Quiz of the Day - Mono Mole

#1. Which statement best describes the results of attempted separation by fractional distillation of cis-1,2-dichloroethene and trans-1,2-dichloroethene?

The two isomers can be separated by fractional distillation, with the cis-isomer boiling at the lower temperature.

The two isomers can be separated by fractional distillation, with the trans-isomer boiling at the lower temperature.

The two isomers cannot be separated by fractional distillation because both isomers have the same boiling point.

The two isomers cannot be separated by fractional distillation because they interconvert rapidly at the temperatures at which they distill.

In the cis isomer, both chlorine atoms are on the same side of the double bond. Since chlorine is more electronegative than hydrogen, the C-Cl bonds are polar. The individual bond dipoles add up vectorially, resulting in a net dipole moment for the molecule (approximately 1.9 D). This polarity leads to stronger dipole-dipole intermolecular forces between cis-1,2-dichloroethene molecules. In the trans isomer, the chlorine atoms are on opposite sides of the double bond. While the individual C-Cl bonds are polar, their bond dipoles are oriented in opposite directions and effectively cancel each other out, resulting in a net dipole moment of zero (or very close to zero). Therefore, trans-1,2-dichloroethene molecules primarily experience weaker London dispersion forces.

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