Photochemical Synthesis of Benzopinacol

3 March 2017

In this reaction, molecules of benzophenone was brought to n((* triplet state where it possibly abstracted hydrogen from isopropyl alcohol and through subsequent reactions of radicals it formed two diphenyl ketyl radical which dimerized into benzopinacol. Subsequently, mixture of synthesized benzopinacol, glacial acetic acid and iodine crystal reacted in an acid-catalyzed rearrangement wherein dehydration of benzopinacol resulted into the formation of carbocation. In this state, an aromatic shifted and then, forming delocalized carbocation.

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Afterward, regenerating the catalyst stabilized the molecule into benzopinacolone. After qualitative and quantitative analysis of the products, it was found out that benzopinacol was successfully synthesized as attested by the IR spectrum which contained OH broad stretch at 3417. 86 cm-1-3460. 30 cm-1 and overtones at 1800 cm-1-1950 cm-1 region. However, inconsistency was observed in the determined melting point of 200(C-210(C which deviated from theoretical 47. 9(C perhaps due to improper use of the apparatus.

Conversely, synthesis of benzopinacolone was not a success as there was no drastic change observed in its IR spectrum to that of benzopinacol and deviation on determined melting point from 175(C-177(C to 190(C-197(C which maybe due to incompletion of reaction. Low yields were also obtained after the experiment as only 18. 16 %( 0. 73g) benzopinacol and 21. 88% (0. 104g) benzopinacolone was collected. I. Introduction Photochemistry is the study of chemical reactions, isomerizations and physical behavior which occurs under the influence of visible or ultraviolet light.

There are two fundamental laws in regard with this principle. First, the Grotthuss-Draper law which states that light must be absorb by the compound so as to initiate photochemical reaction. Second, the Stark-Einstein’s law which states that for each photon of light absorbed by the compound, only one molecule is activated for proceeding reactions. Here, the absorption of visible or ultraviolet light excites the molecules wherein change in molecular orbital occupancy, an increase in energy, change in local distribution and change in charge distribution occurs.

This excitation results in the population of higher vibrational levels where several phenomena may then take place such as the vibrational energy lost may be use to relax the molecule bringing it to zero vibrational level. Another, the excited state may return to ground state by emitting photon. The energy of this emitted light is lower to the initially absorbed light. This radiative decay is called fluorescence if it takes place rapidly from initial to excited state. And, phosphorescence if it occurs slowly by another excited state. And lastly, the molecule may cleave into radicals.

For better illustration, In the Jablonski diagram, shown above, it features possible routes for excited molecule to return into its ground state. In the diagram, electronic states of molecules and transition states are shown. The states are arranged vertically by energy and are grouped horizontally by their spin multiplicity. This visually presents the mechanism in molecule excitation and relaxation. Mostly for aryl ketones like benzophenone, after excitation, it may possibly undergo hydrogen abstraction, bond cleavage or cycloaddtion.

Another principle to be employed in this synthesis is the pinacol rearrangement. In this phenomenon, the molecule is dehydrolyzed in the presence of an acid and thus, formation of carbocation occurs. Then, a shift is observed by one of the atoms to the carbocation. And finally, to stabilize the molecule, catalyst is then regenerated to yield the final product. In this experiment, benzopinacol is to be synthesized through photochemical reaction from benzophenone and benzopinacolone via acid-catalyzed rearrangement of benzopinacol. II. Methodology

In this experiment, benzopinacol was to be synthesized through photochemical reaction and its acid-catalyzed rearrangement product benzopinacolone. Synthesis of Benzopinacol In this synthesis, 2. 0g of benzophenone was dissolved in 50ml isopropyl alcohol in 50ml Erlenmeyer flask. In this solution, one drop of glacial acetic acid was added. It was then filled with isopropyl alcohol up to the brim. After, the flask was stoppered using a well-rolled cork. It was ensured that very little air as possible was trapped inside the flask. It was tightly bind using a parafilm.

The flask was inverted and exposed to sunlight outside the laboratory. After all additional product ceased to form, the reaction mixture was cooled in an ice bath to allow precipitation of benzopinacol. The final product was then filtered off from the solution using a Buchener funnel. Its melting point, yield and infrared spetrum was then obtained. The Acid-Catalyzed Rearrangement of Benzopinacol In this synthesis, in a test tube, 2. 5ml of glacial acetic acid and a small crystal of iodine were placed. In this test tube, 0. 5g of benzopinacol recently synthesized was added.

