Photochemical Synthesis of Benzopinacol and Its Acid-Catalyzed Rerrangement Product Benzopinacolone

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Photochemical Synthesis of Benzopinacol and its Acid-Catalyzed Rerrangement Product Benzopinacolone

M.E.G. Balita
Institute of Chemistry, University of Philippines, Diliman, Quezon City Date Performed: February 18, 2011
Date Submitted: March 4, 2011
In this experiment it is aimed to synthesize benzopinacol through photochemical reaction of benzophenone and, benzopinacolone via acid-catalyzed rearrangement of benzopinacol. In this experiment, mixture of benzophenone, isopropyl alcohol and a drop of glacial acetic acid was exposed to sunlight which in turn, undergone photochemical reaction. 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. 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...
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