Step 2 -Photoisomerization\' The trans-dibenzoylethene is dissolved in a mixture

Question

Step 2 -Photoisomerization' The trans-dibenzoylethene is dissolved in a mixture of ethyl acetate: hexane (1:6). Approximately 7 mL of solvent is required for every 1 mmol of trans-dibenzoylethene. (Gentle heating may be required.) This yellow solution is placed in an Erlemeyer flask, stoppered, and placed in the sunlight. After one week, the solution is placed in an ice bath to help maximize the yield of white crystals consisting of the cis isomer. Shorter reaction time periods have been reported (ca. 5 hrs)

Explanation / Answer

trans-dibenzoylethene is dissolved in a mixture of ethyl acetate and hexane (1:6) because it consists polar C=O group, phenyl rings and ethylene group which should be dissolved in the ethyl acetate but the reaction is a photochemical reaction so much amount of hexane will provide less evaporation of the ethyl acetate and together with this due to slow evaporation it will increase the rate of crystal growth. for making the saturated solution of reaction mixture less amount of ethyl acetate is needed and due to much density in comparison with hexane, it will present in the bottom layer of the flask.

Gentle heating is done to increase the solubility of trans-dibenzoylethene in less amount of ethyl acetate solvent. In 7 ml we can say 1 mL ethyl acetate and 6mL hexane is needed.

Erlemeyer flask with stopper is useful for the crystal growth of the given compound and solvent will not evaporate due to the stopper hence it will generate the vapor pressure inside the flask to grow the crystal of photochemical product.

The solution is kept in an ice bath after one week to get more yield because the ice will provide the cooling to the solution by absorbing the heat of solution will cause the much freezing of solution in the form of a white color crystal.

The shorter reaction time may also possible in the reaction but the quantity of the yield may be different or most probably less.

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