exp5w08 - Experiment 5 Photochemical Isomerization of an...

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Experiment 5 Photochemical Isomerization of an Olefin: cis-1,2-Dibenzoylethylene: (TLC) Mohrig, Chapter 15 In this experiment, the technique of thin layer chromatography (TLC) along with destruc- tive and non-destructive visualization techniques will be used to monitor the progress of a reaction over time. Rotation about carbon-carbon double bonds (–HC=CH–) requires that the pi bond breaks and that the sigma bond remains connected. About 60 kcal/mol of energy is re- quired for this event to occur. If this amount of energy were supplied thermally, decom- position of the alkene would probably compete with its isomerization. There is, however, another source of energy that can supply this energy—light. Alkenes such as 1,2- dibenzoylethylene have polarizable electrons that can interact with light. When this occurs, an excited electronic state of the molecule is generated that leads to geometric changes in the molecule. In this case, the carbon-carbon double bond is twisted. Eventually, the mole- cules “relax” back to their normal electronic state with equal probability of forming the cis or trans geometric isomer. The yellow colored trans isomer, however, is more effective at ab- sorbing the visible light emitted by the lamps used in this experiment. Therefore, the color- less cis isomer accumulates over the course of the experiment. Proof that these two isomers interact differently with light can be seen in their colors; the trans isomer is yellow-orange, while the cis isomer is colorless. Such a photo- chemical isomerization is vital to the biochemistry of your vision. Vitamin A 1 (retinol) is oxidized to Vitamin A aldehyde (trans-retinal) in the liver. An enzyme located in the eye then catalyzes the isomerization of trans -retinal to cis -retinal, which becomes bound to the protein opsin to give rhodopsin. When cis-rhodopsin is exposed to light of 500 nm wave- length, isomerization of the double bond occurs to afford trans-rhodopsin. This change in geometry triggers a nerve impulse, which is interpreted by the brain as vision. trans -1,2-dibenzoylethylene most stable O O cis -1,2-dibenzoylethylene best at absorbing light O O hv please include this completed table in part 1 of your notebook reagents trans-1,2-dibenzoylethylene 95% EtOH light cis-1,2-dibenzoylethylene formula C 16 H 12 O
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