Post lab 10 - Post lab 10 PDF

Preview

Summary

Post lab 10...

Description

Post-Lab 10: Retro and Forward Diels-Alder Reaction Results: Unknown Dienophile Code:

L427H872

Initial Volume of Dienophile:

1.50 mL

Initial Volume of Cyclopentadiene:

2.00 mL

Final Weight of Diels-Alder Product:

1.109 grams

Time spent boiling off starting material:

15 minutes

Unknown Dienophile:

Diisopropyl acetylenedicarboxylate (DIAD)

Percent Yield

Calculations: Finding the Limiting Reagent: (2.00 mL cyclopentadiene) x (0.805 g/mL cyclopentadiene) / (66.1 g/mol Cyclopentadiene) = 0.024 moles of cyclopentadiene (1.50 mL DIAD) x (1.074 g/mL DIAD) / (1 mol/198.22 g DIAD) = 0.0081 moles of DIAD *Diisopropyl acetylenedicarboxylate is the limiting reagent* Percent Yield: (0.081 mol Diels-Alder product) x (264.3 g/mol Diels-Alder product) = 2.14 g (theoretical yield) (actual yield/theoretical yield) x 100 = percent yield (1.109 g diels-alder product recovered) / (2.14 g theoretical diels-alder product) x 100 = 51.82 %

Discussion/Conclusion:

In this experiment, we performed a Diels-Alder reaction by using cyclopentadiene and an unknown dienophile as reagents. At the end of the experiment, we recovered 1.109 grams of the final product and obtained an H-NMR and IR spectrum to determine the identity of the dienophile used as our unknown. The four possible identities of the unknown dienophile used were dimethyl acetylenedicarboxylate (DMAD), diethyl acetylenedicarboxylate (DEAD), dipropyl acetylenedicarboxylate (DPAD), and diisopropyl acetylenedicarboxylate (DIAD). Because the four possible identities of the dienophile used were alkynes, we know that the

product should be a six membered ring with two isolated carbon-carbon double bonds. Therefore, all four dienophiles would yield a product with similar characteristics in the functional group region of the IR spectrum: a C=O stretch, a C=C stretch, and sp3 C-H bands. There would also be a C-O-O single bond stretch in the fingerprint region around 1200 cm -1 for each dienophile used because each dienophile produces an ether functional group in the product. However, ethers do not show any characteristic absorption bands in the functional group region. This was confirmed by our IR spectrum, for we can see the sp3 C-H bands around the 3000 cm-1 region, a C=O stretch around 1700 cm-1, a C=C stretch around 1600 cm-1, and a C-O single bond stretch around 1200 cm -1. Therefore, we cannot determine the identity of our unknown dienophile based on the presence of the absorption bands alone because all possible dienophiles would produce these same absorption bands. We used the H-NMR spectrum to identify the unknown dienophile used because it allowed us to distinguish between all three possible dienophiles used, while the IR did not. We determined that our dienophile was diisopropyl acetylenedicarboxylate (DIAD) because we saw the presence of two isopropyl groups in our product based on the H-NMR. In each isopropyl group, the hydrogen bonded to the carbon which bonds to the two methyl groups splits into a septet around 4.5 to 5.0 ppm because each methyl group has equivalent protons. The methyl hydrogens split into a doublet around 1.25 because all six methyl hydrogens are equivalent and they are bonded to a carbon with one equivalent hydrogen. The n+1 rule (1 neighboring unequivalent hydrogen +1) therefore allows us to say that our dienophile was diisoproyl acetylenedicarboxylate because only this dienophile would yield a product with a doublet near 1.0-1.5 ppm. The products of DEAD and DPAD would both yield products with methyl hydrogens bonded to a carbon with two non-equivalent hydrogens, not one. Therefore those methyl hydrogens would split into a triplet. The product obtained from DMAD would produce a singlet for the methyl hydrogens because the carbon which they are bonded to is not bonded to another carbon with non-equivalent hydrogens. Because our percent yield was 51.82 %, we know that we lost some product because it is very low. The low percent yield could be due to our rough estimation of when 2 mL of cyclopentadiene boiled off. We decided to watch the solution to estimate when about 1.5 mL of liquid remained in the flask. Therefore, we could have either under boiled or over boiled the solution when determining if about 2 mL of cyclopentadiene had evaporated. If we did not wait long enough for the cyclopentadiene to boil off, there would be impurities present in our product because the solution was under-boiled. In the future, we could pay close attention to the amount of liquid in the flask when we the solution begins to boil and try to more accurately determine how much liquid has boiled off throughout the process. Glassware with more precise volume markings every 1 mL would help us determine this with more precision and lead to a greater percent yield....