What is the - Purpose for this experiment and why is it a greener experiment com

Question

What is the

- Purpose for this experiment and why is it a greener experiment compared to traditional ?

- Theoretical yield and atom economy

Compare the “greenness’ of this procedure with a more conventional synthesis using reflux.

I have copied everything I have for this lab.

Diels-Alder/Retro-Diels-Alder Synthesis Via Microwave Irradiation of Diethyl Phthalate

Chemical Concepts:

Diels-Alder reactions; microwave irradiation; reflux; column chromatography; thin-layer chromatography (TLC); rotary evaporation; 1H NMR spectroscopy

Green Lessons:

Energy efficiency; atom economy; solventless reaction; microwave heating of reaction mixture; alternative heat sources

Introduction:

Diels-Alder reactions are one of many thermal cycloaddition reactions studied in organic chemistry. Cycloaddition reactions are pericyclic and involve two or more reactants coming together, often in a concerted step. These reactions are useful as they easily form multiple new bonds and six- membered ring, with good stereochemical control. At temperatures above 1000K, diradicals are also formed in small amounts. In this experiment, you will have the opportunity to perform a cycloaddition reaction that takes place in a microwave, a new method researchers are beginning to focus their attention towards. Instead of performing a reaction at high temperatures using reflux, you will use microwave irradiation to provide the energy to carry out the reaction.

Throughout this experiment, you will gain experience in the reaction chemistry of alkynes and cycloalkenes as well as gain proficiency at using multiple techniques to identify your product. You will also have the opportunity to consider the energy usage differences between reflux and the rapidly developing microwave irradiation. In the course of your investigations, you will also gain experience in the laboratory techniques such as column chromatography, thin-layer chromatography, rotary evaporation, and microwave irradiation.

Diels-Alder reactions are referred to as [4+2] cycloaddition reactions, where the numbers refer to the number of pi electrons involved in the reaction. Two pi bonds are broken in the “diene” reactant

and one pi bond is broken in the “dienophile”, while a new pi bond and two sigma bonds are formed [Figure 1].

The simplest Diels-Alder reaction results in four stereocenters. In a Diels-Alder reaction involving a ring as the starting material, a bridge is formed in the product.

In a variety of Diels-Alder reactions, the product formed is able to break down into a more stable compound, known as a retro-Diels-Alder reaction (cycloreversion). The bicyclic ring in this reaction immediately breaks down, forming an aromatic compound that is greatly favored. The charge and pi electrons in the ring are delocalized through resonance, resulting in energy being spread over a larger area rather than confined to a small area.

This experiment highlights a number of green chemical concepts ranging from solvent-free synthesis to an energy efficient alternate method of heating.

Reaction:

1.) In a 5mL glass vial, weigh out 1,3-cyclohexadiene (0.12 g) and DEAD (0.225 g). Immediately cap the vial. Note that 1,3-cyclohexadiene has a very high vapor pressure.

2.) Irradiate two vials at a time in the microwave (1100 watts) for 5 minutes (this prevents burning). Place the vials in ice in between pulses. Repeat for an additional 5 minutes. If reaction is colored red, do not irradiate for an additional 5 minutes.

3.) Prepare dilute samples of pure diethyl phthalate and DEAD reactant, along with a column while reaction is cooling. Add 8 ?L of DEAD to 0.5 mL of acetone, and 4 ?L of pure diethyl phthalate to 0.5 mL of acetone.

4.) Add 4 ?L of the reaction mixture (from vial) to 0.5 mL of acetone. Save these dilutions for TLC spotting.

5.) Add 1 mL of acetone to a GC/MS vial and 2 ?L of the reaction mixture. Set up a sequence using ASD_NA_FID_MSD method on the GC/MS and let it run. Each test takes ~15 minutes.

Thin Layer Chromatography (TLC)

6.) On a TLC plate, spot DEAD, pure diethyl phthalate, and the reaction mixture dilutions. Make a 1:2 ethyl acetate/heptane mixture to run the plate until the solvent line is 1-2 cm away from the top of the plate. Check your TLC plate under the UV light. Spots indicating product and starting material should be present. An additional spot below the product indicates intermediate. Any other spots are likely to result from side reactions.

7.) Place all reaction vials on a hot plate for 3 minutes at ~100oC. Once the reaction has cooled, dilute 4 ?L of it in 0.5 mL of acetone. Run the TLC plate in the solvent again and check it under the UV light. The intermediate spot should have disappeared if there was one initially.

Purification – Column Chromatography:

8.) Prepare a silica gel column in a large chromatography column (25 mm inside diameter). Insert a small amount of glass wool into the bottom of the column to cover the opening. This stops sand from flowing through the hole. Gently add sand of about 1cm in depth.

