an alkene, usually called the dienophile. It produces a substituted cyclohexene. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon atoms. One reason that the Diels-Alder reaction goes so well is that the transition state has six-delocalised π electrons and thus, is aromatic in character and partially resonance stabilized.

2c) Explain the term “diene”.

The diene component in the Diels-Alder reaction can be open-chain or cyclic and it may have many types of substituents. It must be able to take up the s-cis conformation so that the cyclic flow of electrons in the mechanism may occur. In the Diels-Alder reaction, the σ bond in the reacting diene becomes a π bond in the product; the conformation of that σ bond becomes the configuration of the produced π bond. The yield improves when the reaction temperature is lowered because polymerization side reactions between dienes are prevented.

2d) Explain the term “dienophile”.

The dienophile must have an electron-withdrawing group conjugated to the alkene. There must be some extra conjugation – at least a phenyl group or a chlorine atom – or the cycloaddition will occur with poor yield. The dienophile can be activated by a Lewis acid.

3) How many stereoisomers will be present in the product mixture? What are their relationships?

The Diels-Alder reaction is stereospecific. If there is stereochemistry in the dienophile, then it is faithfully reproduced in the product. Thus, cis and trans dienophiles give different diastereomers of the product. For this experiment, fumaric acid is used as the dienophile. Since fumaric acid has –COOH groups which are trans to each other, only trans-5-norbornene-2,3-dicarboxylic acid will be formed.

Bibliography

Anonymous. Diels-Alder Reaction: Preparation of cis-norbornene-5,6-endo-dicarboxylic anhydride. University of Colorado, Boulder, Department of Chemistry and Biochemistry. [Article retrieved on 23/10/11:http://www.enc.edu/~timothy.t.wooster/courses/CH322/Lab/321%20The%20Diels%20Alder%20reaction.pdf]

Wothers, Peter; Greeves, Nick; Warran, Stuart and Clayden, Jonathan. Pericyclic reactions 1: cycloadditions. Organic Chemistry. Oxford University Press, 2009.