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Hi A-Level/JC Chemistry Tuition Students – Singapore
For the same molecular formula, the cis isomer has a higher boiling point than the trans isomer.
Reasons:
_ The cis isomer is polar while the trans isomer is non-polar.
_ The polar cis isomer has permanent dipole- permanent dipole forces of
attraction between its molecules while the non-polar trans isomer has weak induced dipole-induced dipole forces of attraction between its molecules.
_ More energy is required to overcome the stronger permanent dipolepermanent dipole forces between the molecules of cis isomer.
_ Hence, the cis isomer has a higher boiling point than the trans isomer.
Why is the melting point of the cis isomers lower?
You might have thought that the same argument would lead to a higher melting point for cis isomers as well, but there is another important factor operating.
In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid.
Trans isomers pack better than cis isomers. The “U” shape of the cis isomer doesn’t pack as well as the straighter shape of the trans isomer.
The poorer packing in the cis isomers means that the intermolecular forces aren’t as effective as they should be and so less energy is needed to melt the molecule – a lower melting point.
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