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Synthesis of Snowflake Compound (Hexaphenylbenzene) JAMES A. FLYNN AND ALEXIS SALAZAR

Department of Chemistry and Biochemistry, St. Mary’s University, 1 Camino Santa Maria, San Antonio, Texas 78228

[email protected] [email protected]

April 16, 2018

   Abstract:

Hexaphenylbenzene could easily be described by its snowflake configuration;

however, in a more technical description, Hexaphenylbenzene is an aromatic compound with a benzene ring as its parent chain, which is further substituted with six phenyls on carbon numbers: one, two, three, four, five, and six. This aromatic compound is partially responsible for the full-color display in our modern mobile devices. The primarily purpose of the lab was to create Hexaphenylbenzene (C42H30) from various starting reagents and benzaldehyde. The experiment's synthesis procedure incorporates different mechanisms, but the most significant is the last stage, the Diels-Alder Reaction to produce the end product, Hexaphenylbenzene. Other key mechanisms used in the procedure was an Aldol Reaction, an Addition/Elimination Reaction, a Benzoin Condensation Reaction, and a Wittig's Reaction. Specific to this experiment, the synthesis procedure produced an overall percent yield of 10.2% of Hexaphenylbenzene and was verified through IR Spectroscopy Analysis. When reviewing this experiment, it is important to note that this synthesis is not the quickest method for producing Hexaphenylbenzene. The experiment's most important quality is the exposure it provides to the scientist performing the experiment. For this sole reason, the experiment would be recommended to a scientist just breaking into the field, and that a more experienced scientist might prefer to do a quicker/higher yielding synthesis procedure.

Figure 1: Bond-Line of Hexaphenylbenzene

    Introduction:

Hexaphenylbenzene is a compound that has been used to synthesize organic light emitting compounds, to aid in illuminating the display screens in both our modern mobile devices, and large electronic devices. The large organic molecules that are in display screens can be easily manufactured further from Hexaphenylbenzene. In other words, Hexaphenylbenzene is a stepping stone in the manufacturing process of each one of these light-emitting compounds. Some common examples of these large organic molecules are 5P-VA, 5P-VTPA, and 5P- DVTPA (blue-emitting compounds), and

they are commonly referred to as Organic Light Emitting Diode (OLED) (1). These blue-light emitting compounds allow for the color blue to display on the screen as well as optimize both the red and green colors of other Organic Light Emitting Diode (OLED). The reason these Hexaphenylbenzene derivatives are used in modern display screens is that of their unique thermal properties. The thermal properties allow for these compounds to operate at a low failure rate, a long-life cycle, and a low power requirement. Improvements have been made in the structure of the Hexaphenylbenzene derivatives, over the years. Resulting in this

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Results and Discussion:

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Scheme 1:

Synthesis Procedure for Hexaphenylbenzene

The products of the Wittig reaction were a mixture of trans- and cis-stilbenes

molecule exhibition of, an external quantum efficiency of 21.6%, a power efficiency of 54.3 in W-1, and an operational lifetime of 50% (2).

In the previous paragraph, Hexaphenylbenzene has shown its importance to developing an OLED. So, the experiment that follows presents a synthesis scheme that can produce Hexaphenylbenzene, at a decent overall yield. The relevance of the scheme below might only be beneficial to that of a scientist breaking into the field of organic chemistry, and would not by any means be recommended for an experienced scientist using this procedure for an industrial use. The scheme just isn't efficient enough. The scheme below utilizes the following mechanisms to obtain the final product, Hexaphenylbenzene: A Diel-Alder Reaction, an Aldol Reaction, an Addition/Elimination Reaction, a Benzoin Condensation Reaction, and a Wittig's Reaction.

prepared from Benzyltirphenyphosphonium chloride(1) and benzaldehyde(2). When the 10 ml of sodium hydroxide was added the color of the solution turned from yellow to and orange color. When water and dichloromethane was added there was a bottom orange layer (organic product) and a clear top layer. A total of 15 ml of water was added to the solution in order to better separate the layers. The melting point obtained for the product was between 91-94 degrees Celsius. The reaction resulted in 1.106 g of crystals of stilbene(4), with a percent yield of 62.5%.

