Post Lab #9 - I earned an A in this lab class. PDF

Preview

Summary

Experiment #9: Preparation of meso-Tetraphenylporphyrin Name: Danielle Curtis Lab Partners: Virginia Van Grod and Yefrain Munoz TA: Katrinah Tirado Introduction/Background Porphyrins are a class of molecules that are derived from the basic molecule porphin, sometimes called porphyrin A porphyrin mol...

Description

Experiment #9: Preparation of meso-Tetraphenylporphyrin Name: Danielle Curtis Lab Partners: Virginia Van Grod and Yefrain Munoz TA: Katrinah Tirado

Introduction/Background Porphyrins are a class of molecules that are derived from the basic molecule porphin, sometimes called porphyrin.1 A porphyrin molecule is a rigid, square-planar molecule that is composed of four pyrrole rings, which are connected to form one larger product.1 To clarify, a pyrrole ring is a 5-membrered heterocyclic ring that contain a nitrogen atom.1 This type of molecule is a highly conjugated system of alternating double and single bonds throughout the entirety of the molecule, resulting in aromaticity. 1 As a result of this aromaticity throughout, the structure of this molecule is highly stabilized.1 The porphyrin molecule has its four N atoms facing the center of the ring.2 This orientation allows the molecule to capture a metal ion, forming a very stable organometallic complex, also known as a metalloporphyrin2. It is this particular property that makes the molecule important in many biological systems.2 For example, chlorophyll is a porphyrin derivative with a magnesium atom located at its central position, thus making it a metalloporphyrin.2 Moreover, heme is a porphyrin derivative with an iron atom located at its central position, therefore making it a metalloporphyrin as well.2 Porphyrins are very biologically relevant molecules because they have the ability to perform very diverse functions.2 For example, they can act as intermediates in the transfer of electrons from oxidizing to reducing agents, they can convert light energy into chemical energy during photosynthesis, they can transport oxygen from air to the cells and they have the ability to play a number of roles in various biosynthetic pathways.2 In order for a compound to be considered aromatic, it must adhere to all of Huckle’s rules.3 Therefore, it must be cyclic, planar, fully conjugated and contain 4n+2 pi

electrons (where n = any integer).3 Looking at the structure of tetraphenylporphyrin, this experiments desired product, it is evident that the structure is cyclic, planar and fully conjugated. Therefore, they only rule left to satisfy is the 4n+2 pi electron rule. After counting the double bonds, which contain 2 pi electrons each, it is evident that tetraphenylporphyrin contains 46 pi electrons. If n=11, the 4n+2 pi electron rule states that there should be 46 pi electrons. Therefore, it can be concluded that tetraphenylporphyrin is in fact an aromatic compound because it is cyclic, planar, fully conjugated, and its number of pi electrons adhere to the 4n+2 rule, giving 4(11)+2=46 pi electrons. In this experiment, pyrrole reacts with benzaldehyde, in the presence of a glacial acetic acid catalyst, to produce meso-tetraphenylporphyrin via an aromatic electrophilic substitution reaction.2 As illustrated in the reaction mechanism (figure 1), pyrrole acts as the nucleophile to attack the elctrophile, which is benzaldehyde.3 This reaction occurs for each mol of benzaldehyde and pyrrole, which is 4 and 4 respectively, to produce 1 mesotetraphenylporphyrin molecule.2

Benzaldehyde

Pyrrole

meso-Tetraphenylporphyrin

Figure 1: Mechanism for Synthesis of meso-Tetraphenylporphyrin

** Other side products will occur if the mol ratio is off. For example, if there are 3 mols of pyrrole and 2 mols of benzaldhyde instead of the 4:4 ratio, then an undesired product would form.** Figure 2: Possible Side Product Reaction Mechanisms

Experimental Section: 1.4 mL of pyrrole, 2.0 mL of benzaldehyde, and 40 mL of glacial acetic acid were added to a round bottom flask. A drop was placed on a TLC plate. Boiling stones were added to the flask and the flask was attached to a reflux condensor. The solution was refluxed for 1 hour. After refluxing, the solution was cooled to room temperature and a dot was placed on the TLC plate.

The contents were allowed to cool to room temperature and the mixture was transferred to a separatory funnel. The round bottom flask was washed with dichloromethane and this was transferred to the separatory funnel as well. The dichloromethane (bottom layer) was extracted in a beaker. The dichloromethane layer was washed several times with DI water and dried with anhydrorous sodium sulfate.

The solution was decanted into a vacuum Erlenmyer flask and dried via vacuum evaporation. The weight of the product was determined and the percent yield was calculated.

The TLC was placed in the TLC chamber containing a dichloromethane solvent and allowed to run. The Rf values of both were recorded. The round bottom flask was placed in an ice bath and dark purple crystals were allowed to form. The crystals were filtered using vacuum filtration and washed with cold methanol to remove the boiling stones. After the crystals were dried, they were weighed and the percent yield was calculated.

