Experiment 6

Experiment 6 - Experiment 6: Photochemical Isomerization of

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7 Experiment 6: Photochemical Isomerization of cis-1,2- dibenzoylethylene and Thin Layer Chromatography (TLC) ( Mohrig Chapter 15) In this experiment, the technique of thin layer chromatography (TLC) along with de- structive 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 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 isomerize (see margin for definitions of bolded words). 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, how- ever, 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 photochemi- cal 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 ex- posed to light of 500 nm wavelength, isome- rization 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. 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 2 C 16 H 12 O 2 equiv 1 excess excess 1 MW 236.27 mg/mmol 236.27 mg/mmol den vol 2.0 mL mass 25 mg 25 mg mmol. 0.105 mmol 0.105 mmol mp trans -1,2-dibenzoylethylene most stable O O cis -1,2-dibenzoylethylene poor at absorbing light O O hv 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
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This note was uploaded on 04/09/2010 for the course CHEM 6a taught by Professor Pettus during the Winter '07 term at UCSB.

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Experiment 6 - Experiment 6: Photochemical Isomerization of

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