9.) Prepare a slurry of 20 g of silica gel in 60 mL of 1:2 ethyl acetate/heptane. Swirl the flask to make sure the silica gel is thoroughly suspended in the solvent. Then quickly and carefully pour the suspension into the column. Open the stopcock and allow the silica gel to settle and the solvent to drain. Collect the solvent in the same beaker and rinse any remaining silica gel into the column. Collect the solvent in a clean beaker and using a pipet, rinse the column of silica gel.

10.) When 2 inches of solvent is remaining on top of the packaged silica gel, close the stopcock and evenly add 1 cm layer of sand on top to protect the top surface of the column.

11.) Drain the solvent down until the top layer of the sand becomes dry. Close the stopcock and using a 9 inch pipet, add the entire solution of the reaction mixture on top of the sand. Put the vial aside for now. Prepare a new solvent solution of 90 mL of 1:2 ethyl acetate/heptane. This will be used to move the reaction sample through the silica gel and separate the compounds.

12.) Use a clean pipet and add a small amount of the solvent into the reaction vial to rinse out any remaining reaction mixture. Pipet the mixture on top of the sand layer and open the stopcock for a few seconds to let the reaction mixture reach the top of the silica gel.

13.) Close the stopcock and carefully pipet the solvent on top, careful to not disturb the surface. Once the solvent is 5 cm above the sand, carefully pour the remaining solvent against the column wall to avoid disturbance.

14.) Run the column at maximum flow rate and collect the first 35 mL of solvent in a beaker. This should be clean solvent, although it is a good idea to include it in TLC testing. Label 50 test tubes and collect 1.0 mL of solvent in each.

Thin Layer Chromatography (TLC)

15.) Cut 2 TLC plates with dimensions ~9 cm (height) by 11 cm (length). Use the same capillary tube to spot the fractions. After each fraction, dip the capillary tube in a small beaker filled with acetone and flip it upside down. Hold it on a Kimwipe to drain the liquid and repeat once more.

16.) Spot every 5th fractions on the TLC plate and spot each fraction at least 5 times (as it is diluted in the solvent). This is to narrow down the range of test tubes that contain compounds. There should be a total of ~10-12 fraction spots on the plate. At one end of the TLC plates, spot the diluted pure product and DEAD mixtures made as reference points (total of ~12-14 spots).

17.) Make a new solvent mixture of 1:2 ethyl acetate/heptane. Run the TLC plates in the solvent mixture until the solvent line reaches 1 cm below the top of the TLC plate. Mark the observed spots with a soft pencil and calculate their Rf values.

18.) Once you have determined fractions that obtain compounds, spot a few fractions before the first spot showing compound, and a few fractions after the last spot showing compound on the TLC.

19.) Rinse with acetone and pre-weigh a 25 mL round-bottom flask and combine all the fractions containing the same spots. You do not need to collect fractions containing DEAD, which will elute first from the column, as it is the least polar. DO NOT fill the flask more than half way full, as chance of bumping becomes more likely.

20.) Close the flask with a rubber septum and fold down the top of the septum. Save this flask for next lab sections.

Rotary Evaporation

21.) If a large volume of solvent has been collected, perform multiple rotary evaporations. Fill the chamber with dry ice and acetone. Attach the flask to the rotary evaporator and lower into warm water. Evaporate for 5 minutes until all of the solvent has been removed. Record final mass of flask and determine product yield.

Characterization – Gas Chromatography & 1H NMR:

22.) Prepare your sample for gas chromatography to verify purity. For the entire class, add 2 ?L of pure product in 1.0 mL of acetone in a GC/MS vial. Students should add 2 ?L of their purified product to 1.0 mL of acetone in another GC/MS vial. Set up a sequence and let it run while NMR samples are obtained.

23.) Add 0.8mL of deuterated chloroform (CDCl3) and 1 drop of tetramethylsilane (TMS) into the flask containing pure product. Swirl the contents and transfer the solution into a clean NMR tube with a pipet. Obtain the spectrum of the product.

Explanation / Answer

1. The method used for the diels alder reaction here is greener in terms of less waste generated after the process. The mthod used no solvent for the reaction which makes it more eco-friendly and green process. On the other hand, the traditional method of running this reaction involves reflux condition in an organic solvent. The solvent is discarded as a waste once the reaction is completed and product is separated. This thus generates a lot of waste and makes the traditional process not green in nature.

2. Theoretical yield = (0.12 g/80.13 g/mol) x 202.37 g/mol = 0.30 g

Atom eonomy, 1 mole of cyclohexadiene would produce 1 mole of diels alder product

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