Experiment_5

Experiment_5 - Experiment 5 Photochemical Isomerization of...

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Experiment 5: Photochemical Isomerization of cis-1,2- dibenzoylethylene and Thin Layer Chromatography (TLC) Qualitative Analysis (Mohrig Chapter 15) In this experiment, the technique of thin layer chromatography (TLC) along with 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 breaking the pi bond. About 60 kcal/mol of energy is required for this event to occur. If this amount of energy were supplied thermally, the alkene would probably decompose before it would isomerizes . There is, however, another source 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 molecules “relax” back to their normal electronic state with equal probability of forming the cis or trans geometric isomer. The trans isomer, however, is more effective at absorbing the visible light emitted by the lamps used in this experiment. Therefore, the 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 photochemical isomerization is vital to the biochemistry of 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 wavelength, 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. Now you know… Isomerization is the process of transforming a molecule whereby it has the exact same atoms, but they differ in their arrangement in space. Homogenous m e a n s t h a
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This note was uploaded on 09/28/2010 for the course SCI mcdb 1a & taught by Professor Bush during the Spring '10 term at UCSB.

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Experiment_5 - Experiment 5 Photochemical Isomerization of...

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