The addition/elimination reaction was to convert stilbene(4) to diphenylacetylene(7). When Pyridinium per bromide was added to stilbene and acetic acid the solution turned a bright orange color. The melting point obtained from the product (diphenylacetylene) was a between 61-62 degrees Celsius. We obtained a mass of 1.069 g of crystals for the addition reaction. During the elimination reaction once the solution had been heated for a while the solution looked a yellowish-brown color. When removed from the heat the solution appeared a dark tan color. The crystals obtained from this reaction appeared white. The mass of the elimination product was 0.431 g and the melting point of stilbene dibromide was 235- 245 degree Celsius. The percent yield of the reaction turned out to be 63.9%.

For the oxidation of Benzoin(3) to Benzil(5), once the nitric acid was added it looked like a yellow foggy color. It then turned into a clear orange color, and finally a dark red color and the glass of the reflux apparatus appeared orange and was releasing orange fumes. We started the temperature at around 65 volts and reflux began around 7 min after heating. The one struggle we had all experiment was during this experiment we weren’t able to recrystallize; however, if we were to do it again, we would not cool the reaction down so fast. The melting point of

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the Benzoin(3) was 133-135 degrees Celsius and the mass of crystals were 3.153 g resulting in a 70.1% yield.

Once the potassium hydroxide was added, during the preparation of the Tetraphenylcyclopentadienone(8), the solution turned purple then black a few moments later. After recrystallization, the product was purple and shiny. The melting point obtained from this product was 214-220 degree Celsius and the mass of the product obtained was 1.45 g resulting in a 79.3% yield.

The synthesis of Hexaphenylbenzene(9) was done by performing a Diels-Alder reaction. The Diels-Alder reaction is a one-step reaction of a conjugated diene and a dienophile, which is reversible. When solution was placed in a round bottom flask it was a mix of white and black powder, when the stirrer was placed in the flask the solution began to look sticky, it then quickly began to dissolve into a purple liquid. The solution eventually began to appear red. The reflux began when red droplets fell back into solution. Once the solution was removed from heat we added 1 ml of diphenyl ether and the product was completely dissolved. The crystals obtained were a brownish-red color, when vacuum filtered, the crystals were white. More diphenyl ether was added to take away brown stains. The result of this reaction was 0.839 g of Hexaphenylbenzene and a 75.4% yield with an overall percent yield of the entire synthesis 10.2%.

Conclusion:

In conclusion, the experiment takes a longer synthetic approach to yielding of Hexaphenylbenzene; however, the trade-off for a longer synthetic process is that the experiment retains more value for a developing scientist. To elaborate on that, the longer synthetic approach provides the scientists with a laboratory experience for the following mechanisms: a Wittig's Reaction, a

Benzoin Condensation Reaction, an Addition/Elimination Reaction, an Aldol Reaction, and a Diels-Alder Reaction. With the exposure to these mechanisms, the synthesis could result in a lower overall yield such as 10.2%, which was obtained in our experiment. If you were to perform a more condensed synthesis the overall yield could result in a greater yield. Furthermore, our experiment wouldn't be recommended for industrial use for there is a more efficient synthesis that would be more beneficial for that application. Although the experiment is long, it is a flexible synthesis reaction, allowing for modifications to the reactions and procedures. One might choose to modify the procedure, for the sole purpose of synthesizing a different compound, like the compounds mentioned in the introduction for use of the color emitting in model devise displays.

Experimental Section:

General Experimental Methods:

The experimental methods used to obtain these products include the use of a sand bath on a hot plate for both melting substances and for reflux, vacuum filtration with a Buchner funnel for the collection of the crystals, purification of crystals and recrystallization. The IR and the melting points of the crystals for each experiment were obtained. The IR used to obtain certain data was Bruker’s Alpha machine. We used a melting point machine which was made by Mel-Temp II and the glassware kit was, PYREX’s Organic Chemistry Kit.

Synthesis of Benzoin:

Figure 3: Benzoin bond line drawing

    

A 1.5 ml of a 5-molar sodium hydroxide was cooled and 0.80g of thiamine hydrochloride was dissolved in 2.5ml of water. 7.5 ml of 95% ethanol was added to the thiamine and the solution was cooled. 1.5 ml of the 5-molar sodium hydroxide was added to the thiamine solution while swirling in order to keep the solution below room temperature. 5 ml of benzaldehyde was added to the reaction while swirling. The crystals that were formed were washed with 10-15 ml of cold 2:1 mixture 95% ethanol. Crystals were left to air dry for about 15 min. The solid was recrystallized from hot 95% ethanol (8 ml per gram). The melting point obtained for the product was between 91-94 degrees Celsius. We collected 3.153 g of crystals. The melting point of the product was 133-135 degrees Celsius.62.5% yield. IR: 1594.43, 3407.73.

