Hexaphenylbenzene Lab Report-2 PDF

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Synthesis of Hexaphenylbenzene Lab Report...

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Synthesis of Hexaphenylbenzene Purpose: The purpose of this experiment is to understand and perform a multistep parallel synthesis of hexaphenylbenzene Day One... Reaction 1: Synthesis of meso-dibromostilbene:

Reaction 2: Synthesis of Tetraphenylcyclopentadienone:

Day Two... Reaction 1: Synthesis of Diphenylacetylene:

Reaction 2: Hexaphenylbenzene:

SOURCE: All reaction diagrams were pieced together from the UChicago Organic Chemistry Lab Manual by Valerie Keller. Results: 1. Yield of Multistep Synthesis: Synthesis of meso-dibromostilbene : 1.0 g trans - stilbene (5.55E-3 mol) used with 2.0 g pyridinium tribromide (6.25E-3 mol); thus, trans-s tilbene is expected to be limiting reagent. By stoichiometry, we would have a theoretical yield of 1.89 g meso-dibromostilbene. We obtained 1.79 g meso - stilbene, thus there was a very high yield of 94.7%. Synthesis of Tetraphenylcyclopentadienone We used 200 mg of benzil (0.951 mmoles) with 200 mg of 1,3-diphenylacetone (0.951 mmoles). Thus, we expect 0.951 mmoles of Tetraphenylcyclopentadienone, or 0.366 g. We obtained 0.14 g, giving a yield of 38%.

Synthesis of Diphenylacetylene:

We used all 1.79 g of meso - stilbene (5.26E-3 moles) and obtained 0.44g of diphenylacetylene (2.47E-3 moles) for a yield of 47.0%. Synthesis of Hexaphenylbenzene : We used 0.075 g Tetraphenylcyclopentadienone (1.95E-4 moles) with 0.14 g Diphenylacetylene (7.86 E-4 moles). Thus, we should expect 1.95E-4 moles or 0.104 grams of hexaphenylbenzene. We obtained 0.06 g of product, for a yield of 57.7%. 2. We have two reactants in the final synthesis of hexaphenylbenzene: one is diphenylacetylene and the other is tetraphenylcyclopentadienone. We conducted a parallel synthesis of both of these reagents. The synthesis of diphenylacetylene had an overall yield of 94.7% x 47.0%, or 44.5%. The synthesis of tetraphenylcyclopentadienone had an overall yield of 38.0%. Thus, we will use the lower of these two percents, 38.0% in calculating our overall yield for the synthesis of hexaphenylbenzene. In our last calculation, we found that the yield for the synthesis of hexaphenylbenzene was 57.7%. We must multiply this percentage by the lower of the two overall yields in the synthesis of the reagents, which we found to be 38.0%. 57.7% x 38.0% = 21.9% overall yield. 3. Key peaks identified on attached IR spectra. Discussion: 1. Observed Melting Pt.

Literature Melting Pt.

meso- Dibromostilbene

239.8 - 240.6 ℃

241 ℃

Tetraphenylcyclopentadienone

218.8 - 219.3 ℃

219 - 220 ℃

Diphenylacetylene

61.3 - 61.9 ℃

62.5 ℃

All Literature Melting Points were derived from Wikipedia.org Based on our literature melting points, we can see that all of our intermediates were quite pure. Our meso- Dibromostilbene appeared to be a white, sand-like solid, which was in conjunction with the anticipated physical appearance. Our Tetraphenylcyclopentadienone was a blackish-purple solid powder, which also agreed with the literature physical appearance. Finally, our hexaphenylbenzene was a tan sand-like solid, as we'd expect from literature. All literature

physical properties were derived from chemspider.com . Based on these physical properties, we can say our intermediates were quite pure, which is why we had relatively low yield. 2. Tetraphenylcyclopentadienone... -2969.49 cm-1  = aromatic C=C-H stretching -1777.09 cm-1  = carbonyl C=O (ketone) -1366.18 cm-1  = aromatic C=C bonds Hexaphenylbenzene -3061.60 cm-1  = aromatic C=C-H stretching -1 -1494.41 cm = aromatic C-H stretching Essentially, these two molecules have the same peaks, except hexaphenylbenzene is missing the C=O ketone peak. This makes sense, as our tetraphenylcyclopentadienone undergoes retro D-A to remove the carbon monoxide in the bridge to product the final product. This difference shows our reaction was successful as the missing C=O peak indicates hexaphenylbenzene was obtained. 3. The twisted structure of hexaphenylbenzene-- which has the 6 phenyl groups attached perpendicularly to the central benzene-- is advantageous because it prevents steric clash between the neighboring benzyl groups. Preventing steric clashes increases the stability of the structure. In addition, there is a reduction in ring strain by having the phenyl groups perpendicular to the central ring. 4. As we recall from UV-Vis spectroscopy, light in the visible range is absorbed by pi-systems when there is a Π --> Π* transition. This is unlikely in hexaphenylbenzene because all of the pi-bonds partake in an aromatic ring. Due to this, light from the visible range wouldn't have enough energy to cause the transition. Tetraphenylcyclopentadienone, on the other hand, lacks aromaticity in the central cyclopentadiene. As such, the amount of energy needed for a Π --> Π* is lower and able to be attained by visible light. This produces the sharp color we see from the compound. 6. The diene in our synthesis of hexaphenylbenzene was tetraphenylcyclopentadienone. The dienophile was diphenylacetylene. Our diene was slightly electron poor by induction from the aryl groups. This effect is weaker weak because inductive effects are weaker than the typical resonance effects we see. Also, typically our dienes are electron-rich for successful D-A reactions. On the other hand, our dienophile was diphenylacetylene. This dienophile was an alkene, however, there isn't really much of an effect on its strength by the neighboring groups, as resonance would break aromaticity. If anything, the dienophile is a bit electron-poor as well. All of these effects considered, the HOMO-LUMO gap shouldn't be significantly affected because we don't see any resonance effects.

This is important because our final structure isn't going to be very stable and we want to have a retro-Diels Alder reaction to remove carbon monoxide from the bridge of tetraphenylcyclopentadienone.

Sources Cited: 1. UChicago Organic Chemistry Lab Manual 2. Wikipedia.org  pages for meso - Dibromostilbene, Tetraphenylcyclopentadienone, and Diphenylacetylene. 3. Chemspider.com  pages for diphenylacetylene and hexaphenylbenzene....