The solution was then heat to dissolve benzopinacol and further heated for 5 minutes. After a stiff paste product formed, the reaction mixture was cooled to room temperature. The crystalline mass that formed was subsequently reduced to pieces. Using a little ethanol, the paste was thinned. The mixture was then centrifuged and the supernant was decanted leaving the solids in the test tube. Another two centrifugations were performed, note that, small amount of ethanol was added after each decantation. The product formed was then transferred to a filter paper by resuspending the solids in a little cold ethanol.

Then, it was filtered through suction. It was subsequently crystallize by dissolving the filtered solids in a 5ml of 2:1 mixture of toluene and hexane in a pre-weighed vial. The vial was covered with aluminum foil with holes and was left in the fume hood to dry the product. Its melting, yield and infared spectrum was obtained the next meeting. III. Results and Discussion In this experiment, it is aimed to synthesized benzopinacol from benzophenone via photochemical reaction and benzopinacolone through acid-catalyzed rearrangement.

In the synthesis of benzopinacol, benzophenone underwent photochemical reaction. Here, the starting material was exposed to sunlight which supplied enough energy to excite its molecules. Benzophenone upon absorbing light, undergone a rapid intersystem crossing of n((* singlet state to an energetically close (((* triplet state. Then, the latter rapidly decayed into n((* triplet state. This pathway is shown on the following diagram: Figure 1: Benzophenone Excited State In the diagram, excited benzophenone was relaxed through conversion to a triplet state and subsequently relaxed via phosphorescence.

After photoexcitation of benzophenone, hydrogen abstraction reaction subsequently proceeded. Figure 2: Hydrogen Abstraction of Isopropyl Alcohol The n((* triplet state of carbonyl compounds is diradicaloid in nature hence, possibly participate in hydrogen abstraction. In Figure 2, in this mechanism, hydrogen of isopropyl alcohol was abstracted by benzophenone in a n((* triplet state to yield diphenyl ketyl and dimethyl ketyl radical. Figure 3: Radical Transfer Here, radical transfer from the dimethyl ketyl radical to benzophenone occurred in which yielded acetone and another diphenyl ketyl radical.

Figure 4: Dimerization of Diphenyl Ketyl Radicals In this last mechanism, the two previously produced diphenyl ketyl radicals dimerized to form benzopinacol. Prior to the reaction mechanism, as done in the procedures, one drop of glacial acetic acid was added in the reaction mixture. It was done to remove the alkali which maybe present in the mixture that would consequently cause decomposition of the product to benzophenone and benzohydrol. After the synthesis, qualitative analysis was done to the synthesized product to verify its success. Figure 5: IR Spectrum of Synthesized Benzopinacol

Examining the structure of benzopinacol, it was to be expected to have a broad –OH stretch and overtones region. In the infrared spectrum of the synthesized product as shown in Figure 5, there was a broad stretch at 3417. 86 cm-1-3460. 30 cm-1, encircled in blue, which is markedly due to hydroxyl present in the compound. And, an overtones at 1800 cm-1-1950 cm-1 as shown in the Figure5, encircled with red, is mainly due to the presence of aromatic rings. Another qualitative test done was the melting point determination of the final product. Theoretically, the melting point of benzophenone is 47. 9(C.

However, the synthesized product melted at 200(C-210(C. This inconsistency maybe due to improper use of melting point apparatus since there was observed consistency with the infrared spectrum obtained. Quantitatively, the theoretical yield of the product must be 4. 02g however in the synthesis 0. 73g or only 18. 16% was synthesized. Loss of material was maybe due to long storage of the product inside the locker wherein some products spilled out the container. From the synthesized benzophenone, 0. 5g was used to synthesize benzopinacolone via acid catalyzed rearrangement. Figure 6: Benzopinacol Rearrangement

In this disproportionation reaction, glacial acetic acid protonated one of the hydroxyl group, hence giving a positive charge on oxygen. Consequently, the compound was then dehydrated where water was removed and thus forming a carbocation. Then, it underwent pinacolone rearrangement wherein an aromatic migrated to the carbocation previously formed and hence, forming a more stable carbocation intermediate where charge was delocalized in a heteroatom as H+ is attached with oxygen. And since, H+ was the catalyst, it was regenerated thus stabilizing the molecule forming benzopinacolone.

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