Oxidation of Benzoin to Benzil:

Figure 5: Benzil bond line drawing

A mixture of 2g of benzoin and 7 ml of concentrated nitric acid was made. This mixture was heated for about 30 min, this was about the time it took for the nitric oxide gases to no longer be evolved. The flask was cooled while the flask remained covered. Yellow solid was collected and was washed twice with 5 ml of cool water. The product was recrystallized using 95% ethanol. It was dissolved in hot ethanol and water was added drop wise, the product then recrystallized. The melting point of the benzoin was 133- 135 degrees Celsius and the mass of crystals were 1.39g. 70.1% yield. The melting point of the product was 83-85 degrees Celsius. IR: 1441, 1708.

Preparation of Tetraphenylcyclopentadienone:

Figure 8: Tetraphenylcyclopentadienone bond line drawing

A mixture of 0.7g of Benzil, 0.7g of bibenzyl ketone, and 5 ml of absolute ethanol was made. The mixture was heated until the solids dissolved. To provide a slow reflux, 0.1g of potassium and 1 ml of absolute ethanol were added dropwise to the mixture. The mixture was allowed to reflux for 15 min. We then cooled reaction to room temperature. The mixture was then filtered and washed twice with 5 ml of cold 95% ethanol, it was then left to air dry for about an hour. We recrystallized a portion of our product using a 1:1 mixture of 95% ethanol and toluene. The melting point obtained from this product was 214-220 degree Celsius and the mass of the product obtained was 1.45 g. The percent yield was 79.3%. IR: 1441, 1708.

Wittig’s Reaction -Benzaldehyde to E-Stilbene:

Figure 4: E-Stilbene bond line drawing

A mixture containing 2mL of Benzaldehyde, 20 Mil-Mole of Benzyltirphenyphosphonium chloride and 5mL of Methylene Chloride was made. Next, mix reactant and once a liquid solution has become evident add 3.3 mL of 10 M Sodium Hydroxide. Heat solution to establish a gentle reflux. After reflux has occurred for 15 minutes, take the solution and add 3 mL of

   

water and observe organic layers. Now, separate the layers and dry the organic layer using anhydrous sodium sulfate while rinsing with 2-3 mL of Dichloromethane. Next, heat the solution evaporating the methylene chloride. Finally, you will obtain a yellow powder, and proceed to recrystallization with 95% ethanol. The mass of the final product obtained was 1.1g. The percent yield obtained was 61.% The melting point of the product was 92-94 degrees Celsius. IR: 3079.07, 3059.241, and 3021.787.

Conversion of Stilbene to Diphenylacetylene:

Figure 7: Diphenylacetylene bond line

drawing

A mixture composed of 1g of stilbene

was dissolved in 20 ml of glacial acetic acid and was heated. 2 g of Pyridinium per bromide was added while the reaction was warmed. The reaction was left at room temperature for crystallization. The crystals were then isolated. Ice cold methanol was used to rinse flask and wash product.

Be sure to heat the mixture of 1 g stilbene dibromide, 4 g triethylene glycol, and 0.5 g potassium hydroxide pellets. The apparatus was then heated to 160 degrees Celsius and the internal temperature was maintained between 160-170 degrees Celsius. The product was isolated by adding 10 ml of cold water to the diphenylacetylene. The crystals were purified by recrystallization from 95% ethanol. The mass of the elimination product was 0.431 g and the percent yield was 63.9%. The melting point of diphenylacetylene was 61-62 degree Celsius. IR: 1599.26, 3063.

Synthesis of Hexaphenylbenzene:

 Figure 9: Hexaphenylbenzene bond line drawing

 A mixture composed of 0.676g of Tetraphenylcyclopentadienone, 0.676g of diphenylacetylene, and 3.38g of benzophenone was made. The reaction was heated until it began to reflux. When no further lighting in color was observed in the reaction (after about 45 min) the heat was removed and 1 mL of diphenyl ether was added to the solution to prevent solidification of the Benzophenone. Recrystallization occurred by reheating and leaving out at room temperature. The product was collected and washed with toluene. The mass of the final product obtained was 0.839g. The percent yield obtained was 75.4%. IR: 1576, 1600, 3021, 3059, 3081.

  Supporting Information:

Copies of IR spectroscopy obtained after each process are attached.

References:

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(2)

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(3)

Chem Mater. 2004; 8:4522. doi: 10.1021/cm040081o.

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