Forward and Retro Diels PDF

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Ochem 1 Lab...

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Forward and Retro Diels-Alder Reaction Introduction This experiment involves both the retro Diels-Alder reaction and the reverse of that which is the forward Diels-Alder reaction. The Diels-Alder reaction is a [4+2] cycloaddition where a six-membered ring is formed by combining a pi system of a conjugated diene (four pi electrons) with the pi system of a dienophile (two pi electrons). The retro Diels-Alder reaction is the microscopic reverse of the Diels-Alder reaction and heat is required to initiate the reaction with moderate temperature favoring cycloaddition and high temperatures favoring the retro Diels-Alder. One or more electron withdrawing groups adjacent to the alkene or alkyne of the dienophile results in a faster reaction and higher yield. The diene should conform to a s-cis geometry for it to successfully react. When an electron donating group is added to the diene it helps to increase the reaction rate and efficiency. The stereospecificity of the products is strict because the stereochemistry of an alkene dienophile is conserved over the course of the reaction. An alkyne (dienophile) will result in a six-membered ring product with two isolated carboncarbon double bonds. Cyclopentadiene has a strong and unpleasant odor. At room temperature, cyclopentadiene reacts with itself and dimerizes to form the dimer dicyclopentadiene via a DielsAlder reaction. Dicyclopentadiene can be cracked by heating at 170 °C to form the monomer cyclopentadiene. One of the four dienophiles will be given to react with cyclopentadiene to form a bicyclic Diels-Alder product. FTIR and 1HNMR spectroscopies will help determine the identity of the unknown dienophile. The new technique used will be microwave distillation and the previous techniques used are reflux, FTIR spectroscopy, and H NMR spectroscopy. Chemical Equation

Table of Reagents MW (g/mol )

MP (°C)

BP (°C)

Densit y (g/mL)

Dicyclopentadiene

132.2

32-34

170

-

Cyclopentadiene

66.1

-

40

0.786

Dimethyl acetylenedicarboxylate

142.11

-

195-198

1.16

Diethyl Acetylenedicarboxylat e

170.16

-

107-110

1.063

Dipropyl acetylenedicarboxylate

198.22

-

285.8

-

Diisopropyl acetylenedicarboxylate

198.22

-

273.106

-

Compound

Structure

Safety 

Prevent cyclopentadiene from coming in contact with sparks, flames, or hot surfaces

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Prevent inhaling the strong and unpleasant odor of cyclopentadiene Prevent exposure of dicyclopentadiene to eyes; can cause severe irritation. Dicyclopentadiene - hazardous in case of skin contact (irritant), eye contact (irritant), ingestion, inhalation. Cyclopentadiene - Toxic by ingestion, inhalation or through skin contact. Dimethyl acetylenedicarboxylate - hazardous in case of skin contact (irritant), eye contact (irritant), ingestion, inhalation. Liquid or spray mist may produce tissue damage particularly on mucous membranes of eyes, mouth and respiratory tract. Skin contact may produce burns. Diethyl acetylenedicarboxylate - Causes severe skin burns and eye damage. It may be harmful if inhaled. Dipropyl acetylenedicarboxylate - Hazardous in case of skin contact (irritant), eye contact (irritant), ingestion, inhalation. Diisopropyl acetylenedicarboxylate - Hazardous in case of skin contact (irritant), eye contact (irritant), ingestion, inhalation. Make sure snorkels re working properly before dispensing any liquid. Do not breath in any fumes or allow contact with skin. Report any vapor or liquid exposure to the TA immediately. Dispose all liquid and solid waste in appropriate labeled bottle in lab hood. Wear safety glasses, gloves, and lab coat at all times.

Procedure: Retro Diels-Alder reaction    

Add approximately 300 mL of dicyclopentadiene to a 500 mL microwave round bottom flask. Place two microwave absorbing chips and a stir bar into the flask Attach the fiber optic thermodetector to the side arm. Construct a distillation apparatus set-up through the microwave as shown below.

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Set up the microwave program to the temperature profile shown below.

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Start the program and collect the cracked cyclopentadiene in a round bottom flask in the ice bath. Increase the temperature for 10 minutes until the reactant temperature reaches 168 °C. The temperature will remain constant at 168 °C for 30 minutes.

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Procedure: Forward Diels-Alder reaction       

Add 1.50 mL of the assigned dienophile and a spin vane into a 10 mL round bottom flask on an aluminum block. Add 2.00 mL of cold cyclopentadiene dropwise into a flask while stirring within 30 seconds. Put the microscale lid with the rubber septum on loosely. Allow the solution to stir for 20 minutes at room temperature. Take the microscale lid off of the round bottom flask. Turn the temperature to 140 °C and boil off the excess cyclopentadiene while continuing to stir. Don’t exceed 150 °C or you will decompose your product. This process should not take more than 25 minutes. Obtain both FTIR and 1HNMR spectrums of your final product, with the assistance of your TA.

Forward and Retro Diels-Alder Reaction Results     

Unknown dienophile code: Q275MP92 Initial volume of dienophile: 1.5 mL Initial volume of cyclopentadiene: 2.0 mL Final weight of Diels-Alder product: 2.026 g Time spend boiling off starting material: 23 minutes

Calculation Mass in grams DPAD = 1.50 mL x 1.078g = 1.617 g DPAD 1.0 mL

Cyclopentadiene = 2.00 mL x 0.0786 g = 1.57 g cyclopentadiene 1.0 mL Limiting Reagent 1.57 g cyclopentadiene x 1 mol cyclopentadiene x 1 mol of product = 0.0238 mol of product 66.1 g cyclopentadiene 1 mol cyclopentadiene 1.617g DPAD x 1 mol DPAD x 1 mol of product = 0.0082 mol of product 198.22 g DPAD 1 mol DPAD DPAD is the limiting reagent. Percent Yield Actual mass to moles = 2.026 g = 0.0077 moles 264.32 g Percent Yield = 0.0077 moles x 100 = 93.9% 0.0082 moles Discussion/Conclusion The IR spectra shows a C=C absorption band at 1622 cm-1 and C=O absorption band at 1700 cm-1. Also, it shows an aromatic band at 1700 cm-1 and an ester at 2979 cm-1. Finally, it shows the sp3 hybridization C-H bonds at 2940 cm-1 and sp2 hybridization hydrogen at 2872 cm-1. However, IR was not that helpful since all the products had the same functional groups. The HNMR spectrum shows the TMS signal at 0 ppm. The ratio of the integral values is 2:4:2:2:4:6, so this means that there are 20 hydrogens presents in the molecule and the only product formed that has 20 hydrogens is DPAD and cyclopentadiene. Also, this H-NMR has 6 signals which is typical of the product of DPAD and cyclopentadiene. The spilling patterns for HNMR shows a sp3 hydrogen around 4.1 ppm which is a quartet. Also, it shows a doublet around 2.1 ppm which are the hydrogens inside the ring and two allylic hydrogens (doublet) at 3.9 ppm. The two vinylic hydrogens are shown as a singlet around 6.9 ppm and there’s a triplet around 1.0 ppm. The limiting reagent was DPAD (Dipropyl acetylenedicarboxylate) with 0.0082 mol of product. The percent yield of the product was 93.9% which is really good and reasonably high. This is reasonable because the experiment was done within very few steps so the chances for errors were really low as well....