Experiment 10 PDF

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Experiment 10 OCHEM 1
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The Diels (been) Alder’d: a Reaction

Introduction: This lab will focus on performing a Diels-Alder reaction. Diels-Alder reactions are characterized as a [4+2] cycloaddition. This means the reaction will occur by the combination of two pi systems: one of a conjugated diene, which contains four pi electrons, and one of a dienophile, which only has two pi electrons. Overall the reaction creates a different six membered ring. The entire reaction will occur in a one concerted step, so there will not be any intermediates formed at any point in the reaction. This can be seen in the general Deils-Alder reaction below:

A retro Diels-Alder reaction is a microscopic reverse (identical reverse) of a normal Diels-alder reaction. The reaction will require a heat input to start from room temperature due to room temperature being more favorable to cycloaddition and high temperatures being favorable the retro Diels-Alder reaction. In order to carry out the reaction most efficiently, a dienophile with multiple electron withdrawing groups adjacent to the alkene/alkyne will be used. The diene must conform to a s-cis geometry in order to react successfully. The stereospecificity of the products in this reaction is strict because the stereochemistry of an alkene dienophile is maintained throughout the reaction. In addition, the use of electron donating groups on the diene will aid in the efficiency and speed of the reaction. Using an alkyne as the dienophile yields a six membered ring with two isolated carbon-carbon double bonds as product. Lastly, it is important to note cyclopentadiene (our starting compound) at room temperature will slowly react with itself to form dicyclopentadiene via a Diels-Alder reaction. The compound can also be reversed back (cracked, as seen below) to cyclopentadiene by heating at 170  C. In this lab we will be given one of four different dienophiles to react with the cyclopentadiene to undergo the Diels-Alder reaction, we will then use IR and ¹H NMR to determine which of the four dienophiles we used in the lab.

The Diels (been) Alder’d: a Reaction **Individual Balanced Equations and Mechanisms on Separate Page** Reagent Table- Physical Data:

Compound

Dicyclopentadiene Cyclopentadiene Dimethyl acetylenedicarboxylat e (DMAD) Diethyl acetylenedicarboxylat e (DEAD) Diisopropyl acetylenedicarboxylat e (DIAD) Dipropyl acetylenedicarboxylat e (DPAD)

Molecular Weight (g/mol) 132.2 66.1 142.11

Boiling Point ( ℃)

Melting Point ( ℃)

Density (g/mL)

170 40 195-198

32.5 ~~ ~~

~~ 0.786 1.16

170.16

107-110

~~

1.063

198.22

273.106

~~

~~

198.22

285.8

~~

~~

Reagent Table- Safety & Structure Info:

Compound Dicyclopentadiene

Structure

Safety ● Flammable liquid and vapor ● Harmful if swallowed ● Causes skin irritation

Cyclopentadiene

● Flammable liquid and vapor ● Harmful if swallowed ● Causes skin and serious eye irritation ● Fatal if inhaled

The Diels (been) Alder’d: a Reaction Dimethyl acetylenedicarboxylate

● May cause skin, eye, or respiratory irritation ● Flammable

Diethyl acetylenedicarboxylate

●

Causes eye burns

●

Causes skin burns

●

Causes chemical burns to the respiratory tract

● Keep away from sources of ignition Diisopropyl acetylenedicarboxylate

● Causes skin and eye burns ● Causes chemical burns to the respiratory system ● Combustible liquid and vapor

Dipropyl acetylenedicarboxylate

● Harmful if inhaled or swallowed ● Causes skin and eye irritation

Additional Safety Information: ● General safety guidelines should be followed: Use caution around glassware, chemicals, and toxic, caustic liquids. ● Avoid contact or inhalation of all hazardous or toxic chemicals; wear appropriate protective wear (gloves, lab coat,safety glasses), as well as avoid all sparks, flames, or hot surfaces, especially when handling flammable substances. ○ All exposure should be reported to TA ● Again, all compounds used in this experiment are flammable and toxic

Procedure: Retro Diels-Alder Reaction:

The Diels (been) Alder’d: a Reaction 1. Add ~300 mL of dicyclopentadiene to a 500 mL microwave round bottom flask. a. Put two microwave absorbing chips and a stir bar in the flask. 2. Attach the fiber optic thermodetector to the side arm. Construct a distillation apparatus set-up through the microwave as shown by the diagram below 3. Set microwave program to the temperature profile. 4. Start the programs and collect freshly cracked cyclopentadiene round bottom flask, sitting in an ice bath. a. Ramp for 10 minutes until reactant temperature reaches 168 °C. Temperature should remain at 168 °C for 30 minutes. Forward Diels-Alder Reaction: 1. Add 1.5 mL of the dienophile assigned by TA, plus a spin vane to 10 mL round bottom flask on an aluminum block. 2. Add 2.0 mL cold cyclopentadiene, dropwise into the flask while stirring a. **Do within 30 seconds. 3. Put lid on loosely and allow solution to stir for 20 minutes at room temperature. 4. Take the lid off the round bottom flask. 5. Turn heat up to 140 °C to boil off excess cyclopentadiene while continuing to stir a. Do NOT exceed 150 °C or product will decompose b. This process should not take longer than 25 minutes. 6. Obtain an IR and HNMR spectrum of the final product

Balanced Equations

The Diels (been) Alder’d: a Reaction

Mechanisms

The Diels (been) Alder’d: a Reaction

The Diels (been) Alder’d: a Reaction

Post Lab Write-Up Data & Analysis:

Data Type

Value

Unknown Dienophile

X493Q278

Initial Volume of Dienophile

1.50 mL

Initial Volume of Cyclopentadiene

2.00 mL

Final Weight of Product

2.049 g

Time Spent Boiling off Starting Material

~25 min

IR and HNMR Interpretations

The Diels (been) Alder’d: a Reaction In reality, the IR spectrum obtained from this lab was not very useful for identifying the unknown compound. The pictured spectrum was primarily used to exclude the possibility of the unknown containing an isopropoxy or methoxy substituent, but beyond that, its main function was to confirm the isolated product was one of those predicted by the reaction mechanisms. Firstly, although the strong band at ~1704 cm-1 is characteristic of C=O stretching in an aldehyde or some sort of acid (carboxylic or other), it is more likely to be that the bicyclic alkene ring stabilized the carboxyl group, causing a lower wavenumber that typically expected by an amide (~1680-1690). None of the unknowns contained such an aldehyde or acid group, and the lack of C-H stretching of an aldehyde at 2830-2695 cm-1 as well as the missing broad carboxylic acid stretch, suggest it is an ester. The fact that the compound does contain an ethoxy group is confirmed by the presence of the strong C-O stretch at about 1233 cm-1 as well as the series of sp3 C-H stretches around 2800-2900 cm-1 (suggesting a carbon chain). The presence of the chain stretches rule out the possibility of the compound containing an isopropoxy or methoxy group. Lastly, the C=C double bonds in the bicycloalkene are shown at the ~1625 cm-1 stretch even though sp2 C-H stretches are not seen in the IR. Total confirmation of the compound requires further analysis of the HNMR spectrum.

Working from right to left, the first band shown (A) contains 6 H’s and three peaks, suggesting two methyl groups. The next band seems to be a single quartet (D) containing 2 H’s, but it is actually two separate bands that merged together. With a higher resolution NMR machine, it may be possible to distinguish this. The pattern is caused by the unique position that the two hydrogens are in. Being at the top of the bicyclic ring (that all of the possible products

The Diels (been) Alder’d: a Reaction contain), each hydrogen is in a slightly different environment, with one theoretically being harder to vibrate than the other. Therefore the band should show two identical peaking patterns (one slightly downfield of the other) with a high resolution spectrum. A single peak is seen next (C). This is most likely vibration from the two center hydrogens of the bicyclic ring, although the expected, more “complex”, expected splitting is not seen. The single broad peaking could be assumed to be a product of the relatively low resolution of the spectrum. To continue, the quartet (B) containing four H’s (~4.15 ppm) confirms prior speculations of the compound containing two methyl groups. This splitting is characteristic of four CH2 hydrogens being split by three surrounding hydrogens (now certainly from methyl groups). Lastly there is a broad singlet at about 7 ppm. The band is created by the last two hydrogens labeled “E” and closely resembles the pattern of the upfield singlet labeled “C”. So, even though the spectrum shows a singlet, it would be reasonable to assume a higher resolution spectrum would show a doublet due to the hydrogens being surrounded by one other “different” hydrogen each. With this information combined with the IR spectrum above, it is possible to identify the unknown product as diethyl bicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate and the unknown starting dienophile as DEAD (abbreviated). Data Type

Value

Limiting Reactant

DEAD

Theoretical Yield

2.21g Product

Percent Yield

47.5%

Limiting Reactant/ Theoretical Yield Calculations **All reactions occur in a 1:1 reactant:product ratio** Using Dienophile:

1.50 mL DEAD

1.063 g DEAD ∗mol DEAD mL ∗1 mol Product 170.16 g ∗236.26 g 1 mol DEAD =2.21 g Product mol Product

Using Diene: 2.00 mL Cyclopentadiene

The Diels (been) Alder’d: a Reaction 0.786 g Cyclopentadiene ∗mol Cyclopentadiene mL ∗1mol Product 66.1 g ∗236.26 g 1 mol Cyclopentadiene =5.62 g Product mol Product The dienophile, DEAD, is the limiting reactant in this experiment, so the theoretical yield of product is 2.21 g.

Percent Yield Calculation Percent Yield ¿

Actual Yield 2.05 g ∗100=92.8 % ∗100= 2.21 g Theoretical Yield

Discussion/Conclusion: The percent yield of the product was approximately 92.8% which is high and a reasonable result in this particular experiment. This is because there are very few steps taken to get the product, which (presumably) would naturally decrease the chances of errors. As there was some loss of product, one possible source of error would be during the transfer of the dienophile from the graduated cylinder to round bottom flask. A little of the dienophile may have been left in the graduated cylinder, which means that we didn’t precisely start off with 1.50 mL of DEAD but just slightly less than that. The small difference starting material would account for the minor overall loss in product. Other sources of error could be attributed to human error. One of which would be not adding cyclopentadiene to the dienophile in under 30 seconds (the specific consequences of which are unknown beyond intervening with the reaction), and another may be that the solution was over-heated at the evaporation stage. In this case, a little of the product may have decomposed during heating. In future tests, it would be best to add exactly 1.50 mL of dienophile directly to a measuring container that could also be used to complete the reaction. In addition, using a thermometer during the evaporation stage would help to prevent overheating of the solution and decomposition of product. Overall, the experiment was successful on three accounts: one was the percent yield being relatively high; another was that both spectra (IR and HNMR) being reasonably legible and sensible in their readings; and the last was that our unknown starting dienophile was identified and the subsequent product isolated and collected....