While the above solution was refluxing, 0.021 g of tetraphenylporphyrin was added to a new 100 mL volumetric flask. The remaining space in the flask was filled with glacial acetic acid, until the fill line was reached. 10 mL of this solution was removed from the volumetric flask and transferred to a 25 mL round bottom flask with 0.21 g of copper(II)acetate. . Boiling stones were added to the flask and the contents were refluxed for 5 minutes.

Chemicals Used: Name of Chemical IUPAC Name Formula Molar Mass Melting Point Boiling Point Density Safety

Pyrrole Pyrrole C4H5N 67.091 g/mol -23C 129C 0.967 g/cm3 > Skin/eye irritant > Skin permeator > Severe overexposure can lead to death > Flammable

Benzaldehyde Benzaldehyde C7H6O 106.124 g/mol -57.12C 178.7C 1.044 g/cm3 > Skin/eye irritant > Skin permeator > Combustibile

Chemical Structure

Table 1: Table of Chemicals – Starting Material and Reagent Name of Chemical IUPAC Name Formula Molar Mass Melting Point Boiling Point Density Safety

Chemical Structure

Acetic Acid Acetic acid CH3COOH 60.05 g/mol 16.6C 117.9C 1.05 g/cm3 > Skin/eye irritant > Corrosive to skin/eyes > Skin permeator > Flammable

Meso-Tetraphenylporphyrin Meso-tetraphenylporphyrin C44H30N4 614.752 g/mol 300C N/A 1.27 g/cm3 > Skin/eye irritant

Table 2: Table of Chemicals – Catalyst and Product

Results: Crystals appeared fine, purple and shiny Appearance of Tetraphenylporphyrin Product Mass of Tetraphenlporphyrin 0.15 g Crystals 5% Percent Yield of Tetraphenylporphyrin 0.75 Rf of Tetraphenylporphyrin (initial) Rf of Tetraphenylporphyrin (1 0.5; 0.55; 0.575; 0.625; 0.675; 0.75; 0.875; 1.0 ** See Figure 3 – there are several bands hr reflux) 0.07 g Mass of Copper Complexed Tetraphenylporphyrin Percent Yield of Copper 304.48% Complexed Tetraphenylporphyrin Table 3: Data Collected from the Tetraphenylporphyrin and the Metalloporphyrin

Figure 3: TLC Plate Used to Calculate Rf Values of Tetraphenylporphyrin CALCULATIONS: 1.4 mL pyrrole x 0.967 g = 1.3538 g pyrrole 1 mL 2.0 mL benzaldehyde x 1.044 g = 2.088 g pyrrole 1 mL

Calculation1: Converting mL of Reagents to g

1.3538 g pyrrole x 1 mol pyrrole x 1 mol product = 0.00504 mol product 67.091 g/mol pyrrole 4 mol pyrrole

2.088 g benzaldehyde x 1 mol benzaldehyde x 1 mol product = 0.00492 mol product 106.124 g/mol benzaldehyde 4 mol benzaldehyde

*Therefore, benzaldehyde the limiting reagent Calculation 2: Determining the Limiting Reagent of Tetraphenylporphyrin Product

0.00492 mol product x 614.752 g/mol product = 3.024 g product 1 mol product

Calculation 3: Determining Theoretical Yield of Tetraphenylporphyrin

% Yield = Actual Yield x 100% = 0.15 g product x 100% = 5.0% Theoretical Yield 3.024 g product Calculation 4: Determining % Yield Of Tetraphenylporphyrin Product

0.021 g tetraphenylporphyrin x 1 mol tetraphenylporphyrin x 1 mol metallo. = 0.000034 mol metal. 614.752 g/mol tetraphenylporphryin 1mol tetraphenylporphyrin

0.21 g copper x 1 mol copper 181.63 g/mol copper

x 1 mol metallo. = 0.001156 mol product 1 mol copper

*Therefore, tetraphenylporphyrin is the limiting reagent Calculation 5: Determining the Limiting Reagent of Metalloporphyrin Product

0.000034 mol product x 676.282 g/mol product = 0.02299 g product 1 mol product

Calculation 6: Determining Theoretical Yield of Metalloporphyrin Product

% Yield = Actual Yield x 100% = 0.07 g product x 100% = 304.48% Theoretical Yield 0.02299 g product Calculation 6: Determining % Yield Of Metalloporphyrin

Rf of Initial = Distance Spot Travels = 3 cm = 0.75 Distance Solvent Travels 4 cm Calculation 7: Determining Rf of Initial Tetraphenylporphyrin Product

Rf of Refluxed Product = Distance Spot Travels = 2 cm = 0.5 Distance Solvent Travels 4 cm = Distance Spot Travels = 2.2 cm = 0.55 Distance Solvent Travels 4 cm = Distance Spot Travels = 2.3 cm = 0.575 Distance Solvent Travels 4 cm = Distance Spot Travels = 2.5 cm = 0.625 Distance Solvent Travels 4 cm

= Distance Spot Travels = 2.7 cm = 0.675 Distance Solvent Travels 4 cm = Distance Spot Travels = 3.0 cm = 0.75 Distance Solvent Travels 4 cm = Distance Spot Travels = 3.5 cm = 0.875 Distance Solvent Travels 4 cm = Distance Spot Travels = 4.0 cm = 1.0 Distance Solvent Travels 4 cm Calculation 8: Determining Rf of Tetraphenylporphyrin After Reflux

