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Organic lab photochemical isomerization of an alkene. Trans-1,2-dibenzoylethylen

ID: 1004790 • Letter: O

Question

Organic lab photochemical isomerization of an alkene.

Trans-1,2-dibenzoylethylene was purified, mixed with ethanol and exposed to light, after isolation, we got cis-1,2-dibenzoylethylene crystals. According to TLC analysis, cis is more polar than trans which makes sense considering dipole moments in molecules. However, the melting point of the cis compound is higher than the trans, this confuses me. If trans is more stable, and usually trans isomers take longer to melt, how is this possible? please explain!!

Explanation / Answer

The stability of trans form indicates with respect to a chemical reaction but not for M.P/B.P. In general M.P/B.P depends on the intermolecular attractions.

Among the cis and trans forms, cis form is having more intermolecular attractions and this is the reason to have more Melting point.

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