Discussion: Throughout the experiment, the supposed tetraphenylporphyrin product was dotted onto the TLC plate initially and after an hour of reflux. Once the TLC was run in a dichloromethane solvent, which is polar, the initial Rf was found to be 0.75. However, after refluxing the solution for one hour and dotting the contents on the TLC plate, many bands formed on the TLC plates and many Rf values were obtained (as seen in figure 3). The determined Rf values were 0.5, 0.55, 0.575, 0.625, 0.675, 0.75, 0.875 and 1.0. Benzaldehyde is polar molecule and dichloromethane is a slightly polar solvent, thus it can be ascertained that benzaldehyde will move fairly high up the TLC plate. Tetraphenylporphyrin is initially thought to be a non-polar substance because of the N atoms in its center. However, its structure is complicated by the polarity of its substitutent groups. Therefore, because tetraphenylporphyrin has more polar substitutents than benzaldehyde it can be ascertained that the Rf value of tetraphenylporphyrin will ben higher than that of benzaldehyde. This is proven by the obtained Rf values; 0.75 for the initial dot and, the largest Rf value, 1.0 for the tetraphenylporphyrin product. The other Rf values from the various bands on the TLC plate could be indicative of other products that may have formed during the experiment. At the end of the first part of the experiment, a percentage yield was calculated for the tetraphenylporphyrin product. According to the calculations above, benzaldehyde was the limiting reagent and gave a theoretical yield of 3.024 g of tetraphenylporphyrin. However, only 0.15 g of the supposed product, tetraphenylporphyrin was obtained, thus resulting in a percent yield of about 5%. Although this percent yield is extremely low,

according to the literature it is a common occurrence during an experiment of this type. This low percent yield is most likely due to the occurrence of side products, resulting from the reaction of the incorrect number of mole of pyrrole reacting with the incorrect number of moles of benzaldehyde. However, it is also possible that some of the product was lost throughout the experiment, such as when transferring the product from one piece of glassware to another or when measuring out initial reagents. At the end of the second part of the experiment, a percentage yield was calculated for the metalloporphyrin, copper complexed tetraphenylporphyrin. According to the calculations above, tetraphenylporphyrin was the limiting reagent and gave a theoretical yield of 0.02299 g copper complexed tetraphenylporphyrin. However, the actual obtained weight was 0.07 g, therefore giving a percent yield of 304.48%. This percent yield is an extremely inaccurate percent yield and is significantly greater than 100%. The percent yield for this product is likely this high because it the solvent was not sufficiently evaporated from the product, thus giving an inaccurate weight. It is important to perform TLC throughout the experiment because it is an effective and inexpensive way to ensure that the reaction is proceeding completely. TLC allows for Rf values of compounds to be determined. An experimentor can then compare the Rf values of initial products and final products to determine if the reaction proceeded correctly and if the desired product was formed. Rf values will vary depending on the polarity of the compounds and whether they will be moved further up the TLC plate by the TLC solvent.

Conclusion: The information from this data revealed that the synthesis of tetraphenylporphyrin from pyrrole and benzaldehyde is in efficient because the percent yield of the product is typically extremely low. Moreover, this experiment also revealed that the use of TLC plates is an effective way to monitor whether or not a reaction is proceeding correctly, as well as whether the desired product has been obtained. The information learned throughout this experiment is important because it has many real world applications. For example, porphyrins and their derivatives are imperative to many biological functions required to sustain life.4 However, porphyrins and their biomedical applications are also currently being explored in the context of photodynamic therapy. 4 Metalloporphyrins, similar to the one synthesized in this experiment, are very suitable for medical imaging and therapy.4 Moreover, the use of metalloporphyrins in these photodynamic therapies often leads to more efficient phototherapies and potentially decreases the severity of side effects.4 Overall, despite the percent yield being extremely low for the tetraphenylporphyrin and then extremely high for the metalloporphyrin, this experiment did in fact accomplish what it set out to do. This experiment was an excellent example of how aromatic electrophilic substitution could be used to synthesize a porphyrin and then how this porphyrin could become a metalloporphyrin by reacting with copper(II)acetate. Moreover, this experiment also did a good job of reiterating how running a TLC is an inexpensive and effective way to determine whether the reaction is proceeding correctly or that the desired product has formed. These skills can then be applied to understand

concepts outside of the lab, for example when analyzing certain biological functions, and be used innovatively, like how porphyrin is now being used in phototherapies.

References: [1] Chemistry of Porphyrin. org-chem.org. Accessed March 29, 2018.

[2] Wildegirma, S. Experimental Organic Chemistry Lab Manual; University of South Florida: Tampa, FL, 2016; P. 92-95 [3] Porphyrins: Their Biological and Chemical Importance. JAMA. 1955;157(16):1454. doi:10.1001/jama.1955.02950330094033 [4] Huang, H., Song, W., Rieffel, J., and Lovell, J. F. (2015) Emerging applications of porphyrins in photomedicine. Frontiers in Physics 3. Accessed March 29, 2018....